Individual day-to-day variations in plasma amino acid levels in healthy persons.

Fasting plasma levels of the large neutral amino acids (LNAA;L-forms) andL-tryptophan (TRP) ratios were determined in thirteen healthy volunteers (7 males, 6 females) on five consecutive mornings, and the same procedure was repeated for each individual three months later. We found characteristic overall ranges for the parameters studied, and, in addition to certain differences between the sexes, considerable inter- and intraindividual daily variations. Although the individuals showed statistically identifiable mean levels that, in the majority of subjects, were maintained over a period of three months, it is concluded that the degree of intraindividual variability does not allow us to regard a single value as "characteristic" for a given individual. This should be borne in mind in particular in follow-up studies of plasma LNAA in patients with, for example, depressive disorders.