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General Considerations for Feeding and Diet Formulation
A laboratory animal's nutritional status influences its ability to reach its genetic potential for growth, reproduction, and longevity and to respond to pathogens and other environmental stresses. A nutritionally balanced diet is important both for the welfare of laboratory animals and to ensure that experimental results are not biased by unintended nutritional factors.
Laboratory animals require about 50 nutrients in appropriate dietary concentrations. Tables detailing the estimated minimum nutrient requirements of laboratory animals are presented in this report. It is important to recognize that the estimated requirements in these tables have been determined under specific restrictive conditions. Feed palatability and intake, nutrient absorption and utilization, and excretion can be affected by physicochemical characteristics of feeds such as physical form, sensory properties, naturally occurring refractory or antinutritive compounds, chemical contaminants, and conditions of storage. Many biological factors also affect nutrient requirements.
Factors Affecting Nutrient Requirements
Genetics
Genetic differences among species, breeds, strains, stocks, sexes, and individuals may affect nutrient requirements. For example, the lack of L-gulonolactone oxidase (a key enzyme required for the synthesis of ascorbic acid) in some species is apparently the consequence of genetic mutation (Chatterjee, 1978). L-gulonolactone oxidase activity differs among rodent species, among rat strains, and between sexes within rat strains (Jenness et al., 1980). A mutant rat has even been discovered that, like the guinea pig, lacks L-gulonolactone oxidase and has an obligatory dietary requirement for ascorbic acid (Mizushima et al., 1984; Horio et al., 1985). There is evidence that mouse strains may differ in requirements for riboflavin, pantothenic acid, and other nutrients (Fenton and Cowgill, 1947; Lee et al., 1953; Luecke and Fraker, 1979). Genetic differences in growth potential among species, strains, and sexes may influence the daily requirements for amino acids and other nutrients that are incorporated into tissues (Fenton, 1957; Goodrick, 1973).
Stage Of Life
Nutrient requirements change during stages of the life cycle, especially in response to growth, pregnancy, or lactation. Synthesis of tissues or products requires amino acids, fatty acids, minerals, glucose, or other substrates as well as increased amounts of vitamins and associated cofactors. Research on farm animals demonstrates that rates of growth and of milk production affect nutrient requirements (National Research Council, 1984, 1985, 1988, 1989). The same is probably true for laboratory animals; however, few conclusive studies have been reported. As a result, for most nutrients, it is not currently possible to establish separate requirements for various stages of life for individual laboratory animal species.
Environmental Impacts
Nutrient requirements are usually studied under controlled conditions with minimal diurnal or seasonal variation in temperature, light cycle, or other environmental conditions. Marked modification in these conditions may alter nutrient requirements. For example, exposure to temperatures below the lower threshold of the thermoneutral zone increases energy requirements as animals are obliged to expend energy to maintain a constant body temperature. The consequent increase in food intake may permit the feeding of diets of lower nutrient density without decreasing nutrient intakes. High temperature, disturbing stimuli, social conflict, or other environmental factors that reduce food intake may necessitate diets higher in nutrient concentrations to maintain adequate nutrient intakes.
Housing types can also affect the amounts of nutrients needed in diets. For example, laboratory rodents maintained in either galvanized cages or cages with solid bottoms may have a lower dietary requirement for zinc because of the availability of zinc from the feces and cage materials. Solubilized minerals in drinking water (such as copper from copper water lines) may affect the amounts of these minerals that must be supplied by the diet. If laboratory animals ingest bedding or other "nonfood" materials, these may provide an unintended source of some nutrients or toxins. In studies of the requirements of laboratory animals for constituents that might be needed at extremely low concentrations, even the air supply may be a significant source of contamination.
Microbiological Status
Under normal rearing conditions, laboratory animals harbor populations of microorganisms in the digestive tract. These microorganisms generate various organic constituents as products or by-products of metabolism, including various water-soluble vitamins and amino acids. The extent to which these nutrients contribute to the nutrition of the host may be substantial but varies according to species, diet composition, and rearing conditions. In the rat and mouse, most of the microbial activity is in the colon, and many of the microbially produced nutrients are not available to the host unless feces are consumed, as is common for rats and other rodents (Stevens, 1988). Prevention of coprophagy may require an increase in the nutrient concentrations that must be supplied by the diet. The loss of some or all microbial symbionts in animals free of specific pathogens and germ-free animals, respectively, may also alter microbial nutrient synthesis and, thereby, influence dietary requirements. Adjustments in nutrient concentrations, the kinds of ingredients, and methods of preparation must be considered when formulating diets for laboratory animals reared in germ-free environments or environments free of specific pathogens (Wostmann, 1975).
Research Conditions
Experimental procedures may produce stress or otherwise alter food intake. For example, surgical procedures or test substances in diets may lead to anorexia, necessitating the provision of more palatable diets or diets with elevated nutrient concentrations. Experimental protocols that require restriction of the amount of food offered alter the intakes of all nutrients unless dietary concentrations are altered to account for changes in food consumption.
Nutrient Interactions
Alterations in dietary energy density usually cause a change in feed intake. If high-energy diets are used, it may be necessary to increase nutrient concentrations in the diet to compensate for decreased food consumption. Other interactions occur between nutrients, such as competition for absorption sites among certain minerals that share common active transport systems. Thus in formulating diets containing unusual nutrient concentrations, the potential effects on other nutrients must be considered and adjustments made in nutrient concentrations, if appropriate.
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