Neutral amino acid plasma levels in healthy subjects: effect of complex carbohydrate consumed along with protein.

Six healthy fasting females each received, on different days, 0, 25, or 50 g of complex carbohydrate in the form of white spaghetti containing 11-12% protein along with 200 ml water. Blood was collected before and at intervals for 4 hours after the spaghetti ingestion and assayed for plasma levels of large, neutral amino acids (LNAA), insulin and glucose. The area under the curve for the increase of serum insulin was four times greater after the 50-g dose compared to the 25-g dose of carbohydrate, but the respective areas under the curves for the increases of plasma glucose were identical. The plasma ratio tryptophan/LNAA varied significantly and increased 10% above basal level and 25-30% above control level after either carbohydrate dose, but only the difference for the 50-g dose was significant. Although the main effect for plasma ratio tyrosine/LNAA was also significant, the increases of this ratio above control level were marginal. The main effect for the dose of complex carbohydrate for valine, isoleucine and leucine was significant, and the areas above the curves for the decreases of individual branched-chain amino acids were of the same size after the two doses. Plasma tyrosine, phenylalanine and tryptophan showed no significant variation. The study has shown that consumption of complex carbohydrate along with 11-12% protein (the recommended dietary concentration) slightly increases the plasma ratio tryptophan/LNAA for 4 hours, and that the effect is practically maximal after ingestion of 25 g of the test meal.