Carcass components at first estrus of rats on high-fat and low-fat diets: body water, protein, and fat.

Carcass analysis data show that weanling rats fed high-fat (HF) and low-fat (LF) diets until the day of first estrus had similar body compositions at estrus, although the HF rats had estrus significantly earlier and at a lighter body weight (P less than 0.01) than the LF-diet rats. Total water as percent of wet weight, protein as percent of wet weight, and the water/protein ratios of the HF- and LF-diet rats did not differ significantly, whereas the absolute amounts of body water and protein of the two diet groups differed significantly (P less than 0.01), in accord with means (+/- SEM) of total body water as percent of wet weight were 66.2 +/- 0.3% for the HF rats and 66.4 +/- 0.3% for the LF-diet rats, whereas the mean absolute total body water for the two diet groups was 69.7 +/- 2.2 g and 81.1 +/- 2.4 g, respectively (P less than 0.01). Carcass fat of the HF rats, 15.6 +/- 1.0 g, was identical with that of the LF-diet rats, 15.6 +/- 0.9 g. The HF rats were therefore relatively fatter, 14.6 +/- 0.6 wet weight %, than the LF-diet rats, 12.6 +/- 0.4 wet weight % (0.05 greater than P greater than 0.01). The percentage of water in the fat-free wet weight of the HF rats, 77.5 +/- 0.3%, was significantly (P less than 0.01) greater than that of the LF-diet rats, 76.1 +/- 0.3%. These data and the high percentages of total water of body weight show that at first estrus the HF- and LF-diet rats had not as yet attained an adult body composition, similar to the human female at menarche. Within each diet group, the percentages of total water/body weight, protein/wet weight and fat/wet weight, or fat/dry weight, did not change significantly with increasing age of estrus, whereas each absolute carcass component--body water, protein, and fat--increased significantly with increasing age of estrus. The carcass data support G.C. Kennedy's hypothesis that a metabolic signal, related to fat stores, is a signal for puberty in the rat and are in accord with the hypothesis that a critical body composition of fatness is necessary for estrus in the rat, as in the human female. The greater relative fatness of the HF-diet rats may be associated with higher levels of estrogen in the HF rats than in the LF-diet rats.