Meal composition and plasma amino acid ratios: effect of various proteins or carbohydrates, and of various protein concentrations.

We examined the effects of meals containing various proteins and carbohydrates, and of those containing various proportions of protein (0% to 20% of a meal, by weight) or of carbohydrate (0% to 75%), on plasma levels of certain large neutral amino acids (LNAA) in rats previously fasted for 19 hours. We also calculated the plasma tryptophan ratios (the ratio of the plasma tryptophan concentration to the summed concentrations of the other large neutral amino acids) and other plasma amino acid ratios. (The plasma tryptophan ratio has been shown to determine brain tryptophan levels and, thereby, to affect the synthesis and release of the neurotransmitter serotonin). A meal containing 70% to 75% of an insulin-secreting carbohydrate (dextrose or dextrin) increased plasma insulin levels and the tryptophan ratio; those containing 0% or 25% carbohydrate failed to do so. Addition of as little as 5% casein to a 70% carbohydrate meal fully blocked the increase in the plasma tryptophan ratio without affecting the secretion of insulin--probably by contributing much larger quantities of the other LNAA than of tryptophan to the blood. Dietary proteins differed in their ability to suppress the carbohydrate-induced rise in the plasma tryptophan ratio. Addition of 10% casein, peanut meal, or gelatin fully blocked this increase, but lactalbumin failed to do so, and egg white did so only partially. (Consumption of the 10% gelatin meal also produced a major reduction in the plasma tyrosine ratio, and may thereby have affected brain tyrosine levels and catecholamine synthesis).(ABSTRACT TRUNCATED AT 250 WORDS)