Influence of Four-Week Glutamine Supplementation on Gut Microbiota Across a Training Period in Cross-Country Skiers

The gut microbiota is increasingly recognized as an important factor related to intestinal homeostasis and athlete health, yet the effects of L-glutamine supplementation on gut microbiota composition in endurance athletes during training remain unclear. This randomized, double-blind, placebo-controlled crossover trial examined whether 4 weeks of L-glutamine supplementation influenced gut microbiota composition and gastrointestinal responses in 19 collegiate cross-country skiers during a training period. Stool samples were collected at baseline, week 2, and week 4 during each intervention period, and gut microbiota composition was analyzed by 16S rRNA gene sequencing targeting the V4 region. Shannon alpha diversity increased significantly over time, and Bray-Curtis beta diversity showed significant time-related changes, with no significant supplementation or interaction effects. At the genus level, L-glutamine supplementation was associated with a transient increase in Faecalibacterium and a decrease in Collinsella at week 2, though these changes were not sustained at week 4. Positive correlations among selected short-chain fatty acid-producing genera were also observed under the L-glutamine condition. Fecal calprotectin, gastrointestinal symptoms, stool form, and bowel movement frequency were not significantly affected by supplementation. These findings suggest that gut microbiota dynamics in collegiate cross-country skiers were characterized by time-dependent shifts across the intervention period, whereas L-glutamine supplementation was associated with transient genus-level shifts and microbial correlation patterns without adverse gastrointestinal responses.