Bifidobacterium longum BB536 is associated with improvements in gastrointestinal symptoms and odor-related metabolites in microbiota-defined subgroups of male athletes consuming a high-protein diet: exploratory randomized double‑blind placebo‑controlled trial

BACKGROUND: BB536 on gastrointestinal symptoms, gut microbiota, and odor‑related metabolites in male athletes consuming a high‑protein diet. METHODS: ) 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: ‑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.