Effect of various oral glucose doses on plasma neutral amino acid levels

Six healthy, nonobese, fasting subjects each received, on different days 0, 6, 12.5, 25, or 50 g of glucose (Glucola) in a total volume of 100 ml. Blood was taken at intervals and assayed for plasma levels of the branched-chain amino acids (valine, isoleucine and leucine); the other major large neutral amino acids (LNAA) (methionine, phenylalanine, tyrosine and tryptophan); and, in some cases, insulin and glucose. Insulin levels were significantly elevated 30 min after consumption of 12.5, 25, or 50 g of glucose, and were higher after the 50 g dose than after 12.5 g. Changes in plasma glucose concentrations were small and did not correlate with glucose dose. Mean percent reductions of LNAA tended to exhibit dose-dependence, most clearly observed after 120 min. In some subjects as little as 6 g of glucose transiently decreased LNAA concentrations. Branched-chain amino acids were most sensitive, decreasing by 35%-41% after 50 g of glucose. Plasma tryptophan concentrations fell only by 23%, hence the ratio of plasma tryptophan to other plasma LNAA (which affects brain serotonin synthesis) increased significantly.