Effects of Vitamin D Supplementation on Serum 25-Hydroxyvitamin D Concentrations and Physical Performance in Athletes: A Systematic Review and Meta-analysis of Randomized Controlled Trials

Background There is currently no systematic review examining the effects of vitamin D supplementation among athletes. A rigorous systematic review and meta-analysis is important to provide a balanced view of current knowledge on the effect of vitamin D on serum 25-hydroxyvitamin D [25(OH) D] concentrations and physical performance. Objectives This systematic review of randomized controlled trials (RCTs) evaluated the effects of oral vitamin D supplementation on serum 25(OH) D concentrations and physical performance in athletes. Methods Multiple electronic databases were searched, and study eligibility, methodological quality assessment, and data extraction were completed independently and in duplicate. Studies were stratified by baseline vitamin D sufficiency, season, and latitude. A cut-off of 30 ng/ml (75 nmol/l) of 25(OH) D was used for sufficiency. Absolute mean differences (AMDs) between vitamin D and placebo using random effects analysis, and heterogeneity using Q statistic and I 2 index, were calculated. AMD with 95% confidence interval (CI), p value, and I-2 are reported. Results In total, 13 RCTs (2005-2016) with 532 athletes (vitamin D 311, placebo 221) were eligible. A total of 433 athletes (vitamin D 244, placebo 189) had complete outcome data. Among athletes with baseline values suggesting insufficiency, vitamin D supplementation led to significant increases from 3000 IU (AMD 15.2 ng/ml; 95% CI 10.7-19.7, p < 0.0001, I-2 = 0%) and 5000 IU (AMD 27.8 ng/ml; 95% CI 16.9-38.8, p < 0.0001, I 2 = 78%) per day at >45 degrees latitudes. Both doses led to sufficiency concentrations during winter months. Among athletes with baseline vitamin D suggesting sufficiency, serum 25(OH) D sufficiency was maintained from different doses at both latitudes. Of 13 included trials, only seven measured different physical performances and none demonstrated a significant effect of vitamin D supplementation during 12 weeks of follow-up. Conclusion Despite achieving sufficiency in vitamin D concentrations from >= 3000 IU supplementation, physical performance did not significantly improve. Between-study heterogeneity was large, and well-designed RCTs examining the effect of vitamin D supplementation on serum 25(OH) D concentrations, physical performance, and injuries in different sports, latitudes, ethnicities, and vitamin D status are needed.