Effects of 10-Week Moderate-Intensity Aerobic and High-Intensity Interval Running Training on Serum Tryptophan, Serum 5-HT, and Kynurenine-Pathway Markers in Recreationally Active Young Men: An Exploratory Non-Randomized Controlled Pilot Study

Background/Objectives: The chronic, modality-specific effects of exercise training on tryptophan (TRP) metabolism are unclear. This exploratory non-randomized controlled pilot study examined whether 10 weeks of moderate-intensity aerobic running or high-intensity interval running (labelled ‘anaerobic’) alters serum serotonin (5-HT; principal outcome) and kynurenine-pathway markers. Methods: Twenty-four recreationally active young men were allocated by the investigators to control, aerobic or anaerobic groups (n = 8 each). Fasting serum TRP, 5-HT, kynurenine-pathway metabolites and immunoreactive enzymes were measured by ELISA before and after training and compared by baseline-adjusted ANCOVA. Results: Serum 5-HT differed between groups (F(2,20) = 7.60, p = 0.004, η2p = 0.43) and was higher after aerobic training than in controls (+127 ng/mL; 95% CI 59–194); the difference from anaerobic training (+73 ng/mL) was not significant after Holm adjustment (p = 0.071). No secondary analyte showed a consistent between-group difference, and kynurenic acid increased in all groups, including controls. Conclusions: Aerobic running was associated with higher serum 5-HT than no training, but superiority over high-intensity interval running was not established. Because serum 5-HT is largely platelet-derived, most 5-HT values were extrapolated above the highest calibrator, the original absorbance data were not retained and training adaptations were not verified, this finding is preliminary.