Factors affecting plasma amino acid concentrations in control subjects.

Plasma amino acid concentrations were determined in groups of normal control subjects under fasting and postprandial conditions. Differences were observed that were found to be, to a considerable extent, sex-related. Fasting females had lower concentrations than fasting males of several amino acids, although postprandial differences in concentrations between the sexes were not significant. Under the dietary conditions normal for a British population, no significant diurnal variation of plasma amino acid concentrations was found. Reducing the dietary protein by 30% failed to affect plasma amino acid concentrations; however, increasing the dietary protein by 50% resulted in a significant increase in the plasma concentrations of several amino acids, and induced a significant degree of variation throughout the day in six, proline, half-cystine, methionine, valine, leucine, and isoleucine. The importance of these results with regard to the interpretation of population mean values and disease diagnosis is discussed.