(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)CO2 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)CO2/(12)CO2 ratio in the breath was monitored for 180 min. The cumulative recovery rate during the 180-min test (CRR0-180; %), area under the Δ(13)CO2-time curve (AUC; %*min), and the maximal Δ(13)CO2 (Cmax; %) 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 Cmax 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.