Vitamins for rats is a three-part series that includes: A Guide to Vitamins for Rats, Vitamins for Gym Rats and Vitamins for Pet Rats. Accessory items will also be available including products like vitamin C supplements and other vitamins that cater specifically to rat owners.
Do you want to make your pet rats healthier and happier? These vitamins are specially formulated for your pet’s needs.
vitamins for rats
Vitamins are food for your rats; thiamine, riboflavin, niacin and more. Rats need vitamin B-12 daily to remain healthy.
Shop our selection of rodent supplements that are packed with vitamins, minerals, and other nutrients required by your rattie. These quality rodent supplements will give your pet a healthy meal of vitamins and minerals that boost their health and reduce the risk of illness.
what vitamins do rats need
Nutrient Requirements of the Laboratory Rat
The laboratory rat (Rattus norvegicus) has long been favored as an experimental model for nutritional research because of its moderate size, profligate reproduction, adaptability to diverse diets, and tractable nature. It is now the species of choice for many experimental objectives because of the large body of available data and the development of strains with specific characteristics that facilitate the study of disease and other processes.
Go to:
Origin Of The Laboratory Rat
The laboratory rat is a domesticated Norway rat, which in nature is one of the most widespread and abundant of the more than 70 species of the genus Rattus (family Muridae). The albinos of the Norway rat were first domesticated in Europe in the early 19th century and came into use as experimental animals shortly thereafter (Lindsey, 1979). The Norway rat is not indigenous to Europe; however, it is believed to have originated in Asia and to have taken advantage of human movement in expanding its range worldwide (Nowak, 1991). The origin and historical development of the major strains of laboratory rats have been reviewed by Lindsey (1979).
In the wild, the Norway rat exhibits both territorial and colonial behavior and typically occupies underground burrows (Calhoun, 1963). Females produce 1 to 12 litters per year, and those in a colony nurture their young collectively. The Norway rat is omnivorous, eating a wide variety of seeds, grains, and other plant matter as well as invertebrates and small vertebrates (Nowak, 1991). Other than the fact that it lacks a gallbladder, the rat’s digestive tract resembles that of other omnivorous rodents in that the stomach contains both nonglandular and glandular regions, the small intestine is of moderate length, and the cecum is relatively well developed (Bivin et al., 1979; Vorontsov, 1979).
Go to:
Growth And Reproductive Performance
Growth and reproductive performance are two key indicators of dietary adequacy. It is important that investigators monitor performance of experimental animals in relation to expected patterns of weight gain and reproduction. Given the large number of strains and different genotypes, it is not possible to describe a single growth pattern or reproductive performance applicable to all laboratory rats. Poiley (1972) summarized body weight gains from birth to about 24 weeks of age for 18 inbred strains and 3 outbred strains of rats. Examples of his mean values for 5 major inbred strains (Brown Norway, Fischer 344, Long-Evans, Osborne-Mendel, and Sprague-Dawley) are illustrated in Figure 2-1. Males gain weight more quickly and become larger than females of the same strain, but there are considerable differences in growth rates and adult body mass among the strains. In view of ongoing genetic selection of the strains of rats, and improvements in diets, the mean growth rates shown in Figure 2-1 may not represent desired performance at present. However, Reeves et al. (1993a) found similar growth rates in Sprague-Dawley rats fed a commercial rat diet for 16 weeks. Investigators should obtain expected or desired growth curves for their experimental rats from the supplier or breeding colony from which animals originate.
TABLE 2-1. Some Reproductive Characteristics of Representative Strains of Inbred and Outbred Rat Colonies Maintained at the National Institutes of Health.
TABLE 2-1
Some Reproductive Characteristics of Representative Strains of Inbred and Outbred Rat Colonies Maintained at the National Institutes of Health.
Reproductive characteristics such as age at the time reproduction begins, fertility, litter size, growth rates of suckling young, and preweaning mortality also vary among strains. C. T. Hansen (Veterinary Resources Program, National Center for Research Resources, National Institutes of Health, personal communication, 1993) recently summarized selected reproductive parameters of rat strains maintained in breeding colonies at the National Institutes of Health, including 37 monogamously mated inbred strains, 42 harem-mated congenic strains, 5 harem-mated mutant stocks, and 3 monogamously mated outbred stocks. Differences in the percentage of sterile matings, litter size, and preweaning mortality were evident among major inbred strains such as Brown Norway, Fischer 344, Osborne-Mendel, Sprague-Dawley, and Wistar (Table 2-1). Outbred strains tended to have higher reproductive performance (Table 2-1). Thus the expected reproductive performance of rats in experimental studies may vary according to strain and system of breeding.
FIGURE 2-1. Mean body weight of male and female rats of five inbred strains: _ Brown Norway; _ , Fischer 344; _ Long-Evans; _ Osborne-Mendel; and . FIGURE 2-1 Mean body weight of male and female rats of five inbred strains: Brown Norway; _ , Fischer 344; _ Long-Evans; _ Osborne-Mendel; and _. Sprague-Dawley. SOURCE: Data adapted from Poiley (1972).
Go to:
Estimation Of Nutrient Requirements
Although the nutrient requirements of the laboratory rat are better known than those of other laboratory animals, there can be considerable disparity in estimated requirements as a consequence of the criteria used (Baker, 1986). For example, the amounts of nutrients required to sustain maximum growth of young rats may be different from the amounts needed to maintain tissue concentrations or to maximize functional measures such as enzyme activities. Moreover, nutrient requirements are not static; they change according to developmental state, reproductive activity, and age. There is also evidence of differences in requirements between males and females as well as among various inbred and outbred strains. The nutrient requirements listed in this chapter represent average values, but they may not suffice in all circumstances. There is a need for further research that will identify the sources of variation in nutrient requirements.
Recommended nutrient concentrations in this report have not been increased to allow a margin of safety for variation in dietary ingredients or for differences among rats. The data on which requirements are based were reported from many different laboratories that used various colony management practices. One may assume that the recommendations are adequate for rats in most laboratory conditions, but particular experimental protocols such as maintenance of germ-free colonies or testing of experimental drugs (see Chapter 1) may alter the requirements for one or more nutrients. In some cases sufficient data were available to differentiate the nutrient requirements for adult maintenance from those for growing, pregnant, or lactating rats; hence, estimates of requirements are provided for maintenance, growth, and reproduction (Table 2-2). If data available were insufficient to determine requirements, adequate concentrations are reported on the basis of long-term feeding. If cited papers provided nutrient intakes per day but did not specify dietary concentrations, the values have been converted to dietary content by assuming a dietary intake of 15 g/rat/day for growing rats or adult rats at maintenance, 15 to 20 g/rat/day during pregnancy, and 30 to 40 g/rat/day during lactation.
TABLE 2-2. Estimated Nutrient Requirements for Maintenance, Growth, and Reproduction of Rats.
TABLE 2-2
Estimated Nutrient Requirements for Maintenance, Growth, and Reproduction of Rats.
Examples Of Diets For Rats
The type of diet and its nutrient composition will vary according to experimental objectives (see Chapter 1). Most practical diets include nutrient concentrations that exceed requirements as a margin of safety. Examples of natural-ingredient diets based on detailed formula specifications and used successfully to maintain rat colonies at the National Institutes of Health and at other facilities are provided in Table 2-3. The ingredient specifications, however, have not been updated for some years and are not entirely in agreement with recommendations in this report.
TABLE 2-3. Examples of Natural-Ingredient Diets Used for Rat and Mouse Breeding Colonies at the National Institutes of Health.
TABLE 2-3
Examples of Natural-Ingredient Diets Used for Rat and Mouse Breeding Colonies at the National Institutes of Health.
As indicated in Chapter 1, natural-ingredient diets do not offer the same control over nutrient concentrations or potential contaminants as do purified diets. An example of a purified diet that has often been used in rat studies is given in Table 2-4. However, as there has been concern about the standardization and concentrations of some constituents, as well as the clinical observation of nephrocalcinosis in females when this diet is used (Reeves, 1989; Reeves et al., 1993a), a change in formulation is warranted. A committee of the American Institute of Nutrition (AIN) has reformulated and tested new purified diets for the growth (AIN-93G) and maintenance (AIN-93M) of rats and mice (Reeves et al., 1993a,b; Reeves et al., 1994). These diets are included in Table 2-5.
TABLE 2-4. Example of a Commonly Used Purified Diet (AIN-76A) for Rats.
TABLE 2-4
Example of a Commonly Used Purified Diet (AIN-76A) for Rats.
TABLE 2-5. Examples of Recently Tested Purified Diets for Rapid Growth of Young Rats and Mice or for Maintenance of Adult Rats and Mice.
TABLE 2-5
Examples of Recently Tested Purified Diets for Rapid Growth of Young Rats and Mice or for Maintenance of Adult Rats and Mice.
Go to:
Energy
Purified diets containing 5 to 10 percent fat have gross energy (GE) values of about 4.0 to 4.5 Mcal/kg (17 to 19 MJ/kg). The digestible energy (DE) of most purified diets ranges from 90 to 95 percent of GE (Hartsook et al., 1973; McCraken, 1975; Deb et al., 1976). The metabolizable energy (ME) varies from 90 to 95 percent of DE (Hartsook et al., 1973; Pullar and Webster, 1974; McCraken, 1975; Deb et al., 1976). These values may be somewhat lower when diets formulated from natural-ingredients are used (Yang et al., 1969; Peterson and Baumgardt, 1971a). The addition of cellulose to natural-ingredient diets decreases energy digestibility (Yang et al., 1969; Peterson and Baumgardt, 1971a) even though 15 to 60 percent of the cellulose is digested (Conrad et al., 1958; Yang et al., 1969; Peterson and Baumgardt, 1971b). Some of the decrease in energy digestibility is caused by the low digestibility of cellulose and some is caused by increased fecal nitrogen losses (Meyer, 1956).
In general the rat will consume food to meet its energy requirement (Brody, 1945; Mayer et al., 1954; Sibbald et al., 1956, 1957; Yoshida et al., 1958; Peterson and Baumgardt, 1971b; Kleiber, 1975). Yoshida et al. (1958) reported that the daily caloric intake remained constant when the diet contained from 0 to 30 percent fat. A proportionate increase in consumption of diet occurs when the diet is diluted with inert materials. A maximum concentration of 40 percent dilution of the diet could be made for weanling female rats before caloric intake was reduced, whereas 50 percent dilution could be made for mature females (Peterson and Baumgardt, 1971b). The energy requirement of the lactating female is high, and dilution of the diet with only 10 percent of inert material results in a significant decrease in DE intake (Peterson and Baumgardt, 1971b). Inadequate dietary protein may decrease energy intake (Menaker and Navia, 1973).
Temperature, age, and activity influence the energy requirement of the rat. The lower critical temperature of the fasting rat is 30°C (Swift and Forbes, 1939; Brody, 1945; Kleiber, 1975). The lower critical temperature is that environmental temperature below which heat production must be increased to maintain body temperature. The basal metabolic rate of the rat can be estimated from the general formula
Image img00001.jpg
where Hkcal is the heat production in kcal per day, BW is the body weight in kilograms, and 72 is the average heat production (kcal) per kg0.75 of 26 groups of rats studied (Kleiber, 1975). This formula is valid only for estimating the basal heat production of mature animals. If kilojoules (kJ) are used as the unit of energy, the formula is Hj = 301 BWkg0.75.
Maintenance
The maintenance energy requirement can be generally defined as that portion of the total energy requirement that is separate from the needs for growth, pregnancy, and lactation. Animals fed at maintenance are in energy equilibrium. Advantages and disadvantages of the methods by which the maintenance energy requirement may be estimated have been reviewed by van Es (1972). The maintenance energy requirement is usually expressed as energy required per unit of body weight in kilograms to the 0.75 power (BWkg0.75), and is based on the work of Brody (1945) and Kleiber (1975).
An estimate of the maintenance requirement for the adult rat (300 g) can be made by increasing the value for basal heat production (300 BWkg0.75; Kleiber et al., 1956) by approximately 20 percent (Morrison, 1968) to cover the expected requirement for activity in a laboratory setting. This value, which is in net energy units, can be converted to ME units by dividing by 0.75, based on an efficiency of 75 percent in conversion of ME to net energy (McCraken, 1975). The daily maintenance energy requirement in ME units for adult rats, based on these assumptions, is 114 kcal ME/BWkg0.75 (477 kJ ME/BWkg 0.75). This value is remarkably similar to direct estimates of the maintenance energy requirement in ME units at 100 kcal (418 kJ; Pullar and Webster, 1974), 106 kcal (444 kJ; McCraken, 1975), 130 kcal (544 kJ; Deb et al., 1976), and 91 kcal (381 kJ; Ahrens, 1967). However, the requirement for fat rats (e.g., obese Zucker) is approximately 15 percent lower than the requirement for normal rats (Pullar and Webster, 1974; Deb et al., 1976). As this difference in maintenance energy requirement between the adult normal and adult obese Zucker is not determined by body weight, the practice of estimating maintenance energy requirement solely from body weight must be used with caution (Pullar and Webster, 1974; Webster et al., 1980).
It is well established that resting heat production per BWkg0.75 is greater in working and producing animals than in nonworking animals (Brody, 1945). A portion of this difference is attributable to differences in amount of intake. Walker and Garrett (1971) demonstrated that a decrease in food intake of rats results in a decrease in energy expenditure and in the maintenance energy requirement.
Previous plane of nutrition also influences maintenance energy requirement. Rats fed at a high plane of nutrition (39.6 g/BWkg0.75/day) for a 3-week period had a 38 percent greater heat production when compared to rats on a low plane (28.8 g/BWkg0.75/day) of nutrition (Koong et al., 1985). Experiments with sheep indicate that changes in organ mass associated with amount of intake and sequence of feeding may be largely responsible for altered heat production (Koong et al., 1985). Thus an accurate prediction of the maintenance energy requirement of the rat requires consideration of the amount of intake, previous nutritional history, physiological state, and other factors including the composition of the body (see Baldwin and Bywater, 1984, for a review). Nonetheless, data suggest that maintenance energy requirements of the rat will be met in most cases by consumption of 112 kcal/BW kg0.75/day (470 kJ/BWkg0.75/day).
Growth
It is difficult to estimate the energy requirement for growth because of variation in the composition of weight gain (Meyer, 1958; Schemmel et al., 1972; Hartsook et al., 1973; McCraken, 1975; Deb et al., 1976) and in the energetic efficiency of net protein and fat synthesis. Kielanowski (1965) used a multiple regression model that partitioned total ME intake (MEI) into a component proportional to body weight, representing maintenance energy requirement, a component proportional to energy gain as fat, and a component proportional to energy gain as protein or lean body mass. Separation of the efficiencies of energy gain in fat and lean is problematic, however, leading to inconsistent and sometimes biologically impossible results (see Chapter 3, which deals with the mouse). Pullar and Webster (1974) attempted to circumvent some of these difficulties by using obese and lean Zucker rats, which partition energy differently between gain in fat and in lean at all stages of growth. The energetic efficiencies of fat and net protein synthesis were estimated at 65 and 43 percent, respectively. These estimates assumed that the maintenance energy requirement remained constant. In a subsequent experiment that did not require assumptions about maintenance energy requirements, Pullar and Webster (1977) determined the energetic efficiencies of fat and net protein deposition as 73.5 and 44.4 percent, respectively. Kielanowski (1976) concluded from a review of previous work that the energetic efficiency of net protein deposition in growing rats is approximately 43 percent.
The requirement of the rat for maintenance and growth can be met by diets with a wide range of energy densities. Peterson and Baumgardt (1971b) reported that weanling and mature rats consumed 225 and 150 kcal DE/BWkg0.75 (940 and 630 kJ/BWkg0.75), respectively, when the energy density of the diet varied from 2.5 to 5.0 kcal DE/g (10.5 to 20.9 kJ/g). When the energy density in the diet fell below 2.9 kcal/g (12.1 kJ/g), the weanling rat could not meet its energy requirement. The mature rat could meet its energy requirement until DE density fell to values below 2.5 kcal DE/g (10.5 kJ/g). These values are equivalent to diluting the diet with 40 and 50 percent inert material for weanling and mature rats, respectively. During the 4-week growth period after weaning at 21 days postpartum, the average daily energy requirement is at least 227 kcal/BWkg0.75) and may be greater. A diet con taining at least 3.6 kcal ME/g (15.0 kJ ME/g) will meet the energy requirement for maintenance and growth if rats are allowed free access to food and the diet is not deficient in other nutrients.
Gestation And Lactation
The energy requirement for gestation appears to be 10 to 30 percent greater than that of the mature but nonreproductive female rat (Morrison, 1956; Menaker and Navia, 1973). Food intake of rats fed diets adequate in protein increased 10 to 20 percent (Menaker and Navia, 1973) or 20 to 30 percent during the first days of gestation and up to 140 percent by days 16 to 18 of gestation (Morrison, 1956). Total heat production in pregnant rats increased approximately 10 percent above that of nonpregnant female rats (Brody et al., 1938; Kleiber and Cole, 1945; Morrison, 1956; Champigny, 1963). Approximately one-third of the 100 to 201 kcal (420 to 840 kJ) stored during gestation is deposited in fetal tissues (Morrison, 1956). Restriction of the diet during gestation decreases the size and viability of the young and may induce resorption (Perisse and Salmon-Legagneur, 1960; Berg, 1965). Protein appears to be more critical than energy for satisfactory reproduction (Hsueh et al., 1967; Menaker and Navia, 1973).
The daily ME requirement of the rat is about 143 kcal/BWkg0.75 (600 kJ/BWkg0.75) in early gestation and may increase to 265 kcal/BWkg0.75 (1,110 kJ/BWkg0.75) during the later stages of gestation. Lactating rats consume from two to four times more energy than nonlactating female rats, and the magnitude of increase depends on the number of offspring being nursed (Nelson and Evans, 1961; Peterson and Baumgardt, 1971b; Menaker and Navia, 1973; Grigor et al., 1984). Some of the increase in measured intake late in the lactation period may be caused by the consumption of diet by the litter, but this is not significant until about 15 to 17 days postpartum. In spite of the large increase in feed consumption, however, rats are generally in negative energy balance at peak lactation. Losses of both body fat and protein can occur. In general it appears that lipid is stored in the maternal body during gestation and then mobilized to support the lactation process (Sampson and Janson, 1984). Naismith et al. (1982) concluded that hormonal factors were responsible for the storage of lipid during gestation and the mobilization of body fat during lactation. They suggested that body fat supplies a major portion of the energy required to support lactation. Sainz et al. (1986) fed lactating rats diets containing 12, 24, and 36 percent protein and 4.50, 4.37, and 4.04 kcal ME/g (18.8, 18.3, and 16.9 kJ/g). Although fed ad libitum, the lactating rats lost body fat from days 7 to 14 of lactation regardless of diet. It is difficult to establish an energy ”requirement” for the lactating rat based on body energy change. At peak lactation, rats rearing 8 pups produced about 41 g milk/day, representing a milk energy output of about 239 kcal/BWkg0.75/day (1,000 kJ/BWkg0.75 /day) (Kametaka et al., 1974; Oftedal, 1984). It is likely that during peak lactation, the dam’s daily ME requirement to support lactation will be at least 311 kcal/BWkg0.75 (1,300 kJ/BWkg0.75), but the ME requirement will vary with litter size.
Go to:
Lipids
Lipid is an important component of the rat diet because it provides essential fatty acids (EFA) and a concentrated energy source, aids in the absorption of fat-soluble vitamins, and enhances diet acceptability.
Essential Fatty Acids
n-6 Fatty Acids
The rat requires fatty acids from the n-6 family as a component of membranes, for optimal membrane-bound enzyme function, and to serve as a precursor for prostaglandin formation (Mead, 1984; Dupont, 1990; Clandinin et al., 1991). Linoleic acid [18:2(n-6)] cannot be synthesized endogenously but can be elongated and desaturated to form arachidonic acid [20:4(n-6)]; thus, the requirement for n-6 fatty acids can be met through dietary linoleic acid. Early estimates of the requirement for n-6 fatty acids were made by using growth and skin condition as response criteria. The requirement for growing male rats was estimated to be 50 to 100 mg/day and that of growing female rats to be 10 to 20 mg/day (Greenberg et al., 1950). That the requirement differs between sexes is in agreement with early observations that male rats are more susceptible than female rats to the development of EFA deficiency signs (Loeb and Burr, 1947). A series of later experiments indicated that other lipid components of the diet [e.g., oleic acid (Lowry and Tinsley, 1966) and cholesterol (Holman and Peiffer, 1960)] increased the need for n-6 fatty acids. Also, Holman (1960) demonstrated that more linoleic acid is needed in high-lipid diets versus low-lipid diets in order to meet the EFA requirement of the rat. Therefore it has become common practice to express the requirement as a percent of dietary ME rather than as a percent of diet weight. The Δ6 desaturase enzyme involved in the synthesis of long-chain polyunsaturated fatty acids uses the fatty acid substrates in the following order: n-3 > n-6 > n-9 (Johnston, 1985).
Holman (1960) found that the ratio of 20:3(n-9) to 20:4(n-6) (also called the triene:tetraene ratio) was relatively constant in tissues until clinical signs of EFA deficiency began to develop. Because this biochemical measure was both objective and relatively stable (the precise ratio varied slightly among tissues), Holman (1960) initiated the use of the triene:tetraene ratio to indicate inadequate EFA status. Diets high in n-3 fatty acids also lower the triene concentration (Morhauer and Holman, 1963; Rahm and Holman, 1964) because of the competition for desaturation mentioned above. Therefore the triene:tetraene ratio is a valid indicator of EFA status only when n-6 fatty acids are the principal unsaturated dietary fatty acids. Pudelkewicz et al. (1986) used this technique (in conjunction with dermatitis and growth) to estimate the linoleic acid requirement of growing female and male rats to be 0.5 percent and 1.3 percent of dietary ME, respectively.
The requirement for pregnancy is met by diets adequate for growth, while that for lactation is somewhat higher. Deuel et al. (1954) conducted two experiments to estimate these requirements. In the first experiment, 0, 10, 40, 100, 200, 400, and 1,000 mg cottonseed oil (approximately 50 percent linoleic acid) were fed daily. The number of litters born and the average number of pups per litter increased and then plateaued at 10 and 100 mg dietary cottonseed oil (e.g., 5 and 50 mg linoleate), respectively. When pure linoleate was fed as the fat source in a second experiment in which rats were provided with 0, 2.5, 5.0, 10, 20, 40, and 80 mg linoleate daily, the maximum litters born and average number of pups per litter increased and then plateaued at 2.5 and 20 mg linoleate, respectively. This higher amount of linoleic acid intake will be achieved if the pregnant rat consumes 18 g/day and the diet contains 0.25 percent of dietary ME as linoleic acid.
The requirement for lactation also can be estimated from these experiments. Indicators of successful lactation (the maximum average weight per pup in a litter at 21 days and minimum percent mortality between 3 and 21 days) reached a plateau between 100 to 200 mg cottonseed oil (50 to 100 mg linoleate) in the first experiment. In the second experiment, these criteria for successful lactation reached a maximum at 80 mg of linoleate. Because greater amounts of linoleate were not tested, it is not possible to determine whether the optimal requirement for lactation is more than 80 mg/day. In addition, the authors did not indicate the range of experimental error; therefore, a precise requirement cannot be established. Assuming a requirement of 100 mg linoleate/day, a lactating rat consuming 35 g diet/day will consume sufficient linoleate for lactation (100 mg/day) if the diet contains 0.68 percent of dietary ME as linoleate (approximately 0.30 percent of the diet).
Bourre et al. (1990) proposed that the n-6 fatty acid requirement be established as the percent of dietary linoleate that results in a constant concentration of tissue arachidonic acid. They found that constant concentrations of arachidonic acid were achieved at 150 mg (nerve tissue), 300 mg (testicle), 800 mg (kidney), and 1,200 mg (liver, lung, heart) linoleate/100 g diet. The minimal requirement then was expressed as 1,200 mg linoleic acid/100 g diet (or approximately 2.5 percent of dietary ME) because some tissues require a higher concentration of arachidonic acid to reach a plateau. Because the authors did not relate the arachidonic acid concentration plateau to any functional phenomena, the validity of this method of estimating the n-6 fatty acid requirement remains to be established.
n-3 Fatty Acids
The essentiality of the n-3 fatty acids has been equivocal until recently. It was initially demonstrated that the n-3 fatty acid, α-linolenic acid [18:3(n-3)], could substitute, in part, for the requirement of n-6 fatty acids (Greenberg et al., 1950). Tinoco et al. (1971) failed to show any change in growth of rats raised for three generations on diets lacking in n-3 fatty acids in comparison to litter mates fed diets containing 1.25 percent linoleate and 0.25 percent linolenate. However, the sequestering of n-3 fatty acids in specific tissues (retina, cerebral cortex, testis, sperm; Tinoco, 1982) and the tenacity with which they retain these fatty acids, despite the variation in dietary concentration (Crawford et al., 1976), led many researchers to speculate that n-3 fatty acids were required for some function in the body.
Bernsohn and Spitz (1974) fed rats lipid-free diets for 4 months and measured slightly decreased amounts (38 percent of control values) of monoamine oxidase and 5′-mononucleotidase in cerebral cortex that responded to dietary α-linolenic but not to linoleic acid. Retinal function may be negatively impacted in offspring of rats fed a low linolenate oil for two to three generations (Okuyama et al., 1987). Lamptey and Walker (1976) found reduced exploratory behavior in second generation rats fed safflower oil. A large percent of safflower oil is linoleic acid, and a very small percent is linolenic acid. This was confirmed in studies by Enslen et al. (1991) who showed a reduction in exploratory behavior in 16- to 18-week-old rats from dams fed safflower oil 6 weeks prior to mating and throughout gestation and lactation. These researchers found that when rats were switched to α-linolenic acid at weaning, they did not recover exploratory behavior, further suggesting a specific requirement for n-3 fatty acids during development.
Yamamoto et al. (1987, 1988) found a reduction in brightness discrimination learning in offspring from rats fed safflower oil through two generations in comparison to rats similarly fed perulla oil, a rich source of α-linolenic acid.
Bourre et al. (1989) measured impairment of nerve terminal Na+, K+ -ATPase activity, brain 5′-nucleotidase, and 2′,3′-cyclic nucleotide-3′-phosphodiesterase in offspring from rats fed 1.8 percent sunflower oil in comparison to those from dams fed 1.9 percent soybean oil through two generations. They estimated the requirement for n-3 fatty acids as the least amount of α-linolenic acid in the diet that resulted in a higher concentration of brain n-3 fatty acids. Using this methodology, they estimated a requirement of 2 g/kg food (or 0.4 percent of the total dietary ME). These experiments indicate that n-3 fatty acids are required; further study is needed to determine the amount below which functional impairment occurs.
Although a requirement has not been defined, it may be advisable to include a source of n-3 fatty acids when dietary oils such as sunflower or safflower are fed through two or more generations. Homeostatic mechanisms appear to sequester n-3 fatty acids to protect the rat from n-3 deficiency during short-term dietary deprivation.
Signs Of EFA Deficiency
A deficiency of EFA results in a plethora of gross clinical signs, anatomical changes, and physiological changes as discussed by Holman (1968, 1970). Classical overt signs include diminished growth, dermatitis, caudal necrosis, fatty liver, impaired reproduction, increased triene:tetraene ratio in the tissue and blood, and increased permeability of skin, with impaired water balance. There are many other less noticeable but equally severe changes that have been reported, including kidney lesions and a decrease in urine volume (Sinclair, 1952), lipid-containing macrophages in the lung (Bernick and Alfin-Slater, 1963), increased metabolic rate (Wesson and Burr, 1931), decreased capillary resistance (Kramar and Levine, 1953), and aberrant ventricular conduction (Caster and Ahn, 1963).
The EFA content of the rat’s diet prior to the feeding of EFA-deficient diets affects reserve stores of EFA (Guggenheim and Jurgens, 1944). Weanling rats rapidly exhibit signs of EFA deficiency after consuming a lipid-free diet for 9 to 12 weeks, while mature rats may require an extensive period of starvation, then refeeding of a lipid-free diet in order to develop EFA deficiency signs (Barki et al., 1947). Dermal signs resulting from EFA deficiency were reported after feeding adult rats a lipid-free diet for 35 weeks (Aaes-Jorgensen et al., 1958). Pups from dams fed EFA-deficient diets exhibit the most severe signs of EFA deficiency and usually die within 3 days to 3 weeks after birth, depending on the duration of the EFA feeding to the dam (Guggenheim and Jurgens, 1944; Kummerow et al., 1952). Other dietary factors [e.g., cholesterol (Holman and Peiffer, 1960) and 18:2(n-6) trans,trans fatty acid (Hill et al., 1979; Kinsella et al., 1979)] accelerate the development of EFA deficiency in rats fed EFA-deficient diets. Males may develop signs of EFA deficiency more quickly than females because males have a greater EFA requirement than do females (Morhauer and Holman, 1963; Pudelkewicz et al., 1968). Prevention of coprophagy will accelerate the development of EFA deficiency in rats fed lipid-free diets (Barnes et al., 1959a).
The relative capacities of n-6 and n-3 fatty acids to alleviate some of the deficiency signs are shown in Table 2-6. The classical signs of EFA deficiency appear to be more amenable to amelioration by n-6 fatty acids. It was hypothesized that the sole function of linoleic acid was as a precursor to arachidonic acid [20:4(n-6)] (Rahm and Holman, 1964; Yamanaka et al., 1980). Linoleic acid concentration is much lower than that of arachidonic acid in membrane lipids (Sprecher, 1991). However, linoleate-rich O-acyl sphingolipids have been identified in the epidermis of pigs and humans (Gray et al., 1978). The structures of pig and human epidermal acyl ceramide and acyl glucosyl ceramide were confirmed by Wertz et al. (1986). Hansen and Hensen (1985) fed EFA-deficient rats oleic [18:1(n-9)], linoleic [18:2(n-6)], columbinic [18:3(n-6)], α-linolenic [18:3(n-3)], and arachidonic [20:4(n-6)] acid esters and measured epidermal sphingolipids and trans-epidermal water loss. Only n-6 fatty acid esters restored the water barrier; however, among n-6 fatty acids, only linoleate was esterified in substantial amounts in the sphingolipids. The authors suggested that columbinate and arachidonate result in linoleate mobilization from other tissues for incorporation into the epidermal sphingolipids. The relationship between linoleate-containing epidermal sphingolipids and trans-epidermal water loss awaits further study.
TABLE 2-6. Relative Ability of n-6 and n-3 Fatty Acids to Alleviate Several Signs of EFA Deficiency in Rats.
TABLE 2-6
Relative Ability of n-6 and n-3 Fatty Acids to Alleviate Several Signs of EFA Deficiency in Rats.
Digestibility Of Lipids
Most commercial sources of dietary lipid consist exclusively of triglycerides and contain a high percentage of 18-carbon fatty acids. During digestion, lipase activity releases fatty acids from the 1 and 3 positions of the triglyceride. Free fatty acids and 2-monoacyl glycerol are absorbed.
Digestibility differs among lipid sources. Crockett and Deuel (1947) demonstrated that lipid digestibility was reduced when the melting point of the lipid was greater than 50° C. Fatty acid composition also may affect digestibility. Mattson (1959) showed that digestibility was reduced with increasing content of simple triglycerides (same fatty acid at each position) composed of 18-carbon saturated fatty acids. Saturated fatty acids and monounsaturated trans-fatty acids of 18-carbon chain length or longer are poorly absorbed as free fatty acids but easily absorbed in the 2-monoacyl glycerol form (Linscher and Vergroeson, 1988). Increasing the number of double bonds in the fatty acid improved absorption; increasing the fatty acid chain length decreased absorption (Chen et al., 1985, 1987a). Very long-chain n-3 fatty acids were poorly hydrolyzed in in vitro experiments (Brockerhoff et al., 1966; Bottino et al., 1967) but were well absorbed in unesterified forms (Chen et al., 1985).
The rate of triglyceride digestion and subsequent absorption of fatty acids depends on the nature of the fatty acid (chain length and number and position of double bonds) and its molar frequency and position in the triglyceride (Apgar et al., 1987; Nelson and Ackman, 1988). Tables of fatty acid composition of dietary fats that include their positional specificity are available (Small, 1991).
Utilization of dietary lipid may be affected by other dietary components (Vahouny, 1982; Carey et al., 1983). Estimates of utilization by using the lipid source in the absence of other dietary ingredients may not correctly define the utilization of lipid in a complex diet. Nelson and Ackman (1988) reviewed the literature on the use of ethyl esters of lipid to study absorption and concluded that absorption and transport may not be identical to naturally occurring (triglyceride) lipid sources. The digestibility of many dietary lipids has been determined and certain of these experimental results are summarized in Table 2-7.
TABLE 2-7. Digestibility of Some Selected Dietary Fats.
TABLE 2-7
Digestibility of Some Selected Dietary Fats.
Dietary Lipid Concentration
Better growth, reproduction, and lactation performance result when rats are fed diets in which lipid content is increased from 5 to 40 percent (Deuel et al., 1947). These observations and those of Forbes et al. (1946a,b) led Deuel (1950, 1955) to conclude that 30 percent lipid is the optimal dietary concentration. These response criteria alone are no longer considered sufficient to establish the optimal amount of dietary lipid.
Maximum growth also is associated with a decrease in longevity (French et al., 1953). Rats fed diets containing 10 or 20 percent corn oil had higher growth rates of a transplantable mammary tumor than those fed diets containing 2 or 5 percent corn oil (Kollmorgen et al., 1983). Rolls and Rowe (1982) demonstrated diminished growth and survival of pups suckled by dams fed high-lipid diets. Another study showed that rats consuming diets containing 3 or 20 percent lipid had superior reproduction (total numbers of offspring; percent of young weaned) compared to rats consuming diets containing 36 and 50 percent lipid (Richardson et al., 1964). It appears, then, that maximum growth should not be the only predictor of optimal dietary lipid content.
Data from the following experiments serve as justification for maintaining the previously recommended amount of 5 percent dietary lipid for both males and females during rapid growth and for adult females during reproduction and lactation (National Research Council, 1978). Swift and Black (1949) showed that the greatest improvement in energy retention occurred when dietary lipid content was increased from 2 to 5 percent; additional increments in energy retention were smaller when lipid content was above 5 percent. Deuel et al. (1947) reported that the greatest reduction in number of days required to reach puberty occurred when the percentage of lipid in the diet was increased from 0 to 5 percent. Relatively small changes occurred when lipid was greater than 5 percent of the diet. Burns et al. (1951) demonstrated that 5 percent lipid was satisfactory for absorption of carotene and vitamin A. Loosli et al. (1944) reported only slight improvement in weight gain of rat pups when lactating females were fed diets that contained more than 5 percent lipid. Furthermore, many lipids provide sufficient EFA when included in the diet at these concentrations. Reeves et al. (1993b: pp. 1941-1942) noted the following:
Bourre et al. (1989, 1990) used the method of dietary titration of 18:2(n-6) and 18:3(n-3) to determine linoleic and linolenic acid requirements, respectively. They used tissue saturation of 20:4(n-6) and 22:6(n-3) to make the assessments and concluded that 12 g of linoleic acid and 2 g of α-linolenic acid per kilogram of diet were the minimal requirements for the rat. This amounts to approximately 3 percent soybean oil in the diet. However, to reach the plateau for maximal concentrations of these fatty acids in many tissues of growing rats, an amount of fat equivalent to 5-6 percent soybean oil was required.
Lee et al. (1989) suggest that a n-6:n-3 ratio of five and a polyunsaturate:saturate (P:S) ratio of two are the points of greatest influence on tissue lipids and eicosanoid production. Bourre et al. (1989) suggested that the optimal n-6:n-3 ratio is between one and six. Soybean oil is a source of dietary fat that may meet these criteria. The oil contains about 14 percent saturated fatty acids, 23 percent monounsaturated fatty acids, 51 percent linoleic acid, and 7 percent linolenic acid. This gives a n-6:n-3 ratio of seven, and a P:S ratio of approximately four. The fatty acid composition of commercial lipid sources must be monitored because of the widespread practice of hydrogenation and the emergence of new cultivars with different fatty acid compositions.
Go to:
Carbohydrates
Although no definite carbohydrate requirement has been established, rats perform best with glucose or glucose precursors (such as other sugars, glycerol, glucogenic amino acids) in their diets. Diets containing 90 percent of dietary ME from fatty acids and 10 percent from protein were unable to support growth of young male rats. The substitution of neutral fats (soybean oil) for fatty acids or the addition of glycerol equivalent to that in the triglyceride allowed growth but not at rates equivalent to that achieved with a 78 percent starch diet (Konijn et al., 1970; Carmel et al., 1975). When carbohydrate-free diets containing 80 percent of dietary ME from fatty acids and 20 percent from protein were fed, rats were capable of weight gain, but growth increased when the diet was supplemented with glucose or neutral fats (Goldberg, 1971; Akrabawi and Salji, 1973). Rats fed low-protein (10 percent of dietary ME), carbohydrate-free diets were hypoglycemic and demonstrated abnormal glucose tolerance curves (Konijn et al., 1970; Carmel et al., 1975); rats fed higher protein (18 percent of dietary ME), carbohydrate-free diets had normal blood glucose concentrations but still demonstrated slightly abnormal glucose tolerance curves (Goldberg, 1971). When neutral fats replaced fatty acids in carbohydrate-free diets (20 percent of dietary ME from protein), growth did not improve when rats were allowed to eat ad libitum but was greater with diets containing the neutral fats when rats were meal-fed once daily (Akrabawi and Salji, 1973).
Over wide ranges of dietary fat:carbohydrate ratios (0.2 to 1.4, ME basis), the heat increment was found to be constant at 47.5 percent of ME, indicating that carbohydrate and lipid are used with equal efficiency (Hartsook et al., 1973).
A large number of carbohydrates can be used by the rat. Those most commonly used in rat diets include glucose, fructose, sucrose, starch, dextrins, and maltose. (See “Fiber” section for a discussion of fiber sources.) These carbohydrate sources support similar rates of growth; however, in diets adequate in other respects, fructose (and sucrose as a source of fructose) can lead to several abnormalities when compared to glucose or glucose-based polymers. Because the initial metabolic steps in fructose utilization are mediated by fructokinase and aldolase B, fructose metabolism bypasses the control of glycolysis at phosphofructokinase and, thus, increases the flux through glycolysis. Feeding of fructose or sucrose leads to increases in liver weight, liver lipid, liver glycogen, and activities of liver lipogenic enzymes: glucose-6-phosphate dehydrogenase, malic enzyme, ATP citrate lyase, and fatty acid synthetase (Worcester et al., 1979; Narayan and McMullen, 1980; Michaelis et al., 1981; Cha and Randall, 1982; Herzberg and Rogerson, 1988a,b). Hypertriglyceridemia associated with fructose feeding has been attributed to both increased hepatic synthesis (Herzberg and Rogerson, 1988b) and decreased peripheral clearance (Hirano et al., 1988) of triglyceride. Increases in kidney weight and nephrocalcinosis also were observed when diets containing 55 percent sucrose (Kang et al., 1979) or 63 percent fructose (Koh et al., 1989) were fed. Starch was more easily metabolized than sucrose by rats fed low-protein diets (12.5 percent casein; Khan and Munira, 1978) or protein-free diets (Yokogoshi et al., 1980). Essential fatty acid deficiencies may be exacerbated by high sucrose diets (Trugnan et al., 1985).
Poor performance and cataract formation occurred in rats fed lactose or galactose (Day and Pigman, 1957); diarrhea was also observed in weanling rats fed α- or β-lactose (Baker et al., 1967). Xylose is toxic to rats; lens opacity and diarrhea were observed in rats fed diets containing 15 percent or more xylose (Booth et al., 1953). Sorbose, a slowly absorbed sugar, decreases feed intake and growth rate when added to rat diets but appears to supply the rat with a significant amount of energy, presumably, in part, as end products of hindgut fermentation (Furuse et al., 1989). Mannose (up to 8 percent of the diet) improved growth of rats fed a carbohydrate-free diet, suggesting that it can be metabolized, at least in low concentrations (Keymer et al., 1983). Leucrose [D-glucosyl-α(1–5)D-fructopyranose, a bond isomer of sucrose] appears to be metabolized as well as sucrose (Ziesenitz et al., 1989). Of the sugar alcohols, lactitol and xylitol decrease feed intake and growth when added to diets at 16 percent of dry matter, although rats appear to adapt, at least in part, to these two sugar alcohols within 2 weeks (Grenby and Phillips, 1989). Sorbitol can be metabolized by rat liver (Ertel et al., 1983).
A series of experiments defined the rats’ need for carbohydrate for successful reproduction. In all these experiments a low-protein diet was required in order to demonstrate the need for carbohydrate. Rats fed carbohydrate-free diets [ME = 4.25 kcal/g (17.8 kJ/g), 12 percent of dietary ME from protein] were unable to maintain pregnancy. Although 78 percent of embryos were normal following 6 days of gestation for rats fed carbohydrate-free diets (compared to 91 percent for controls), only 25 percent (control = 89 percent) were classified as normal following 8 days and 0.6 percent (control = 90 percent) following 10 days of the carbohydrate-free diet. By day 12 of gestation, all embryos from rats fed carbohydrate-free diets had been resorbed (Taylor et al., 1983). A carbohydrate-free diet [ME = 4.11 kcal/g (17.2 kJ/g), 10 percent of dietary ME from protein] fed to gestating rats had to be supplemented with 4 percent carbohydrate (as glucose or an equivalent amount of glycerol) to maintain pregnancy to term, 6 to 8 percent glucose to produce normal maternal weight gain and normal fetal weight, and 12 percent glucose to produce fetal liver glycogen concentrations one-half as large as controls fed a 62 percent carbohydrate diet (Koski et al., 1986). Survival was poor for pups from dams fed low-glucose diets (9.5 percent protein) from day 9 of gestation through day 7 of lactation. From dams fed 6 percent or less glucose, no pups survived 7 days postpartum. From those dams fed 8 or 12 percent glucose, pup survival at 7 days was 6 and 30 percent, respectively. Control pups whose dams were fed 62 percent glucose diets had 93 percent survival (Koski and Hill, 1986). Poor rat pup survival caused by feeding dams a low (4 percent)-glucose diet (10 percent of calories from protein) could be markedly improved by feeding dams a high-carbohydrate diet for the final 2 days of gestation and through lactation (Koski and Hill, 1990). Lactation is not supported by carbohydrate-free diets. Milk production can occur for rats fed 6 percent glucose diets, but the milk contains low concentrations of carbohydrate and lipid, which is associated with retarded postnatal growth of pups (Koski et al., 1990). In general, fructose appears to be an adequate source of carbohydrate in diets fed to pregnant rats. However, when low (4 percent)-carbohydrate diets are fed during lactation, neither fructose- nor glycerol-supplemented diets will support deposition of as much fetal liver glycogen as 4 percent glucose diets (Fergusson and Koski, 1990). Essential fatty acid deficiencies may be more likely to occur in gestating rats fed sucrose-based (61.5 percent) diets than in those fed glucose-based diets (Cardot et al., 1987).
Go to:
Protein And Amino Acids
In establishing the protein requirements at different stages of life, three factors must be considered: (1) energy concentration in the diet, (2) amino acid composition of the protein (see Appendix Table 2), and (3) bioavailability of the amino acids.
Protein And Growth
Protein requirements are most accurately expressed as a protein:energy ratio to take into account the large differences in energy concentration that may occur among diets. In earlier studies that used egg protein as a highly digestible and balanced source of amino acids, the minimal amount of protein required for maximum weight gain in young rats was 25 to 31 mg/kcal GE (6.0 to 7.4 mg/kJ GE) (Hamilton, 1939; Barnes et al., 1946; Hoagland et al., 1948; Mitchell and Beadles, 1952). Similar results were obtained with pure amino acid mixtures (Rose et al., 1948) and with casein supplemented with sulfur amino acids (Breuer et al., 1963; Hartsook and Mitchell, 1956); as expected, greater amounts of protein were required when unsupplemented casein was used (Yoshida et al., 1957). These data indicate that a dietary protein concentration of 10 to 15 percent is required for maximum growth when a low-fiber diet containing a balanced amino acid pattern, 5 percent fat and 4 kcal ME/g (17 kJ ME/g), is fed. The 1978 edition of this report concluded that the protein requirement for maximum growth of the rat is 12 percent when highly digestible protein of balanced amino acid pattern is used.
Computation of the percentage of dietary protein required for maximum growth when the diet contains a mixture of proteins requires that both the content and bioavailability of the amino acids in the different proteins be considered. Historically, most methods have assumed that protein quality is constant over a range of dietary protein concentrations. For example, in the slope-ratio procedure, test proteins are fed at several concentrations and the value of a protein is determined by linear regression (Hegsted and Chang, 1965). However, the value of protein as used for maintenance differs from the value of protein as used for growth, and the difference is not linear (Phillips, 1981; Finke et al., 1987a,b, 1989; Mercer et al., 1989; Schulz, 1991). Finke et al. (1987a,b, 1989) used a four-parameter logistical model to describe the effect of utilization of protein from a variety of sources on growth rates and nitrogen gain of young rats (Sprague-Dawley strain). Although 1.11 times more casein than lactalbumin was required to achieve 95 percent of the maximum nitrogen gain, 1.43 times more casein than lactalbumin was required to achieve maintenance or zero nitrogen gain. In another comparison, twice as much soybean protein as lactalbumin was required to support 95 percent of the maximum nitrogen gain, but only 1.54 times as much soybean protein was required to meet maintenance needs. Thus, use of nonlinear models to describe an animal’s growth response to dietary protein or protein mixtures indicates that the relative value of protein is not constant and that the value of a protein for maintenance may not predict its value for growth. This may be an expression of the different amino acid patterns required for maintenance versus growth. Finally, nonlinear response models, in which marginal efficiency (response per unit input) changes with response level, seem to be more accurate than linear (constant marginal efficiency, i.e., broken stick) models in predicting the relative capacity of proteins to support maximum gain or nitrogen retention.
In determining the relative value of proteins to support growth using nonlinear models, it is important to include test diets that produce a maximal response or response plateau so that the “diminishing-returns” portion of the response curve can be defined. The weight gain response per unit of protein added (diminishing-returns) is expected to vary with type of dietary protein. By applying a saturation kinetics model, Mercer et al. (1989) used data from Peters and Harper (1985) to demonstrate that 19 percent unsupplemented casein (about 17 percent crude protein) in the diet was necessary to give 95 percent of the maximum growth response and that about 26 percent unsupplemented casein (23 percent crude protein) was needed to produce 100 percent of the maximum growth response. Given that 1.11 times as much casein as lactalbumin is required to support maximum gain, the requirement for 95 percent maximum growth response of rats fed lactalbumin is about 15 percent crude protein. This concentration is adopted as the protein requirement for rats fed a diet containing a balanced protein source and 4 kcal ME/g (17 kJ ME/g). Additional studies with nonlinear-response models and rapidly growing rat strains are needed to refine this requirement. In practice, natural-ingredient diets that contain 18 to 25 percent crude protein have supported high rates of postweaning growth.
Protein And Maintenance
Although protein requirement declines with age after weaning, the problem has not been studied extensively (Forbes and Rao, 1959; Hartsook and Mitchell, 1956). Hartsook and Mitchell (1956) estimated from carcass analyses that the requirement declined from about 28 percent of the diet (14 mg net protein1/kJ GE) at 30 days of age to 10 percent (about 5 mg net protein/kJ GE) at 50 days of age. The higher value agrees with that calculated from analysis of rat milk (Luckey et al., 1954). Using carcass nitrogen as the dependent variable, Sheehan et al. (1981) found that a dietary protein concentration averaging 4 percent was required for 12-month-old Sprague-Dawley female rats. Baldwin and Griminger (1985) were able to maintain nitrogen balance in 12- and 24-month-old male rats with an amino acid mixture simulating casein provided in the diet at 4.5 percent to 6.0 percent. Dibak et al. (1986) found that minimal concentrations of casein and wheat gluten of 4.86 percent and 7.12 percent were required for positive nitrogen balance of 6-month-old male rats. Therefore the maintenance requirement is about 5 percent protein when the source is of high-quality. In natural-ingredient diets a concentration of 7 percent crude protein is suggested by Bricker and Mitchell (1947).
Amino Acids And Growth
As with estimation of the protein requirement, it is necessary to consider the energy concentration of the diets when estimating the amount of each amino acid needed to support growth (Wretlind and Rose, 1950; Rosenberg and Culik, 1955). The sample amino acid patterns given in Table 2-8 are intended for use in a diet that contains 5 percent fat. Extrapolation of the requirements to diets of different caloric densities can probably be safely made by maintaining a constant amino acid:energy ratio and allowing for variations in amino acid digestibility (Kornberg and Endicott, 1946; Guthneck et al., 1953; Schweigert and Guthneck, 1953, 1954; Lushbough et al., 1957; Rogers and Harper, 1965).
TABLE 2-8. Examples of Amino Acid Patterns Used in Studies with Purified Diets Containing 5 Percent Fat.
TABLE 2-8
Examples of Amino Acid Patterns Used in Studies with Purified Diets Containing 5 Percent Fat.
Amino acid requirements are related to dietary protein concentration (Grau, 1948; Almquist, 1949; Brinegar et al., 1950; Becker et al., 1957; Bressani and Mertz, 1958). In general, the requirement for an amino acid, expressed as a percent of the diet, tends to increase as dietary protein concentration increases but may remain constant or decrease slightly when expressed as percent of protein (Forbes et al., 1955; Bressani and Mertz, 1958).
As with protein quality assessment, a nonlinear model best describes the growth response of rats fed varying amounts of amino acids (Yoshida and Ashida, 1969; Heger and Frydrych, 1985; Gahl et al., 1991). A nonlinear model most accurately describes the diminishing-returns portion of the response curve. Heger and Frydrych (1985) and Gahl et al. (1991) used different nonlinear models to assess the maintenance and maximum response of young rats to dietary concentrations of individual amino acids. Gahl et al. (1991) added incrementally a mixture of amino acids to the diet to obtain a growth response rather than adding a test amino acid to a diet devoid of that test amino acid to obtain a growth response. The test amino acid was incorporated into the amino acid mixture at a concentration 35 percent below that of the other amino acids. Addition of incremental amounts of the mixture to the diet was used to obtain a growth response to the test amino acid. This approach ensured that the limiting amino acid remained first limiting. Figure 2-2 shows the response curves for nitrogen gain as a function of lysine and sulfur amino acid intake. These curves were generated from data reported by Gahl et al. (1991) and Benevenga et al. (1994). Similar curves for each indispensable amino acid were used to generate the requirement estimated to support growth (Table 2-9). Estimates were also made for the amount of amino acid required for nitrogen gain based on carcass nitrogen gain. The estimated amino acid requirements based on nitrogen gain were 1.1 to 1.7 times those required for weight gain. Because weight gain per se does not reflect a change in body composition, nitrogen gain may be a more dependable response criterion. The substitution value of tyrosine for phenylalanine and cystine for methionine could not be estimated from the results used to generate the requirements shown in Table 2-9. The replacement of phenylalanine by tyrosine was determined by Stockland et al. (1971) by comparing the growth of rats fed diets containing phenylalanine alone as part of an amino acid mix and with five phenylalanine:tyrosine ratios. They found the requirement for phenylalanine alone was 0.70 percent of the diet, while that for phenylalanine plus tyrosine was 0.69 percent of the diet. Tyrosine without phenylalanine would not support growth, and at least 0.38 percent L-phenylalanine had to be in the diet for tyrosine to be of benefit. Tyrosine could provide 45 percent of the aromatic amino acid requirement. Estimates of the replacement value of cystine for methionine have been made (Sowers et al., 1972; Stockland et al., 1973). The requirement for methionine alone was 0.49 percent of the diet and cystine could replace 48 to 58 percent of methionine. The diet had to have at least 0.17 percent methionine for cystine to be of benefit. Rat growth was between 4 and 5.5 g/day in these studies. The estimates of these replacement values may not be applicable to rats with high growth potential such as those used by Gahl et al. (1991).
FIGURE 2-2. Nitrogen gain response curves generated using the parameter estimates for the logistic equation as described by Gahl et al.
FIGURE 2-2
Nitrogen gain response curves generated using the parameter estimates for the logistic equation as described by Gahl et al. (1991) for rats fed diets limiting in one indispensable amino acid. Each data point represents the mean (SEM) for four rats.
TABLE 2-9. Comparison of National Research Council Estimates of Indispensable Amino Acid Requirements of Rats for Growth.
TABLE 2-9
Comparison of National Research Council Estimates of Indispensable Amino Acid Requirements of Rats for Growth.
The estimates for indispensible amino acids reported in earlier editions of this publication are, on average, 23 percent lower than the current estimates (see Table 2-9 for comparisons). The reasons for this difference is the methods used to estimate the requirement. Estimates reported earlier (National Research Council, 1972, 1978) were based on significant differences between means by use of a multiple-range test, which produced values similar to results obtained with broken-stick models. Estimates made in this way will be lower than those made by the four-parameter logistical model and, as observed in the guinea pig (Chapter 5), may underestimate the requirement for indispensable amino acids by about 20 percent. A reanalysis of the original data of Benevenga et al. (1994) using these two analytical approaches gave the following requirement estimates, as grams per kilogram of diet, for 95 percent of Rmax (maximum rate) for growth versus multiple-range tests of group means, respectively: arginine, 4.3, 2.7; histidine, 2.8, 2.3; isoleucine, 6.2, 5.3; leucine, 10.7, 7.9; lysine, 9.2, 7.4; total sulfur amino acids, 9.7, 7.5; total aromatic amino acids, 10.1, 7.2; threonine, 6.2, 4.5; tryptophan, 2.0, 1.6; valine, 7.4, 7.5 (M. Gahl and N. J. Benevenga, University of Wisconsin, personal communication, 1994).
The difference between the total nitrogen requirement and the essential amino acid nitrogen requirement should be made up with mixtures of nonessential amino acids. Stucki and Harper (1962) reported that amino acid diets that contained both essential and nonessential amino acids supported greater growth in rats than diets that contained only essential amino acids. Ratios of essential amino acid nitrogen to nonessential amino acid nitrogen of 0.5 to 4.0 were satisfactory in diets that contained 9.4 to 15.0 percent protein. Arginine, asparagine, glutamic acid, and proline must be included in the nonessential amino acid mixture to support maximum growth (Breuer et al., 1964; Hepburn and Bradley, 1964; Ranhotra and Johnson, 1965; Rogers and Harper, 1965; Adkins et al., 1966; Breuer et al., 1966; Newburg et al., 1975). The responses to these amino acids are presumed to reflect the inability of the rat to synthesize the quantities required for rapid growth. However, as pointed out by Breuer et al. (1964) and Crosby and Cline (1973), rats appear to adapt to diets devoid of certain of the nonessential amino acids and resume nearly maximum growth.
It is evident that specific requirements for the nonessential amino acids cannot be given because of the metabolic relationships among them. Therefore, the values given in Table 2-8 represent a pattern that has been used successfully in studies with purified diets. The value of 40.0 g/kg for glutamic acid is based on the data of Hepburn and Bradley (1964) and Breuer et al. (1964); that for asparagine is 4.0 g/kg, as found by Breuer et al. (1966) to be required for maximum growth. Proline at 4 g/kg is the concentration used by Adkins et al. (1966). To raise the dietary crude protein limit to that planned, a mixture of alanine, glycine, and serine can be used.
Amino acid imbalances and antagonisms can result in increased requirements for individual amino acids, an area reviewed by Harper et al. (1970) and Benevenga and Steele (1984). The effects of imbalances and antagonisms on the requirement for maximum growth may be small or nonexistent if dietary protein concentration is adequate, but the effect in diets that contain suboptimal concentrations of protein may be considerable. The immediate response of an imbalance is decreased food intake (Harper et al., 1970).
Amino Acids And Maintenance
The determination of amino acid requirements for adult rats is difficult because of the flat dose-response curves that occur for many amino acids (Smith and Johnson, 1967; Said and Hegsted, 1970). The indispensable amino acid requirements for maintenance of adult rats are based on reports by Benditt et al. (1950), Smith and Johnson (1967), and Said and Hegsted (1970). The data for each amino acid from these reports were averaged and are expressed on the basis of metabolic body size as follows (mg/BWkg0.75): histidine, 23.5; isoleucine, 90.4; leucine, 53.1; lysine, 32.2; methionine, 67.2; phenylalanine, 54.5; threonine, 53.1; tryptophan, 15.6; and valine, 67.1. Assuming a basal energy requirement of 117 kcal/BWkg0.75 (490 kJ/BWkg0.75) for a 300-g rat, these data have been incorporated into Table 2-2 as g/kg of diet.
Gestation And Lactation
Nelson and Evans (1953) reported that 5 percent protein as unsupplemented casein was the minimum amount needed to support reproduction, while optimal performance occurred at 15 to 20 percent. Later, Nelson and Evans (1958) reported that 18 percent casein supported maximum growth in suckling young but that 24 percent was required to provide for weight gain in the dam during lactation. Supplementary cystine was added to both diets. Sucrose was used as the source of carbohydrate, a factor that may have influenced food intake, and thus protein utilization, at the lower protein concentrations in their studies (Harper and Elvehjem, 1957; Harper and Spivey, 1958). Gander and Schultze (1955) reported that 15 to 16 percent protein derived from a combination of casein, methionine, and mixed cereals supported reproduction and lactation in rats. More recently, Turner et al. (1987) compared the protein requirements for growth and reproduction in Sprague-Dawley female rats. Protein intakes of 8.6 percent (as whole-egg powder) delayed puberty but met the needs for subsequent reproductive function except pup weight. Concentrations of 15.6 percent (the highest used in their study) were needed for maximum growth from weaning to breeding. Subsequent response surface analysis revealed that a concentration of 21 percent would have been needed for maximum responses with a minimum concentration of between 9 to 11 percent whole-egg powder. It seems that the net protein requirement for gestation and lactation as a percentage of the diet does not differ significantly from that for growth of weanling rats (see Table 2-2).
The amino acid requirements for gestation and lactation have not been studied in depth. A concentration of 0.11 percent tryptophan in diets that contained 1 or 2 percent nitrogen (6.25 to 12.5 percent crude protein) was found to be adequate to support normal pregnancy in rats (Lojkin, 1967). Nelson and Evans (1958) reported that the sulfur amino acid requirement for lactation was 1 percent of the diet, one-half of which could come from cystine. Newburg and Fillios (1979) reported an apparent requirement for dietary asparagine in pregnant rats because its omission from the diet may have been associated with impaired neurological development of pups. Data are inadequate at this time to conclude that the concentration of amino acids in the diet required to support gestation and lactation exceed that required for growth in young rats.
Signs Of Protein Deficiency
Protein deficiency in young rats results in reduced growth, anemia, hypoproteinemia, depletion of body protein, muscular wasting, emaciation, and, if sufficiently severe, death. In adults a loss of weight and body nitrogen occurs (Cannon, 1948), and chronic deficiency may lead to edema (Alexander and Sauberlich, 1957). Estrus becomes irregular and may cease, fetal resorption occurs, and newborns are weak or dead. A lack of protein for pregnant and lactating rats may result in offspring that are stunted in growth (Hsueh et al., 1967) and have reduced concentrations of DNA and RNA in various tissues (Zeman and Stanbrough, 1969; Ahmad and Rahman, 1975). Low-protein diets also result in reduced food intake (Black et al., 1950). The reproductive capacity of the male is impaired by consumption of diets with inadequate concentrations of protein (Goettsch, 1949).
Removal of a single indispensable amino acid results in an immediate reduction in feed consumption, a situation that can return to normal within a day after replacement of that amino acid. A lack of an indispensable amino acid in the diet tends to be reflected in the concentration of the amino acid in the blood plasma (Longnecker and Hause, 1959; Kumta and Harper, 1962). Lack of specific amino acids has been reported to manifest as specific signs:
lack of tryptophan—cataract formation, corneal vascularization, and alopecia (Cannon, 1948; Meister, 1957);
lack of lysine—dental caries, impaired bone calcification, blackened teeth, hunched stance, and ataxia (Harris et al., 1943; Cannon, 1948; Kligler and Krehl, 1952; Bavetta and McClure, 1957; Likins et al., 1957; Meister, 1957);
lack of methionine—fatty liver (Follis, 1958);
lack of arginine—increased excretion of urinary urea, citrate, and orotate (Milner et al., 1974) and increased plasma and liver glutamate and glutamine (Gross et al., 1991).
The accumulation of a porphyrin-like pigment on the nose and paws has been observed in rats deficient in tryptophan, methionine, and histidine (Cole and Robson, 1951; Forbes and Vaughan, 1954), but this condition is also observed in other deficiency states.
Go to:
Minerals
Recommended mineral intakes have often been based on estimates of the amounts of minerals that promote maximum growth in short-term studies with little consideration of potential toxicological problems and of nutrient interactions. However, high safety margins added to recommended intakes of one mineral may affect requirements for another. For example, ingestion of extra calcium has been found to decrease absorption of iron and zinc (Greger, 1982, 1989), and excess intake of manganese may decrease iron utilization as these two elements are antagonistic (Davis et al., 1990).
Because of concerns about the consequences of feeding purified diets to rats for more than 6 months in studies on aging, hypertension, and cancer, a separate discussion of the role of dietary minerals in the development of nephrocalcinosis follows the discussions of the individual minerals.
Macrominerals
Six mineral elements occur in living tissues in substantial amounts and are commonly called ”macrominerals” to distinguish them from mineral elements present in lesser quantities and designated as “trace elements.” Although this distinction arose historically because of difficulties in the accurate analysis of the latter (Underwood and Mertz, 1987), it is still of some practical use because the trace elements are typically added to diets in premixes rather than via the formulated proportions of the primary ingredients.
Calcium and Phosphorus
The dietary requirements for calcium and phosphorus are closely linked and depend on the availability of each mineral from the dietary source. In the 1978 edition of Nutrient Requirements of Laboratory Animals , the recommendation for the minimal concentration of calcium and phosphorus to maximize bone calcification during growth was 5 and 4 g/kg, respectively. This gives a Ca:P molar ratio of 0.96. However, Bernhart et al. (1969) had shown previously that adequate mineralization could be accomplished with smaller amounts of dietary calcium and phosphorus. They held the dietary molar ratio of Ca:P to 0.91 and noted that Sprague-Dawley rats attained maximum weight gain when fed diets containing as little as 1.6 g Ca/kg, but the rats required at least 3.5 g Ca/kg to attain maximum body accumulation of calcium. In the same experiment, the amount of dietary phosphorus required to maximize body weight was ³1.5 g/kg, and to maximize body phosphorus concentration it was ³3.5 g/kg. Several groups of investigators have observed normal growth and tissue concentrations of calcium and phosphorus in bones and other tissues of rats (RIVm:TOX and Sprague-Dawley strains) fed from 2.85 to 3.2 g P/kg diet (Kaup et al., 1991b; Shah and Belonje, 1991). Ritskes-Hoitinga et al. (1993) fed rats 2.0 and 4.0 g P/kg diet with 5.2 g Ca/kg diet and 0.6 g Mg/kg diet for three successive generations. They found that 2.0 g P/kg diet sustained reproduction but delayed bone mineralization in offspring. Kaup et al. (1991b) noted that growth and bone calcium concentrations in male Sprague-Dawley rats fed 2.8 g P/kg diet increased as dietary calcium concentrations increased from 2.1 to 3.4 g Ca/kg diet but were similar for rats fed 3.6 and 4.4 Ca/kg diet.
These studies suggest that dietary concentrations of calcium and phosphorus at 3.5 and 3.0 g/kg, respectively, with a molar ratio 0.9, would be sufficient. However, other studies have shown that a larger Ca:P ratio is required to prevent specific abnormalities in Sprague-Dawley rats. Draper et al. (1972), for example, showed that a molar ratio of 1.5 was better than 0.8 for the prevention of osteoporosis.
Variations in calcium and phosphorus intake have been associated with soft tissue calcification, especially nephrocalcinosis, in rats. However, a variety of other dietary factors can influence the development of nephrocalcinosis.
The recommendations for calcium and phosphorus intakes reflect the somewhat conflicting needs to maximize growth and maximize bone calcium and phosphorus concentrations without inducing nephrocalcinosis. A Ca:P molar ratio of 1.3 is needed to prevent nephrocalcinosis in female rats. However, a dietary phosphorus concentration of more than 2.0 g/kg diet is needed to prevent inadequate bone mineralization in successive generations. Therefore, the recommended concentrations of dietary calcium and phosphorus under normal conditions for growth and maintenance of nonlactating rats are 5.0 and 3.0 g/kg, respectively.
Considering the demands placed on the dams during lactation, it might be prudent to increase the amount of dietary calcium and phosphorus during this period. It has been estimated that a lactating dam will produce 70 mL of milk per day (Brommage, 1989). This amounts to approximately 200 mg calcium and 140 mg phosphorus transferred to milk in a 24-hour period. Brommage (1989) showed that this demand for calcium and phosphorus was compensated for by increases in food intake and dramatic increases in intestinal absorption of these minerals. Ritskes-Hoitinga et al. (1993) showed that 2 g P/kg diet, compared to 4 g P/kg, sustained reproductive performance but delayed growth and bone mineralization in successive generations of female rats. To help relieve some of the stress of lactation, it is recommended that the dietary calcium and phosphorus be increased by 25 percent (6.3 g Ca, 3.7 g P/kg diet) during this period. This could be especially helpful for those females used in continuous-breeding programs. It should be mentioned, however, that when lactating and nonlactating rats were given a choice of diets containing various concentrations of calcium and phosphorus, the lactating rats chose a diet that contained a Ca:P molar ratio of 2, whereas the nonlactating rats chose a diet that contained a ratio of 1.5 (Brommage and DeLuca, 1984).
Factors Affecting Calcium and Phosphorus Requirements Certain dietary factors can affect the biological availability of calcium and phosphorus and thus affect requirements for them in the diet. When these factors are present in the diet, appropriate adjustments to the dietary concentrations of calcium or phosphorus should be made. Low concentrations of vitamin D in the diet will reduce the absorption of calcium. Kaup et al. (1990) showed reduced phosphorus absorption in rats fed 10 g Ca/kg diet when compared to those fed only 2.5 g/kg. High dietary phosphorus will in turn reduce the apparent absorption of calcium (Schoenmakers et al., 1989) but may depend on the concentration of dietary magnesium (Bunce et al., 1965). Increasing the amount of fat in the diet from 5 to 20 percent reduced phosphorus absorption in older rats but not young rats (Kaup et al., 1990). High-fat diets also have been shown to decrease the absorption of calcium in mature rats but not young rats (Kane et al., 1949; Kaup et al., 1990). Calcium absorption is decreased in rats fed diets containing sources of oxalate (Weaver et al., 1987; Peterson et al., 1992), and phosphorus availability is reduced in diets containing phytate (inositol hexaphosphate) (Taylor, 1980; Moore et al., 1984). Some protein sources, including soybean protein isolates and other plant products, contain phytate. Phosphorus availability from these products should be considered when they are used in an animal’s diet.
Other factors enhance calcium or phosphorus absorption. Bergstra et al. (1993) showed that the absorption of phosphorus was stimulated in rats fed diets high in fructose. Dietary disaccharides such as lactose and sucrose stimulate calcium absorption in rat intestine (Armbrecht and Wasserman, 1976). However, dietary lactose was better than sucrose in improving bone growth and development in intact vitamin D-deficient rats (Miller et al., 1988). Buchowski and Miller (1991) showed that 20 percent lactose added to diets containing a variety of calcium sources such as calcium carbonate, milk, and cheese significantly increased the amount of calcium in the tibia compared to rats fed diets without lactose. The enhancement was evident in 21-day-old rats fed the diets for 8 days but not in older rats.
Protein sources contain varying amounts of phosphorus, and the bioavailability of this phosphorus may not be the same in all sources. It is advisable, therefore, to analyze the source before using it in the diet and to be aware of whether the phosphorus is biologically available. Although few data are available in rats (Moore et al., 1984), data obtained for common feedstuffs in other species may be useful (National Research Council, 1988, 1994).
Signs of Calcium and Phosphorus Deficiency Boelter and Greenberg (1941, 1943) fed 0.1 g Ca/kg diet to young rats for 8 weeks. The rats exhibited growth retardation, decreased food consumption, increased basal metabolic rate, reduced activity and sensitivity, osteoporosis, rear leg paralysis, and internal hemorrhage. Males failed to mate; females did not lactate properly. Day and McCollum (1939) fed 0.17 g P/kg diet to young rats. The animals survived up to 9 weeks and exhibited lethargy, pain, and cessation of bone growth with massive losses of calcium in urine.
When Sprague-Dawley rats were fed diets with moderate restrictions in calcium (2.1 to 3.4 g Ca/kg diet) for 28 days, the rats grew normally but had reduced bone weight and bone calcium concentrations (Kaup et al., 1991b). Improved calcium absorption and reduced urinary calcium losses allowed the rats to compensate for these marginally low calcium intakes, but these mechanisms would not be sufficient to prevent growth retardation if lesser amounts of calcium had been fed.
Chloride
Voris and Thacker (1942) obtained a reduction of 25 percent in growth in a 10-week, paired-feeding comparison of rats fed 0.2 g Cl/kg diet as compared to 2.9 g Cl/kg diet. Picciano (1970) found no increase in weight gain of young rats fed 2 g Cl/kg compared to rats fed 0.5 g Cl/kg. Miller (1926) reported that 5 mg/day (0.17 to 0.25 g Cl/kg diet) was acceptable for reproduction and lactation. On the basis of these limited studies, the estimated requirement is 0.5 g Cl/kg diet, but future work may indicate this can be reduced.
Signs of Chloride Deficiency The rat tenaciously conserves its supply of tissue chloride by reducing drastically the urinary excretion within hours of consuming a diet deficient in the element. As a result, the signs of deficiency develop slowly. Rats fed 0.12 g Cl/kg diet exhibited poor growth, reduced efficiency of feed utilization, reduced blood chloride, reduced urinary chloride excretion, and increased blood CO2 content (Greenberg and Cuthbertson, 1942). Rats fed 0.5 g Cl/kg diet for more than 70 days had reduced growth, reduced tissue chloride concentrations, and extensive kidney damage; but some rats survived for more than a year (Cuthbertson and Greenberg, 1945).
Signs of Chloride Toxicity Rats are also relatively insensitive to excess dietary chloride as judged by growth and tissue composition. Dahl (salt sensitive) and Sprague-Dawley rats fed excess chloride (15.6 to 26.6 g Cl/kg diet) as sodium or potassium chloride grew normally with unchanged chloride concentrations in kidneys and muscle (Whitescarver et al., 1986; Kaup et al., 1991a,c). However, the Sprague-Dawley rats fed 15.6 g Cl/kg had elevated blood pressure and enlarged kidneys (Kaup et al., 1991a,c). Dahl (salt sensitive) rats fed 4.86 g Cl/kg as NaCl also had elevated blood pressures, while rats fed a basal diet or a diet supplemented with NaHCO3 did not (Kotchen et al., 1983).
Magnesium
Magnesium is required for numerous physiological functions in the rat. The amount required in the diet for adequate nutrition of the rat depends on numerous factors that affect availability of magnesium—the most important being the amount of dietary calcium, phosphorus, and vitamin D present. McAleese and Forbes (1961) found that a diet containing 0.1 g Mg/kg supported normal growth in weanling Sprague-Dawley rats. However, a diet containing 0.35 to 0.425 g/kg was required to maintain normal plasma magnesium concentrations of approximately 20 mg/L. More recently, Brink et al. (1991) found no significant difference in plasma magnesium between rats fed 0.4 and 0.6 g/kg diet; however, bone magnesium concentration was slightly (5 percent) but significantly higher in rats fed 0.6 g Mg/kg diet. On the other hand, rats fed only 0.2 g Mg/kg diet had significantly lower plasma and bone concentrations of magnesium than those fed 0.6 g/kg; 20 and 10 percent differences, respectively. Previously, Martindale and Heaton (1964) reported that 0.4 g Mg/kg was not sufficient to maintain normal bone and serum magnesium in adult Sprague-Dawley rats.
Other studies have shown questionable extremes for magnesium requirement. For example, Smith and Field (1963) estimated that a diet containing 0.05 g Mg/kg would replace endogenous losses in hooded Lister rats, but Clark and Belanger (1967) observed that a diet containing 2.5 g Mg/kg was needed for normal bone histology in Holtzman rats. These extremes may be the result of dietary factors that affect magnesium bioavailability. Brink et al. (1991) showed that magnesium absorption in rats fed diets with soybean protein was significantly less than in rats fed similar diets but with casein. Apparently this effect was caused by the presence of phytate in the soybean protein because when phytate was added to a casein diet, there was a similar reduction in absorption of magnesium. On the other hand, when lactose was added to the casein diet, magnesium absorption was enhanced.
Dietary calcium and phosphorus also affect magnesium requirement. O’Dell et al. (1960) showed that high-phosphorus diets enhanced the signs of magnesium deficiency in rats. Bunce et al. (1965) found that magnesium absorption was reduced by high dietary phosphorus. High dietary calcium also will depress magnesium absorption (O’Dell et al., 1960; Hardwick et al., 1987). Brink et al. (1992) have shown, however, that the effects of calcium and phosphorus on magnesium absorption is probably caused by the two minerals complexing magnesium in the gut lumen and rendering it insoluble. They showed that calcium was ineffective when phosphorus was not present and vice versa. [For an excellent review of factors affecting magnesium absorption, see Hardwick et al. (1991).]
Based on the review of these studies, and keeping the dietary calcium concentration at 5 g/kg and phosphorus at 3 g/kg, the dietary requirement for magnesium for growing and mature, nonpregnant rats is set at 0.5 g/kg diet. However, if diets contain factors, such as phytate, that might reduce the absorption of magnesium, a slightly higher dietary concentration might be required. In addition, because of the demands of lactation, an increase in dietary magnesium during this period is recommended. Wang et al. (1971) found that Sprague-Dawley rats were able to sustain gestation and lactation whether fed 0.8 g Mg/kg or 1.9 g Mg/kg diet.
Signs of Magnesium Deficiency Signs of magnesium deficiency in growing Sprague-Dawley rats include vasodilation, hyperirritability, cardiac arrhythmias, spasticity, and fatal clonic convulsions. Vasodilation occurred after about 1 week and often disappeared and reappeared spontaneously. Convulsions occurred between 21 and 30 days (Kunkel and Pearson, 1948; Ko et al., 1962). In Sprague-Dawley rats renal calcification was common and was detected within 2 days after initiating a markedly deficient diet (Reeves and Forbes, 1972).
Tufts and Greenberg (1938) reported that lactating females fed a deficient diet bred successfully but did not suckle their young. Hurley et al. (1976a,b) reported that magnesium-deficient Sprague-Dawley dams resorbed their fetuses or bore malformed pups; they suggested that the malformations resulted from a concomitant zinc deficiency.
Potassium
In two studies, rats grew normally when fed 1.7 to 1.8 g K/kg diet (Kornberg and Endicott, 1946; Grunert et al., 1950). Two other studies indicated that rats may require 5 to 6 g K/kg diet during lactation (Heppel and Schmidt, 1949; Nelson and Evans, 1961); however, diets designed by the American Institute of Nutrition (1977) that contain 3.6 g K/kg support adequate growth and reproduction. Studies to determine potassium requirements for the rat are limited. Until more extensive research has been done, the minimal requirement is estimated to be 3.6 g K/kg diet. The dietary concentration of potassium may be increased to 5 g/kg during lactation, but such an increase does not seem necessary based on available data.
The potassium requirement of different strains of rats may vary. Sato et al. (1991) showed that increasing the dietary potassium to 42 g/kg diet protected spontaneously hypertensive rats against elevation of blood pressure induced by the ingestion of 8 percent sodium chloride. However, Sprague-Dawley rats developed elevated blood pressure when fed 15.5 g Cl/kg diet either as sodium chloride or potassium chloride (Kaup et al., 1991a,b).
Signs of Potassium Deficiency Insufficient potassium markedly reduces appetite and growth. Animals become lethargic and comatose and may die within 3 weeks. They have an untidy appearance, cyanosis, short fur-like hair, diarrhea, distended abdomens with ascites, and are frequently hydrothoracic. Rats fed diets containing 1 g K/kg diet had a symmetrical loss of hair along the back and a 50 percent reduction in hair per follicular group (Robbins et al., 1965). Pathological lesions are widespread with potassium depletion (Schrader et al., 1937; Kornberg and Endicott, 1946; Newberne, 1964). The initial noninflammatory degeneration of myocardial fibers is followed by necrosis and cellular infiltration. Renal lesions include cast formation in proximal convoluted tubules, sloughing of tubular epithelium in the medulla, and accumulation of hyalin droplets in the epithelium of the collecting tubules.
Signs of Potassium Toxicity Pearson (1948) fed weanling rats 5 percent potassium in the diet as potassium bicarbonate, and after 3 weeks observed reduced growth rate. More than 60 percent mortality was observed when dietary magnesium was low and only 17 percent when magnesium was adequate. Drescher et al. (1958) showed electrocardiograph changes when potassium intake exceeded 10 mg/kg body weight. Potassium toxicity can cause hypertrophy of the adrenal zona glomerulosa, sodium depletion, and increased density of mitochondrial cristae in the kidney tubules (Hartroft and Sowa, 1964; Sealey et al., 1970; Pfaller et al., 1974).
Sodium
Grunert et al. (1950) estimated the sodium requirement of the rat to be 0.50 g Na/kg and to be independent of potassium intake. Forbes (1966) found that 0.48 g Na/kg diet was inadequate for maximum weight gain of weanling Sprague-Dawley rats over a 28-day period but that 2.20 g Na/kg gave maximum gains. Intermediate concentrations were not tested.
Pregnant Sprague-Dawley females fed low-sodium diets (0.3 g Na/kg diet) ate less food and showed languor and debility, particularly during the last week of pregnancy; however, reproduction was not seriously impaired (Kirksey and Pike, 1962). Ganguli et al. (1969a,b) suggested that the sodium requirement for gestation and lactation was 0.50 g Na/kg diet. The estimated requirement for growth, maintenance, gestation, and lactation is 0.5 g Na/kg diet.
Signs of Sodium Deficiency The classic sodium deficiency syndrome was described by Orent-Keiles et al. (1937). Rats fed a diet that contained 20 mg Na/kg diet exhibited growth retardation, corneal lesions, and soft bones. Males became infertile after 2 to 3 months, and sexual maturity was delayed in females. Death ensued in 4 to 6 months. At a concentration of 70 mg Na/kg diet, Kahlenberg et al. (1937) noted reduced appetite, poor growth, increased heat production, and reduced stores of energy, fat, and protein.
Signs of Sodium Toxicity Rats are relatively insensitive to excess sodium as indicated by growth and tissue composition. Sprague-Dawley rats fed 10.1 to 11.2 g Na/kg diet as chloride, sulfate, carbonate, and bicarbonate salts grew normally and had concentrations of sodium in bones and kidneys similar to rats fed a basal diet but had higher concentrations of sodium in bone than rats fed the potassium forms of these salts (Kaup et al., 1991a). However, ingestion of excess sodium as NaCl is associated with elevated blood pressure in several different rat strains, including Sprague-Dawley, Dahl (salt sensitive), Dahl (salt resistant treated with deoxycorticosterone acetate), and spontaneously hypertensive rats (SHR) (Kaup et al., 1991a,c; Tobian, 1991).
Trace Minerals
Of the many trace mineral elements found in foods, only seven—copper, iodine, iron, manganese, molybdenum, se lenium, and zinc—have been unequivocally demonstrated to be required by rats. Although there is some evidence that other mineral elements (such as chromium, lithium, nickel, sulfur, and vanadium) may be required, further research is needed to establish requirement amounts. These other elements are treated in the discussion “Potentially Beneficial Dietary Constituents.”
Copper
There is a general consensus that weanling and adult rats housed individually in wire-bottom stainless steel cages have a dietary copper requirement on the order of 5 to 6 mg/kg diet, the value used in the AIN-76A and AIN-93 diets. Johnson et al. (1993) fed weanling male Sprague-Dawley rats casein-starch-based purified diets containing copper concentrations ranging from 0.2 to 5.4 mg/kg for 5 weeks. They found that numerous functional measures of copper status (including platelet cytochrome c oxidase activity, serum ceruloplasmin activity, plasma copper, copper, and zinc-superoxide dismutase activity) were depressed in rats fed diets with copper concentrations at or below 3 mg/kg.
Studies by Klevay and Saari (1993) showed similar results. When weanling male rats were fed dietary copper ranging from 0.2 to 5.2 mg/kg diet for 5 weeks, the concentrations of copper in liver and heart and serum ceruloplasmin activity were depressed in rats fed copper concentrations at less than 4 mg/kg diet. However, Failla et al. (1988) could find no significant differences in copper status of Lewis rats fed an egg white-starch-based purified diet containing 2.1 or 7.0 mg Cu/kg diet. When similar rats were fed diets with sucrose instead of starch, indicators of copper status were significantly reduced at dietary concentrations up to 2.9 mg Cu/kg when compared to diets containing 7.1 mg Cu/kg. These studies suggest that the minimal dietary requirement for copper, to maintain adequate copper status in young growing rats of different strains and different dietary conditions, is more than 4 mg/kg diet.
Spoerl and Kirchgessner (1975a,b) reported that increasing dietary copper from 5 to 8 mg/kg during pregnancy and lactation resulted in an improvement in serum copper, serum ceruloplasmin activity, and liver copper concentrations in dams and their offspring. Cerklewski (1979) reported higher copper concentrations in milk on day 14 postpartum and in pup liver on day 21 postpartum when dams were fed diets containing 9 versus 6 mg Cu/kg diet during pregnancy and lactation. The functional significance of the higher tissue copper concentrations was not investigated.
Based on these data, a dietary copper concentration of 5 mg/kg diet is recommended for growth and maintenance for a variety of rat strains and under different dietary conditions. A dietary concentration of 8 mg Cu/kg diet is recommended for pregnancy and lactation. It should be noted that the requirement for growth and maintenance has not changed from that reported in the 1978 edition of this volume. However, the requirement recommended for pregnancy and lactation has increased from 5 to 8 mg Cu/kg diet. High dietary concentrations of zinc, cadmium, and ascorbic acid may increase the dietary requirement for copper (Davis and Mertz, 1987).
Signs of Copper Deficiency Copper deficiency develops rapidly in young rats and slowly in adult rats fed diets containing less than 1 mg Cu/kg. The copper deficiency develops more rapidly when fructose or sucrose is fed than when starch is fed, and males show a greater sensitivity to copper deficiency than females with respect to both the severity of signs and the duration of time until the onset of signs of deficiency (Fields et al., 1986; Koh, 1990; C.G. Lewis et al., 1990). Copper deficiency during early development can result in significant abnormalities in the cardiovascular, nervous, skeletal, reproductive, immune, and hematopoietic systems (Keen et al., 1982; Davis and Mertz, 1987). Weanling and adult rats fed copper-deficient diets can develop alternations in platelet function, impairments in both the acquired and innate arms of the immune system, altered exocrine pancreatic morphology and function, anemia (if dietary iron is marginal), alterations in thromboxane and prostaglandin synthesis, and impaired cardiovascular function (Cohen et al., 1985; Koller et al., 1987; Kramer et al., 1988; Dubick et al., 1989; Johnson and Dufault, 1989; Babu and Failla, 1990a,b; Allen et al., 1991; Medeiros et al., 1992; Saari, 1992). Weanling rats fed diets containing less than 2.7 mg Cu/kg show impaired neutrophil function within 5 weeks (Babu and Failla, 1990a).
Signs of Copper Toxicity Rats are particularly tolerant of high concentrations of dietary copper. Boyden et al. (1938) observed no adverse effects after feeding rats diets containing 500 mg Cu/kg, rats fed diets containing 2,000 mg/kg showed marked weight loss, and diets in excess of 4,000 mg/kg resulted in severe anorexia and starvation. Evidence of liver and kidney pathology in rats fed diets containing more than 1,000 mg/kg has been reported (Haywood, 1979; Czarnecki et al., 1984; Haywood, 1985).
Iodine
Iodine is regarded as essential for the rat. Utilization of dietary iodine is very high, and absorption occurs all along the gastrointestinal tract. It is concentrated by many tissues (Gross, 1962) but functions primarily as an integral part of the thyroid hormones. The few studies done to determine the iodine requirement generally agree that it is between 100 and 200 µg/kg diet (Levine et al., 1933; Remington and Remington, 1938; Halverson et al., 1945; Parker et al., 1951). Parker et al. (1951) found that 100 to 230 µg I/kg diet was satisfactory for reproduction; natural-ingredient diets that contained 330 µg/kg supported reproduction (Kellerman, 1934). The 1978 edition of this report set the iodine requirement at 150 µg/kg diet. Many commercially available natural-ingredient diets contain this amount and support adequate growth and reproductive performance in rats. There have been no recent studies to suggest that this concentration should be changed.
Signs of Iodine Deficiency The most obvious sign of iodine deficiency in the rat is enlargement of the thyroid glands with the formation of trabecular-type nodules (Taylor and Poulson, 1956). Iodine-deficient rats also had more coarse and less dense hair than controls. Iodine deficiency results in impaired reproduction (Feldmann, 1960). One biochemical sign of iodine deficiency is a decrease in serum concentrations of the thyroid hormone thyroxine (T4) (Abrams and Larsen, 1973); another is a dramatic increase in the concentrations of serum thyrotropin (Pazos-Moura et al., 1991). An increase in the activity of liver type I iodothyronine deiodinase also occurs in iodine-deficient rats. Arthur et al. (1991) showed that the activity of this enzyme in rats fed less than 100 µg I/kg diet was 60 percent higher than in controls fed 1,000 µg/kg.
Signs of Iodine Toxicity The rat has a relatively high tolerance for dietary iodine. Adult female rats fed 500 to 2,000 mg/kg diet during pregnancy had increased neonatal mortality (Ammerman et al., 1964). However, when fed concentrations approaching 500 mg/kg diet, female rats had decreased milk production. The fertility of male rats fed as much as 2,500 mg I/kg diet for 200 days from birth was not affected.
Iron
For weanling and adult rats the iron requirement for growth and maintenance of maximum hemoglobin concentration is on the order of 35 mg/kg (McCall et al., 1962a; Ahlström and Jantti, 1969). For reproduction McCall et al. (1962b) reported that a diet containing 240 mg Fe/kg was adequate for maximum weight gain and hemoglobin concentrations. Ahlström and Jantti (1969) reported that considerably less iron was needed for reproduction: 28 mg Fe/kg for maximum hemoglobin concentration and 58 mg/kg for maximum iron stores in the offspring. Lin and Kirksey (1976) reported that growing, pregnant rats required between 10 and 50 mg Fe/kg diet for maximum hemoglobin concentration and between 50 and 250 mg/kg for maximum fetal liver iron stores. Shepard et al. (1980) reported that diets containing less than 10 mg/kg resulted in loss of embryos and fetuses. Kochanowski and Sherman (1983) have argued that although 35 mg Fe/kg diet is adequate to maximize maternal weight gain and hemoglobin concentrations during pregnancy and lactation, iron concentrations between 75 and 250 mg/kg diet are needed for optimal iron status at the end of lactation and between 150 and 250 mg Fe/kg are needed to maximize iron stores in the pups. Given the above, the recommended dietary iron concentration during pregnancy and lactation is 75 mg/kg diet.
Signs of Iron Deficiency In addition to anemia, iron-deficient rats can be characterized by multiple abnormalities including hyperlipidemia and low tissue carnitine concentrations (Bartholmey and Sherman, 1986), growth failure, elevated resting metabolic rate (Tobin and Beard, 1990; Borel et al., 1991), reduced exercise capacity (Willis et al., 1990), low milk folate concentrations (O’Connor et al., 1990), a compromised immune system including impaired phagocytosis and natural killer cell activity, and reduced antibody production (Hallquist et al., 1992; Kochanowski and Sherman, 1984, 1985).
Signs of Iron Toxicity The general term used to describe iron toxicity in animals is “iron overload.” Chronic consumption of large amounts of dietary iron results in an accumulation of large quantities of iron in various cells and tissues, especially the liver. Wu et al. (1990) fed young (4 month old) and old (20 month old) rats 25 g elemental Fe/kg diet as finely powdered carbonyl iron and found detrimental effects on growth and maintenance of body weight. Within 2 weeks of feeding the high-iron diet, young rats had stopped gaining weight and old rats had lost 12 percent of their body weight. Weight of the old rats continued to decrease for up to 10 weeks of feeding. Large increases in the concentrations of iron in liver and spleen were seen in both age groups. Reductions in concentrations of serum, liver, and heart copper were also observed in the iron-overloaded rats. Britton et al. (1991) observed similar results in rats fed 30 g Fe/kg diet. These large amounts of tissue iron result in lipid peroxidation and cellular damage (Houglum et al., 1990; Wu et al., 1990).
Manganese
There is a paucity of studies that address manganese requirements in rats. Holtkamp and Hill (1950) reported that the optimal manganese intake for growth is between 2 and 5 mg/kg diet; when dietary manganese concentration was increased to 40 mg/kg, the average weight gain was less than in the group fed 5 mg/kg diet. In contrast, Anderson and Parker (1955) reported a faster growth rate in weanling rats fed 50 mg Mn/kg compared to rats fed 5 mg/kg. The manganese requirement for reproduction has not been firmly established. Diets containing 1 mg Mn/kg are inadequate for normal reproduction; litters from dams fed this concentration of manganese are characterized by ataxia (due to inner ear defects), skeletal defects, and a high incidence of early postnatal death (Hurley and Keen, 1987). Litters from dams fed diets containing 3 mg Mn/kg have normal survival and growth rates; however, depending on the strain, they can still be characterized by a high incidence of ataxia (Baly et al., 1986; Hurley and Keen, 1987). An increased incidence of ataxia was not observed in litters from Sprague-Dawley dams fed diets containing 5 mg Mn/kg (C. L. Keen, University of California, Davis, personal communication, 1992). The above data suggest that a dietary concentration of 5 mg Mn/kg is probably adequate for normal growth and development. However, because there is a significant difference in how different strains respond to dietary manganese intake (Hurley and Bell, 1974; Kawano et al., 1987), the requirement is set at 10 mg/kg. It should be noted that this is lower than the recommendation of 50 mg/kg (National Research Council, 1978); however, given the lack of data supporting the need for such a high concentration of manganese in the diet, coupled with the possible negative effects of excess manganese on iron metabolism (Davis et al., 1990), reduction in the manganese requirement is warranted.
High concentrations of dietary iron, calcium, phosphorus, and copper have been reported to increase the requirement for dietary manganese (Hurley and Keen, 1987; Johnson and Korynta, 1992).
Signs of Manganese Deficiency Diets containing less than 1 mg Mn/kg can result in reduced food consumption, poor growth, bone abnormalities, and early mortality. Reproduction can be impaired and is characterized by testicular degeneration in the male and by a delay in the opening of the vaginal orifice and defective ovulation in the female. If reproduction occurs, litters are characterized by ataxia, skeletal defects, marked abnormalities in glucose and lipid metabolism, and a high frequency of early postnatal death (Hurley and Keen, 1987). Manganese deficiency in weanling and adult rats can result in significant alterations in pancreatic exocrine and endocrine functions (Baly et al., 1985; Chang et al., 1990), impaired glucose transport and metabolism in adipose cells (Baly et al., 1990), an increase in tissue lipid peroxidation (Zidenberg-Cherr et al., 1983), abnormal lipoprotein metabolism (Davis et al., 1990; Kawano et al., 1987), decreased hepatic arginase activity (Brock et al., 1994), and marked inhibitions of osteoblast and osteoclast activities resulting in severe bone disease (Strause et al., 1987).
Signs of Manganese Toxicity The postnatal growth of rats is unaffected by dietary manganese intakes as high as 1,000 to 2,000 mg/kg diet, provided dietary iron is adequate. If dietary iron is low (820 mg/kg), dietary manganese concentrations in excess of 1,000 mg/kg can result in reduced weight gain and iron deficiency (Rehnberg et al., 1982). Diets in excess of 3,500 mg Mn/kg can result in severe growth retardation and mortality. Reproductive dysfunction resulting from long-term intake of excess manganese (>1,050 mg/kg) has been reported for both males and females (Laskey et al., 1982). Although the concentrations of dietary manganese needed for overt toxicity are quite high, weanling rats given water containing 55 µg Mn/mL for 3 weeks were reported to have reduced rates of brain RNA and protein synthesis (Magour et al., 1983). The mechanisms underlying the cellular toxicity of manganese have not been clearly identified but may involve manganese-initiated oxidative damage, disturbances in carbohydrate metabolism, and altered intracellular iron metabolism (Keen and Hurley, 1989).