Does chronic polyphenol supplementation modify resistance training-induced strength and muscle morphology adaptations in adults? A systematic review and multilevel meta-analysis of randomized controlled trials.

BACKGROUND: Polyphenol-rich supplements are widely used to support exercise recovery, but whether these recovery-oriented effects translate into long-term resistance-training adaptations remains unclear. OBJECTIVE: To determine whether chronic polyphenol supplementation modifies resistance-training-induced strength, muscle quantity/lean mass, and direct muscle morphology adaptations in adults. METHODS: Following PRISMA guidelines, PubMed, Embase, Web of Science, the Cochrane Library, and Scopus were searched from inception to May 21, 2026. Randomized controlled trials in adults were included if they examined chronic oral polyphenol or polyphenol-rich supplementation combined with structured resistance training vs. placebo, a matched comparator, or resistance training alone. Risk of bias, methodological quality, and certainty of evidence were assessed using RoB 2, PEDro, and GRADE. Pooled effects were estimated in R 4.5.1 using three-level random-effects models with cluster-robust variance estimation. RESULTS: Eleven reports describing 11 randomized trials and 11 independent study clusters, comprising 360 unique participants, were included. Strength adaptations showed a small, statistically uncertain estimate (22 effect sizes, 8 clusters, 257 unique participants; Hedges' g = 0.25, 95% CI -0.01 to 0.51, P = 0.060; 95% PI -0.36 to 0.86; I 2within = 0.0%, I 2between = 34.5%; low certainty). Muscle quantity/lean mass also showed a small, statistically uncertain estimate (13 effect sizes, 9 clusters, 280 unique participants; g = 0.13, 95% CI -0.07 to 0.33, P = 0.211; low certainty). Direct morphology was based on only three clusters and yielded an imprecise estimate close to zero (6 effect sizes, 115 unique participants; g = 0.05, 95% CI -0.21 to 0.30, P = 0.718; very low certainty). Exploratory subgroup analyses, sensitivity analyses, and publication-bias diagnostics did not materially change the primary interpretation. CONCLUSION: Current evidence does not establish a reliable additional effect of chronic polyphenol supplementation on resistance training-induced strength, lean-mass, or direct muscle-morphology adaptations. However, the limited number of independent trials, substantial clinical and methodological indirectness, and low-to-very-low certainty of evidence prevent firm conclusions regarding either benefit or lack of benefit.