Growth after amino acid diets

Montgomery et al.67 fed pigs alternately a tryptophan-deficient diet and the same diet supplemented with tryptophan. The deficient diet depressed food intake, caused changes in plasma amino acid pattern, increased plasma glucose levels, and increased plasma urea levels per unit food intake. Plasma levels of insulin and growth hormone were somewhat elevated after 2 days of feeding the deficient diet. 

Her very low-protein diet was continued, but in order to ensure an adequate supply of essential amino acids for growth she was fed a mixture of the ketoacids of threonine, methionine, leucine, isoleucine and valine. After each feed she again became abnormally drowsy and markedly ketotic, with significant acidosis. Her plasma ammonium ion concentration was within the normal range, and a glucose tolerance test was normal, with a normal increase in insulin secretion after glucose load. 

In the rat, PEM after the growth spurt affects certain aspects of the metabolism of the brain. In rats given a3y casein diet from weaning, the level of histidine was higher, and that of tryptophan lower, than in brains of rats given isoenergetic amounts of a high-protein diet. These differences were correlated with similar differences in the serum levels of these amino acids. 

In rats a diet specifically low in protein, both during and after the growth spurt, changes the concentrations of certain amino acids in the brain. Whether the concentrations of individual amino acids are raised or lowered depends on a number of factors including activity of the Krebs cycle, protein synthesis and hormonal status. It seems that the plasma tryptophan level is particularly sensitive to the protein intake, and that this partly controls the entry of tryptophan into the brain. Entry of tryptophan is also modified by insulin. 

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