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.