Relationship between the dexamethasone suppression test and the L-tryptophan/competing amino acids ratio in depression.

Levels of L-tryptophan (L-TRP), of the competing amino acids (CAA) valine and leucine, and of postdexamethasone cortisol taken at 8 a.m., 4 p.m., and 11 p.m. were determined in serum samples from 140 depressive patients. The relationships between postdexamethasone cortisol values, the CAA, and the L-TRP/CAA ratio were assessed using Pearson and Spearman correlation coefficients and multiple regression. There was a significant correlation between postdexamethasone cortisol and both L-TRP and the L-TRP/CAA ratio. The highest correlation coefficients were obtained for the 8 a.m. cortisol values. Nonsuppressors, in comparison with suppressors, showed a significantly lower L-TRP value and L-TRP/CAA ratio.