Bioconversion-Based Postbiotics Enhance Muscle Strength and Modulate Gut Microbiota in Healthy Individuals: A Randomized, Double-Blind, Placebo-Controlled Trial

Background: Postbiotics produced by kefir lactic acid bacteria through bioconversion of polyphenol-rich extract and whey protein are emerging as promising modulators of gut microbiota and muscle health. This study investigated whether Lentilactobacillus kefiri DH5-derived postbiotics, prepared with Cucumis melo L. and whey protein (KP, Kefir lactic acid bacteria-derived postbiotics), improve muscle strength and gut microbiota composition in healthy adults. Methods: In this 12-week, randomized, double-blind, placebo-controlled trial, participants consumed either KP (6 g/day) or placebo. Handgrip strength, circulating biomarkers, and fecal microbiota profiling (using 16S rRNA sequencing) were analyzed. Correlations between microbial taxa and muscle-related biomarkers were assessed. Results: KP supplementation significantly increased dominant-hand grip strength and plasma irisin and reduced IL-1β concentrations after 12 weeks, whereas IGF-1, lean mass, and non-dominant grip strength showed no significant changes. Gut microbiota profiling revealed enrichment of Bifidobacterium adolescentis, Latilactobacillus sakei, Lentihominibacter hominis, Mediterraneibacter gnavus, Streptococcus anginosus and Phocaeicola plebeius, with concomitant reductions in Lachnospira eligens, Roseburia inulinivorans, Ruthenibacterium lactatiformans and Vescimonas fastidiosa. Notably, relative abundance of Faecalibacterium prausnitzii was positively correlated with plasma irisin concentration. Conclusions: KP supplementation produced a modest within-group improvement in grip strength, potentially through gut–muscle axis modulation involving irisin and anti-inflammation pathways. These preliminary findings suggest that kefir-derived postbiotics may have potential relevance for muscle health.