Methods: We conducted a nested case–control study within the European Prospective Investigation into Cancer and Nutrition (EPIC) that included 893 incident lung cancer cases and 1,748 matched controls. Circulating levels of tryptophan and six of its metabolites were measured and evaluated in relation to lung cancer risk.
Results: Tryptophan (Ptrend = 2 × 10−5) and the kynurenine/tryptophan ratio (KTR; Ptrend = 4 × 10−5) were associated with lung cancer risk overall after adjusting for established risk factors. The ORs comparing the fifth and first quintiles (OR5th vs. 1st) were 0.52 [95% confidence interval (CI), 0.37–0.74] for tryptophan and 1.74 (95% CI, 1.24–2.45) for KTR. After adjusting for plasma methionine (available from previous work, which was strongly correlated with tryptophan), the associations of tryptophan (adjusted Ptrend = 0.13) and KTR (Ptrend = 0.009) were substantially attenuated. KTR was positively associated with squamous cell carcinoma, the OR5th vs. 1st being 2.83 (95% CI, 1.62–4.94, Ptrend = 3 × 10−5) that was only marginally affected by adjusting for methionine.
Conclusions: This study indicates that biomarkers of tryptophan metabolism are associated with subsequent lung cancer risk. Although this result would seem consistent with the immune system having a role in lung cancer development, the overall associations were dependent on methionine, and further studies are warranted to further elucidate the importance of these metabolites in lung cancer etiology.