Am J Clin Nutr. 1981 Mar; 34(3): 377-85.

Plasma neutral amino acid ratios in normal man and in patients with hepatic encephalopathy: correlations with self-selected protein and energy consumption.

Anderson GH, Blendis LM.

Plasma amino acid concentrations and the ratios of tryptophan and tyrosine to other large neutral amino acids (NAA) were related to prior food consumption by normal healthy adults and by adults with hepatic encephalopathy. Previous habitual (3-day) energy consumption by 32 healthy subjects correlated inversely with the fasting plasma tyr/NAA (r = --0.40, p less than 0.05), while the proportion of energy consumed as protein was inversely related to plasma trp/NAA (r = -0.63, p less than 0.001). Postprandial plasma trp/NAA of 11 normal subjects at 2 and 4 h also reflected (r = --0.62 and --0.69, p less than 0.05, respectively) the proportion of food energy chosen as protein at a single meal. Similarly, samples obtained from five patients with hepatic encephalopathy showed that when high plasma trp/NAA ratios occur these are likely to be due to the low protein intake prescribed. However, for these patients the tyr/NAA or the tyr and met/branched-chain amino acid ratios were above normal ranges caused by diet. The conclusion of this study is that the plasma trp/NAA ratio is associated with the proportion of dietary energy consumed as protein, whereas the plasma tyr/NAA ratio is associated with long-term total energy consumption.