Eight-Week Vitamin D Supplementation at 4000 IU/Day Is Insufficient to Improve Speed and Power in Professional Female Soccer Players

Background: Vitamin D optimizes musculoskeletal function and athletic performance, yet optimal supplementation protocols remain unclear. Methods: In this double-blind RCT, 18 professional female soccer players were randomized during autum preparatory period (August-September) to receive vitamin D₃ (4000 IU/day; n=9) or placebo (n=9) for 8 weeks. Outcomes included serum 25(OH)D/1,25(OH)₂D, hematology, RAST, 5/30-m sprints, and CMJ. Results: At baseline, after summer exposure, four players had 25(OH)D ≤ 30 ng/mL, and fifteen had levels between 30–50 ng/mL; none exceeded 50 ng/mL. After eight weeks of supplementation, no significant differences were observed between groups in 25(OH)D, and metabolites (Δ25(OH)D: EG +12.4±8.2 vs. PG +3.1±6.5 ng/mL; p=0.12), perfomance, or morphology. Training improved RAST (p=0.001) and 30-m sprint (p=0.005). Conclu-sions: Vitamin D₃ supplementation at 4000 IU/day for eight weeks did not significantly improve strength, speed, or CMJ performance in professional female soccer players. Persistently suboptimal vitamin D status suggests that higher doses may be required to improve anaerobic capacity. Further studies in this specific population are warranted, and higher supplementation doses, as observed in studies on male football players, may potentially lead to more pronounced improvements in physical performance tests.