Effects of 8-Week High-Dose Vitamin D Supplementation on Physical Performance in Female National League Handball Players During the Competitive Season

Background: Vitamin D insufficiency is common among indoor athletes, but supplementation effects on performance remain uncertain. This randomized, double-blind, placebo-controlled study examined eight-week vitamin D3 supplementation in 20 female national level (second-division) handball players during the competitive season. Methods: Participants were randomized within pairs matched for baseline serum 25-hydroxyvitamin D [25(OH)D] to receive vitamin D3 (5000 IU/day; n = 10) or placebo (n = 10). Serum 25(OH)D, parathyroid hormone, calcium, phosphorus, 10 m and 30 m sprint times, 20 m shuttle-run distance, Wingate peak power, and total work were assessed before and after supplementation. Exploratory between-group comparisons of change used Holm adjustment across nine outcomes. Results: Serum 25(OH)D increased from 26.70 ± 11.12 to 42.96 ± 16.20 ng/mL with supplementation and from 27.05 ± 8.01 to 29.65 ± 7.67 ng/mL with placebo. The between-group difference in change was 13.66 ng/mL (95% confidence interval: 7.32–20.00; Holm-adjusted p = 0.002). No other outcome showed a significant time-by-group interaction after adjustment. Confidence intervals for all between-group performance contrasts included zero. Conclusions: Vitamin D3 supplementation improved vitamin D status, but a supplementation-specific performance benefit was not established. Given the small sample and imprecise estimates, these findings do not establish equivalence or exclude meaningful performance effects.