Multistrain Probiotic Supplementation Combined with a Standardized Diet Did Not Significantly Affect Exercise Performance or Inflammatory Responses in Male Endurance Runners: A Randomized Controlled Trial

Background: Gut microbiota may influence metabolic and inflammatory responses to exercise through the gut–muscle axis, and probiotic supplementation has been proposed to support adaptation and recovery in endurance athletes. However, evidence in trained populations is inconsistent and is confounded by variability in diet, probiotic strains, dose, and intervention duration. We tested whether a multistrain probiotic, administered against a fully standardized diet, would affect exercise performance and inflammatory, metabolic, and muscle-damage responses. Methods: In this randomized, double-blind, placebo-controlled trial, 30 trained male long-distance runners were randomized and 27 completed the study (probiotic [PRO], n = 13; placebo [PLA], n = 14). All participants followed a standardized, normocaloric meal-box diet for four weeks. Aerobic capacity (peak oxygen uptake, VO2peak; primary outcome) was assessed by an incremental treadmill test and anaerobic performance by a 30 s Wingate test, before (PRE) and after (POST) supplementation. Interleukin-6 (IL-6), tumor necrosis factor-α (TNF-α), creatine kinase (CK), and lactate (LA) were measured around exercise. Outcomes were analyzed with linear mixed-effects models; the primary estimand was the group × phase interaction. Results: VO2peak did not differ between groups over time (PLA 55.00 ± 7.60 → 57.33 ± 8.41; PRO 55.40 ± 7.85 → 53.14 ± 5.95 mL·kg−1·min−1; group × phase −3.5 mL·kg−1·min−1, 95% CI −7.0 to −0.1; nominal p = 0.044, not significant after FDR correction, adjusted p = 0.707). No group × phase or group × phase × sampling-time interaction was significant for IL-6, TNF-α, LA, or CK (all interaction p > 0.15), indicating that supplementation did not modify the exercise-induced response of any biomarker; exercise itself robustly increased IL-6, TNF-α, and LA in both groups (p < 0.001). Conclusions: In trained male endurance runners consuming a standardized diet, four weeks of multistrain probiotic supplementation did not significantly affect aerobic or anaerobic performance, LA response, muscle-damage markers, or circulating inflammatory cytokines compared with placebo. Adequately powered trials with prespecified primary endpoints and direct microbiome assessment are needed.