The Journal of Nutrition Vol. 127 No. 5 May 1997, pp. 824S-825S
Copyright ©1997 by the American Society for Nutritional Sciences

Introduction¹

Shirley R. Blakely², Adrianne Rogers^*, and Suzanne Hendrich

U.S. Food and Drug Administration, Washington, DC, ^* Boston University Medical Center, Boston, MA and  Iowa State University, Ames, IA

FOOTNOTES

LITERATURE CITED

Finding the optimal laboratory animal diet to sustain growth and longevity and yield reproducible experimental results is of critical importance in nutritional and toxicological research. Diet is of concern in toxicological and other research with rodents, because diet composition can affect the activity of xenobiotic test substances and alter the outcome and reproducibility of long-term studies. For example, diets can alter the metabolism of test substances, which can result in errors of interpretation of data.

Nutritionally complete natural ingredient diets, either the closed formula commercial diets or the open formula NIH-07 diet, and purified diets, such as the AIN-76A diet, are most commonly used in nutritional and toxicological studies (Rutten and DeGroot 1992). Evaluation of the carcinogenic potential of xenobiotic compounds can differ depending on the diet fed. In general, higher tumor incidences have been reported in rodents fed the AIN-76A or other purified diets compared with rodents fed natural ingredient diets (Cohen et al. 1997, Fullerton et al. 1992). All diets should be formulated on the basis of current knowledge of nutritional requirements for the laboratory animals as recently published by the National Research Council (1995).

The high batch-to-batch variability in the types and quantities of nutrient and non-nutrient components in the natural ingredient diets can increase the variability and reduce the reproducibility of experimental results (Rao 1988). Unrefined materials in the natural ingredient diet, such as corn, soybean meal, fish meal, wheat and oats, may contain pesticide and mycotoxin contaminants (Merrill et al. 1997). Purified diets overcome some of these limitations because they are formulated with defined quantities of highly refined ingredients. This results in higher batch-to-batch reproducibility and minimizes chemical contaminants (Rao 1988). However, there may be important components in natural ingredient diets that are not present in purified diets, or the balance of components in purified diets may not be ideal (Jackson et al. 1997). Much uncertainty surrounds the use of purified diets in long-term studies, in part because the protein levels of laboratory animal diets are 20-25% (by weight), which are adequate for growth and reproduction but are too high for long-term maintenance.

New diets are being developed to provide better outcomes in long-term studies. The new AIN-93M purified diet contains a different source and lower level of protein than the AIN-76A diet and is designed for long-term maintenance (Reeves 1997). The new National Toxicology Program natural ingredient diet (NTP2000) contains less protein than the NIH-07 diet and yields increased survivorship in rodent bioassays (Rao 1997). The term survivorship is defined as the proportion of animals surviving until the end of chronic studies and is based upon toxicological guidelines that require 50% survival of rats until 24 mo of age as a requirement for a valid negative carcinogenic bioassay (Food and Drug Administration 1982). Declining survivorship of the three strains of rodents most commonly employed in toxicity testing, Fisher 344 rats, Sprague-Dawley rats and B6C3F1 mice, has led some scientists to speculate that the future of risk assessment is threatened (Abelson 1995).

Overfeeding, excessive weight gain and increased spontaneous disease are cited as causes of this decreased longevity. Permitting laboratory animals to feed ad libitum may result in overfeeding. Rather than change the macronutrient composition of the diet, some researchers restrict the amount of diet fed by 10-40% of that consumed by ad libitum-fed animals. Numerous studies have shown that moderate to severe restriction (20-40%) can increase survivorship and change the metabolism of toxicants. Two reports in this supplement address this issue (Keenan et al. 1997, McDonald 1997).

The Nutrition and Food Toxicology Research Interest Section (NFT-RIS) of the American Society for Nutrition Sciences presents this supplement on animal diets. Of the numerous appropriate and relevant topics considered, the NFT-RIS chose animal diets for long-term nutritional and toxicological studies because of the broad applicability to practically all research involving rodents. Three underlying themes were the focus of the symposium: 1) the design of diets for chronic rodent studies, 2) the influence of diet on carcinogenesis studies, and 3) the determination of the correct level and balance of nutrients and essential non-nutrient components. The objectives were to define the broad issues related to designing diets for long-term studies, to identify problems and solutions associated with using both types of diets, and to examine innovative approaches to diet formulation that have been successfully used by researchers. There is a need for continued vigilance and research on animal diets used for long-term nutritional and toxicological studies. Such efforts will serve as reminders of the critical influences diet can have on research results and ensure continued advances toward finding the optimal diet.

FOOTNOTES

LITERATURE CITED

• Abelson, P. H. (1995) Editorial: Flaws in risk assessment. Science 270: 215.
• Cohen S. M., Masui T., Garland E. M., Arnold L. L. Effects of diet on urinary bladder carcinogenesis and cancer prevention. J. Nutr. 1997; 127:826S-829S [Medline][Medline]
• Food and Drug Administration, Center for Food Safety and Applied Nutrition. (1982) Toxicology principles for the safety assessment of direct food additives and color additives used in food. Redbook I, pp. 86-90. FDA, Washington, DC.
• Fullerton F. R., Greenman K. L., Bucci T. J. Effects of diet type on incidence of spontaneous and 2-acetylaminofluorene-induced liver and bladder tumors in BALB/c mice fed AIN-76A diet versus NIH-07 diet. Fundam. Appl. Toxicol. 1992; 18:193-199 [Medline][Medline]
• Jackson C. D., Weis C., Miller B. J., James S. J. Dietary nucleotides: effects on cell proliferation following partial hepatectomy in rats fed NIH-31, AIN-76A or folate/methyl-deficient diets. J. Nutr. 1997; 127:834S-837S [Medline][Medline]
• Keenan K. P., Ballam G. C., Dixit R., Soper K. A., Laroque P., Mattson B. A., Adams S. P., Coleman J. B. The effects of diet, overfeeding and moderate dietary restriction on Sprague-Dawley rat survival, disease and toxicology. J. Nutr. 1997; 127:851S-856S [Medline]
• McDonald R. Some considerations for developing diets for mature rodents used in long-term investigations. J. Nutr. 1997; 127:847S-850S [Medline][Medline]
• Merrill A. H., Schmelz E. M., Wang E., Dillehay D. L., Rice L. G., Meridith F., Riley R. T. Importance of sphingolipids and inhibitors of sphingolipid metabolism as components of animal diets. J. Nutr. 1997; 127:830S-833S [Medline]
• National Research Council (1995) Nutritional Requirements for Laboratory Animals, 4th ed. National Academy Press, Washington, DC.
• Rao G. N. Rodent diets for carcinogenesis studies. J. Nutr. 1988; 118:929-931 [Medline]
• Rao G. N. New nonpurified diet (NTP-2000) for rodents in the National Toxicology Program's toxicology and carcinogenesis studies. J. Nutr. 1997; 127:842S-846S [Medline]
• Reeves P. Components of the AIN-93 diets as improvements in the AIN-76A diet. J. Nutr. 1997; 127:838S-841S [Medline]
• Rutten A.A.J.J.L., De Groot A. P. Comparison of cereal-based diet with purified diet by short-term feeding studies in rats, mice and hamsters, with emphasis on toxicity characteristics. Food Chem. Toxicol. 1992; 30:601-610

0022-3166/97 $3.00 ©1997 American Society for Nutritional Sciences