Effects of Oral Vitamin D Supplementation on Physical Performance in Combat-Sport Athletes: A Systematic Review of Randomized Placebo-Controlled Trials

Background/Objectives: Vitamin D plays an important role in bone and muscle health; however, whether supplementation improves physical performance in combat-sport athletes remains unclear. This review examined randomized placebo-controlled trials of oral vitamin D supplementation and objective physical-performance outcomes in healthy combat-sport athletes. Methods: PubMed, Scopus, and the Cochrane Library were searched from inception to 22 January 2026, with supplementary targeted verification searches conducted through 1 September 2026. Trials were eligible when combat-sport athletes were randomized to receive vitamin D or placebo/no vitamin D and objective physical performance was assessed. Two reviewers independently performed study selection, data extraction, and risk-of-bias assessment. Findings were synthesized narratively by prioritizing randomized between-group contrasts, group-by-time interactions, numerical changes, and reported effect sizes. Results: Four trials were included, involving 108 randomized participants and 99 analyzed observations. Individual trials reported statistically significant group-by-time effects for selected measures of isokinetic torque, Wingate peak power, high-velocity knee-extension torque, lower-body peak oxygen uptake, handgrip strength, isometric press-up endurance, and lower-body best repeated-sprint peak power. Published standardized effect-size estimates were available for several selected outcomes; however, confidence intervals for between-group effects were generally unavailable. Related outcomes, including Wingate mean or average power, other torque velocities, upper-body peak oxygen uptake, time to exhaustion, oxygen kinetics, wall-sit endurance, and most repeated-sprint measures, showed no statistically significant between-group interaction. No trial assessed competition performance, and all trials presented at least some risk-of-bias concerns. Conclusions: Individual trials reported statistically significant effects on selected laboratory measures. However, the available evidence is not sufficient to determine whether vitamin D supplementation meaningfully improves physical or sport-specific performance in combat-sport athletes. Correcting low vitamin D status should be distinguished from supplementation for performance enhancement. Further well-designed trials are required.