Body weight, dietary practices, and tumor susceptibility in the rat.

The mature body weight of outbred CD-Crl:COBS CD(SD)BR male rats permitted to select their own diets throughout life was shown to correlate linearly with the frequency of spontaneous tumors. In an analysis of the dietary practices of these animals, a multifactorial model was derived--for which prepuberty and early post-puberty data only were used--that accounted for a large proportion of the variance in mature weight. The level of food or calorie intake proved less informative than the following: the interactive effects of food intake relative to body weight, the proportion of protein and carbohydrate constituents in the diet, the intake of each of these components, and, particularly, the efficiency with which the diet consumed was used for growth during early postweaning life. In a prospective study of other rats maintained on a similar feeding regimen, the model was found to be an accurate predictor of mature weight. Several of the explanatory factors necessary for estimation of body weight also contributed to tumor susceptibility. Nonetheless, both dietary and body weight-related factors in a specific time-related sequence were required to explain variation in spontaneous tumor susceptibility.