Control of
protein intake was studied in young rats that were allowed to choose between either
protein-free and 55%
casein diets or 15% and 55%
casein diets. Animals on the
protein-free vs. 55%
casein regimen exhibited a lower
weight gain, a lower cumulative energy intake and a greater cumulative total
protein intake during the 13-day study compared to rats selecting between 15% and 55%
casein. The daily average proportion of total food selected as
casein by animals choosing between
protein-free and 55%
casein diets increased from 15% to 38% during the course of the study. In contrast, rats choosing between 15% and 55%
casein chose 18-22% of total food as
protein throughout the entire study. Long-term
protein intake or
protein selection did not correlate significantly with whole-brain contents of
5-HT or
5-HIAA. Our results suggest that
protein intake is not regulated at a constant proportion of total calories, but is controlled between a minimum level that will support rapid growth and a maximum that, if exceeded, would require the animal to undergo substantial metabolic adaptation. The mechanism controlling
protein selection may involve diet-induced changes in the brain content of total free indispensable
amino acids.