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

Six healthy fasting females each received, on different days, 0, 6, 12.5 or 25 g of albumin dissolved in 200 ml water. Blood was collected before and at intervals after the albumin ingestion and assayed for plasma levels of large, neutral amino acids (LNAA), insulin and glucose. Insulin increased significantly after ingestion of 12.5 and 25 g of albumin, whereas changes in serum glucose were small and inconsistent. Increases of plasma LNAA concentrations were dose-related and correlated significantly with the molar percentage in the albumin of the respective amino acids at 1 and 2 hours but not at 3 and 4 hours after consumption. Ratio in plasma of tyrosine to other LNAA increased by 20 to 60%, and still at 4 hours the plasma tyrosine ratio was significantly elevated above control level after all 3 albumin doses. The plasma ratio of tryptophan to other LNAA decreased by 30 to 50%, and at 4 hours the plasma tryptophan ratio was still significantly below control level after the 2 larger albumin doses. The marked changes in the plasma tryptophan and tyrosine ratio suggests that the central serotonin and catecholamine synthesis could possibly be affected following ingestion of pure protein.