Nutritional consequences of excess amino acid intake

Various mechanisms respond to intakes of amino acids in excess of those required for normal tissue function. When excessive amounts of amino acids are taken, catabolism by enzymes in the liver and elsewhere is accelerated when intake exceeds requirements. In addition, changes in the free amino acid levels in the brain signal the nervous system centers regulating food consumption, and eating patterns are affected. This central nervous system mechanism may even determine the proportions of protein and of energy--yielding nutrients chosen in the diet through a mechanism regulated by the entry of tryptophan and other neurotransmitter precursors into the brain. These observations on protective mechanisms are considered in relation to effects obtained by feeding disproportionate amounts of amino acids. Intakes of large amounts of amino acids can produce toxicities, in which plasma concentrations of the administered amino acid rise to very high levels. Antagonisms arise from feeding excess of one amino acid that can be relieved by feeding a structurally related amino acid. Finally, amino acid imbalances are produced by adding surpluses of essential amino acids other than the essential amino acid most limiting for growth; the growth depression caused by this addition can be relieved by adding more of the most limiting amino acid to the diet. In all circumstances involving feeding with disproportionate amounts of amino acids, there is evidence of changes in brain amino acid levels. It is suggested that these changes play an important role in initiating protective responses against abnormal amino acid intakes.