Resistance to dietary hyperphagia in juvenile rats.

Adult rats (12 wk of age) fed a wet, high starch diet consumed more energy, gained more weight, and had more carcass fat than rats fed the same diet in a dry form. In contrast, juvenile rats (3.5 wk of age) did not consume more energy, gain more weight, or become fatter when fed the wet, high starch diet than when fed the same diet in dry form. Although rats of both age groups fed high fat diets consumed more energy than rats fed high starch diets, the effect of the high fat diets was much smaller in juvenile than in adult rats. The level of dietary protein (20 vs. 8%) in the diets did not interact with diet type, but juvenile rats fed low protein diets gained less weight than juvenile rats fed standard protein diets. Additional experiments showed that juvenile rats fed a 20% protein diet were capable of increasing fluid intake and energy intake when given sweetened water or insulin, respectively. Thus, low susceptibility to dietary hyperphagia in juvenile rats cannot be explained by assuming that juvenile rats already eat as much as they can. These data are consistent with the possibility that developmental changes in taste responsiveness may contribute to this effect of age.