Concentrations of various tryptophan metabolites are higher in patients with diabetes mellitus than in healthy aged male adults

Tryptophan metabolites in plasma samples from 20 male subjects with type 2 diabetes mellitus (T2DM) and 20 nondiabetic reference males were analyzed by ultra high performance liquid chromatography. Tryptophan levels in the diabetic subjects were significantly lower than those in nondiabetic subjects. The concentrations of 5-hydroxytryptophan, 5-hydroxyindoleacetic acid, kynurenic acid, 3-hydroxykynurenine, 3-hydroxyanthranilic acid, and xanthurenic acid were found to be higher in the diabetic patients. When the diabetic patients were divided into higher- and lower-tryptophan groups, the concentrations of 5-hydroxytryptophan, indole-3-acetic acid, kynurenine, 5-hydroxykynurenine, and kynurenic acid were found to be higher in the diabetic patients with higher tryptophan levels. However, diabetic patients with lower plasma tryptophan levels had higher levels of 5-hydroxyindoleacetic acid than the patients with higher tryptophan levels. These results suggest that tryptophan was metabolized more in T2DM patients than in nondiabetic subjects. In the kynurenine pathway, the degradation of tryptophan seems to be accelerated in patients with higher plasma levels of tryptophan than in patients with lower levels of tryptophan. In the serotonin pathway, when the level of tryptophan is low, the conversion of serotonin to 5-hydroxyindoleacetic acid appears to be accelerated. In conclusion, our results suggest that T2DM patients may be exposed to stress constantly.