Free tryptophan response to intravenous insulin in depressed patients.

Levels of free plasma tryptophan, total plasma tryptophan, and total amino acids were determined at various times after insulin administration in patients with either major depressive disorder or dysthymic disorder and normal control subjects. Prior to insulin administration, there were no significant differences among the three groups in any of the parameters. However, following insulin, free plasma tryptophan levels fell significantly among the patients with major depressive disorder for the first 30 min, but not among normal controls. The rate of decline among the patients with dysthymic disorder was intermediate between those of the patients with major depressive disorder and normal controls. The levels of total tryptophan and amino acids following insulin did not differ significantly among the three groups.