Małgorzata Magdalena Michalczyk, Mariola Gepfert, Robert Roczniok, Grzegorz Zydek
Background: Vitamin D is involved in musculoskeletal function, but evidence that supplementation improves athletic performance remains inconsistent, particularly in athletes with sufficient baseline vitamin D status. Methods: In this double-blind randomized controlled trial, 18 professional female soccer players were randomized during the autumn preparatory period (August-September) to receive vitamin D3 (4000 IU/day; n = 9) or placebo (n = 9) for eight weeks. Outcomes included total serum 25-hydroxyvitamin D [25(OH)D] and 1,25-dihydroxyvitamin D [1,25(OH)2D] concentrations, hematological variables, RAST total sprint time, 5- and 30-m sprint performance, and countermovement-jump outcomes. Results: At baseline, after summer exposure, 25% of participants had insufficient or deficient 25(OH)D concentrations (≤30 ng/mL). The remaining cohort (75%) had sufficient 25(OH)D concentrations (>30 ng/mL). After eight weeks, no statistically significant between-group differences were observed in vitamin D metabolites, hematological variables, or performance outcomes (Δ25(OH)D: SG +12.4 ± 8.2 ng/mL vs. PG +3.1 ± 6.5 ng/mL; p = 0.12). RAST total sprint time (p = 0.001) and 30-m sprint performance (p = 0.005) improved over time. Conclusions: Vitamin D3 supplementation at 4000 IU/day for eight weeks was not associated with additional improvements in muscle strength, sprint performance, or countermovement-jump outcomes compared with placebo. Because most participants had sufficient baseline 25(OH)D concentrations, larger trials in female athletes with confirmed vitamin D insufficiency or deficiency are needed to determine whether individualized supplementation or longer intervention periods provide additional physiological or performance-related benefits. The trial was retrospectively registered at ClinicalTrials.gov (NCT07641075) on 8 June 2026.