Effects of Ground Grape Seed Supplementation on Tryptophan–Kynurenine Pathway Metabolites and Trimethylamine-N-Oxide in Non-Alcoholic Fatty Liver Disease

This study investigated alterations in serum tryptophan–kynurenine (TRP–KYN) metabolites and trimethylamine N-oxide (TMAO) levels associated with ground grape seed supplementation in individuals with nonalcoholic fatty liver disease (NAFLD) using a non-placebo-controlled, non-randomized, open-label, single-arm intervention design. Thirty NAFLD patients and thirty healthy control participants, matched for age and sex, were enrolled; the control group served solely as a baseline comparison and did not receive supplementation. NAFLD patients consumed 5 g of ground grape seeds daily on an empty stomach during two 25-day treatment periods separated by a 10-day interval. Serum samples were collected at baseline and at the end of each treatment period. Metabolite profiling was conducted using liquid chromatography–tandem mass spectrometry. At baseline, serum tryptophan and kynurenine levels in NAFLD patients were significantly lower than those in the control group (FDR-adjusted p = 0.003 for both); levels of 3-hydroxykynurenine, kynurenic acid, and 3-hydroxyanthranilic acid were also significantly lower. In contrast, the KYNA/KYN and QA/KYN ratios were significantly higher, while the KYNA/QA and 3-HAA/3-HK ratios were significantly lower (all q ≤ 0.004). TMAO levels did not differ between groups (q = 0.965). In longitudinal analyses of the NAFLD group, tryptophan levels remained unchanged, while KYN, 3-HK, KYNA, and QA levels, as well as the KYNA/KYN and KYNA/QA ratios, decreased, and the QA/KYN ratio and TMAO levels increased. For all these parameters, the overall time effect was significant after FDR correction (q < 0.05). Owing to the uncontrolled, non-randomized design, these within-group changes cannot be causally attributed to the supplement and may reflect regression to the mean or unmeasured confounding. These preliminary, hypothesis-generating findings indicate a potential association between ground grape seed supplementation and changes in TRP–KYN pathway metabolites and TMAO levels in NAFLD, underscoring the need for validation in adequately powered, randomized, and placebo-controlled trials.