Abstract
BACKGROUND: High‑protein diets are widely used by athletes but may disturb the gut environment and increase production of odor‑related metabolites. Probiotic supplementation has been proposed as a strategy to support gastrointestinal function under such dietary stress. This study aimed to explore the effects of Bifidobacterium longum BB536 on gastrointestinal symptoms, gut microbiota, and odor‑related metabolites in male athletes consuming a high‑protein diet. METHODS: In an exploratory, randomized, double‑blind, placebo‑controlled trial, 60 healthy male athletes (mean age: 18.62 ± 0.75 years; mean BMI: 22.35 ± 1.80 kg/m(2)) consumed a whey protein supplement (70 g/day) together with either BB536 (46 billion CFU/day, as measured at the start of the intervention) or placebo for 4 weeks. Gastrointestinal symptoms, gut microbiota composition, skin‑emitted volatile compounds, and fecal metabolites were assessed. Subgroup analyses based on responder status and baseline enterotype were conducted post hoc to generate hypotheses regarding microbiota‑dependent responses. RESULTS: In the overall cohort, no significant between‑group differences were observed across gastrointestinal outcomes, gut microbiota indices, or metabolite profiles. Within the BB536 group, diarrhea‑related scores improved from baseline. Post hoc analyses suggested that increases in Faecalibacterium were evident among responders. Enterotype‑based patterns also emerged: individuals with Ruminococcus‑dominant microbiota showed higher skin‑emitted short‑chain fatty acids after BB536 intake, whereas those with Faecalibacterium‑dominant microbiota exhibited reductions in odor‑related metabolites, including methyl mercaptan and ammonia. Corresponding fecal metabolite shifts were modest. CONCLUSION: BB536 supplementation was associated with improvements in diarrhea‑related symptoms and odor‑related metabolites in specific microbiota‑defined subgroups. As these findings did not extend to the full cohort, they should be interpreted as exploratory and hypothesis‑generating. Baseline gut microbiota composition may influence probiotic responsiveness, warranting confirmatory trials with prespecified endpoints.