(13)C-tryptophan breath test detects increased catabolic turnover of tryptophan along the kynurenine pathway in patients with major depressive disorder

Altered tryptophan–kynurenine (KYN) metabolism has been implicated in major depressive disorder (MDD). The l-[1-(13)C]tryptophan breath test ((13)C-TBT) is a noninvasive, stable-isotope tracer method in which exhaled (13)CO(2) is attributable to tryptophan catabolism via the KYN pathway. We included 18 patients with MDD (DSM-IV) and 24 age- and sex-matched controls. (13)C-tryptophan (150 mg) was orally administered and the (13)CO(2)/(12)CO(2) ratio in the breath was monitored for 180 min. The cumulative recovery rate during the 180-min test (CRR(0–180); %), area under the Δ(13)CO(2)-time curve (AUC; %*min), and the maximal Δ(13)CO(2) (C(max); %) were significantly higher in patients with MDD than in the controls (p = 0.004, p = 0.008, and p = 0.002, respectively). Plasma tryptophan concentrations correlated negatively with C(max) in both the patients and controls (p = 0.020 and p = 0.034, respectively). Our results suggest that the (13)C-TBT could be a novel biomarker for detecting a subgroup of MDD with increased tryptophan–KYN metabolism.