METHODS

Weanling male and female Osborne-Mendel rats were fed a grain (2) or a high-fat ration containing 60% Crisco.(The high-fat ration was slightly modified from that used by Mickelsen et al.(14). It contained in percent: Crisco, 60; casein, 25; mineral mix (Wesson, modified Osborne-Mendel purchased from General Biorhemicals, Chagrin Falls, Ohio), 5; vitamin mix (purchased from Nutritional Biochemicals, Cleveland, Ohio), 2.2; nonnutritive fiber (a cellulose type, purchased from General Biochemicals), 2.0; Aureomycin(donated by Lederle Laboratories, American Cyanamid Co., Prince ton, NJ), 0.01; liver powder, 2; DL-methionine, 0.25; sucrose, 3.54.) Littermates were distributed by sex between these two ration groups. Five male and five female rats were sacrificed at weaning (23 days) for carcass composition studies. The distribution of the remaining rats is given in Table 1. All rats were born to dams fed the grain ration throughout gestation and lactation. At the time of weaning, the animals were allocated to a group scheduled to be sacrificed at a specified weight (150, 300, 450 g, etc.). Ass@ nment at weaning was essential since there was some viriation in the time required for rats to attain a given weight. As replacements for those rats that did not attain their specified weights, several animals of both sexes were maintained on each of the diets. Additional data are presented on the body composition of four groups of older rats. Ten had been fed the grain and seven the high-fat ration for about 450 days prior to being sacrificed (Table 1).