Effect of diet and animal care/housing protocols on body weight, survival, tumor incidences, and nephropathy severity of F344 rats in chronic studies

Diet is an important environmental factor affecting body weight, survival, and age-related diseases of rodents. The NIH-07 open formula diet was the diet used in the National Toxicology Program's (NTPs) rodent carcinogenicity studies from 1980 to 1994. In 1994 the NTP began using a new diet designated the NTP-2000 diet. This paper compares body weight, survival, tumor incidence, and nephropathy severity in untreated control groups of Fischer 344 (F344) rats fed the NTP-2000 or NIH-07 diets, using data from 22 separate 2-year feed and inhalation studies. The feed studies were conducted in 3 different facilities, and all the inhalation studies were conducted in a single facility. During feed studies, rats were group housed in polycarbonate cages and fed diets in powder (mash) form, while in inhalation studies, rats were housed individually in wire mesh cages, and fed diets in pelleted form. Survival was significantly (p < 0.05) higher in groups fed NTP-2000 diet compared to the corresponding groups fed NIH-07 diet, irrespective of sex or housing conditions. Use of the NTP-2000 diet was also associated with a decreased incidence of pituitary gland tumors in both sexes and decreased incidences of adrenal pheochromocytoma and preputial gland tumors in males. The incidence and severity of nephropathy was also decreased in animals receiving the NTP-2000 diet, especially males. The decreased nephropathy severity and the decreased incidence of pituitary gland tumors are likely the major factors contributing to the improved survival of rats receiving the NTP-2000 diet relative to those given the NIH-07 diet. These data also support earlier findings that decreased incidences of adrenal pheochromocytoma are associated with reduced nephropathy severity in male F344 rats. Throughout the two-year study female rats receiving the NTP-2000 diet were significantly (p < 0.05) lighter than those receiving the NIH-07 diet. However, it is uncertain if this difference can be attributed to the NTP-2000 diet, since implementation of this diet by the NTP approximately coincided with changes in the F344 rat production colony that resulted in somewhat lighter animals being provided to the NTP. Controls from inhalation studies and feed studies differed significantly (p < 0.01) in the incidence of a variety of tumors, irrespective of diet. This suggests that differences in animal care and housing protocols may impact tumor incidence in F344 rats, most notably pituitary gland and testis tumors.