Short-term zinc deficiency in the rat and self-section of dietary protein level

The effects of short-term zinc deficiency in rats on the selection of protein were studied. Rats were given a choice of two isocaloric diets containing either 10 or 50% soybean protein, with or without supplemental zinc. In one experiment 5-week-old rats, deprived of dietary zinc, chose a diet with a lower proportion of protein than those fed diets adequate in zinc. The zinc-deprived rats also ate less protein and less total diet than controls, but the same amount of carbohydrate. In a second experiment, 8-week-old rats deprived of adequate dietary zinc chose diets with less protein and more carbohydrate than controls without altering total food intake. Protein intake was inversely correlated with molar ratio of tryptophan to other neutral amino acids in plasma. Tryptophan and serotonin levels in brain, however, were unchanged in zinc deficiency. Brain tyrosine levels and the ratio of tyrosine to other neutral amino acids in plasma were elevated in zinc-deprived rats while brain catecholamine concentrations remained unchanged. These studies show an interaction between zinc and protein in the regulation of food intake and suggest that this could be a useful model to study the determinants of eating behavior.