Effects of Probiotic Supplementation on Performance Retention During Detraining and Recovery After Simulated Return-to-Training Stress in College Volleyball Players: An Exploratory Randomized, Single-Blind, Placebo-Controlled Trial

Background: The effects of probiotic supplementation on performance and recovery during training transitions remain uncertain. This exploratory trial examined the effects of probiotic supplementation on performance, recovery, and gut microbiota in national-level university volleyball players. Methods: Twenty-eight players were randomized to receive a multi-strain probiotic or placebo (n = 14 per group) for 8 weeks, comprising regular training, tapering and competition, and reduced training. Performance and gut microbiota were assessed before and after supplementation. Participants then completed 200 weighted vertical jumps as a simulated return-to-training stressor, with recovery monitored for 48 h. Results: Before accounting for multiple comparisons, the probiotic group showed smaller declines in shuttle-test performance and standing and approach vertical jump height across the intervention period, with no significant between-group differences in maximal-strength or Wingate outcomes. Following the simulated return-to-training stressor, the probiotic group showed lower muscle soreness and creatine kinase responses and smaller reductions in selected strength-recovery and rate-of-force-development measures. After accounting for multiple comparisons, only lower muscle soreness and a smaller reduction in non-dominant knee extension maximum work at 180°/s at 24 h remained significant. Chao1 richness decreased more in the probiotic group, whereas changes in Simpson diversity and the relative abundances of Firmicutes, Lachnospiraceae, and Bifidobacteriaceae did not differ between groups. Conclusions: Eight weeks of multi-strain probiotic supplementation was associated with selected differences in performance and recovery during a training transition. However, after accounting for multiple comparisons, the evidence was limited to lower muscle soreness and a smaller reduction in non-dominant knee extension maximum work at 24 h. These exploratory findings require confirmation in larger, prospectively registered trials with prespecified outcomes.