Protein and energy consumption, plasma amino acid ratios, and brain neurotransmitter concentrations

Abstract

The hypothesis that long-term protein and energy intakes are controlled, respectively, by brain concentrations of serotonin (5HT) and the catecholamines, dopamine (DA) and norepinephrine (NE) has been proposed by Anderson and associates [2,5]. This control is assumed to be mediated through the influence of diet composition on the availability of the amino acid precursors of the neurotransmitters to the brain. The hypothesis was tested in experiments in which plasma amino acid concentrations and brain neurotransmitters were measured in young rats allowed to choose between two isocaloric diets differing in protein content (15% vs 55% casein or 15% vs 55% wheat gluten) and containing additional amounts of certain amino acids. No consistent correlation was observed between protein consumption and either the plasma ratio of tryptophan to neutral amino acid (Trp/NAA) concentrations or brain 5HT concentration. Similarly, no correlation was observed between energy intake and either the plasma ratios of Tyr/Phe and Tyr/NAA concentrations or the brain concentrations of NE and DA. The most apparent association observed was a strong correlation between total protein consumption and the sum of plasma branched-chain amino acid concentrations. Under the conditions of this experiment, feeding behavior seemed to be associated positively with the needs of the animal for nutrients and negatively with ingestion of an excess of amino acids generally rather than with plasma amino acid concentration ratios and brain neurotransmitter levels.