Low vitamin D is common in athletes over a northern winter, and supplementation corrects it, but it does not improve strength in athletes
Interpretation
Prevalence is consistent: 56% of 2,313 athletes were below 80 nmol/L in a meta-analysis, with higher risk in winter and indoor sports (Farrokhyar 2015), and 62% of 61 UK professional athletes were below 50 nmol/L in winter (Close 2013). For performance, a meta-analysis of 5 RCTs in 163 athletes found supplementation raised 25(OH)D but did not significantly improve strength (Han 2019); one small trial in deficient professionals reported sprint and jump gains (Close 2013). Gaps: no endurance-performance or triathlete trials, mostly young male athletes, inconsistent thresholds.
Practical implication
Follow public-health advice for a modest daily winter supplement and stay under the 100 microgram (4,000 IU) daily upper limit; treat vitamin D as a health basic, not a performance aid. Testing and treatment doses are for a clinician. Extrapolated from young, mostly male athletes in other sports.
Sources
5 sources, independently verified.
23 studies, 2,313 athletes: 56% (95% CI 44-67%) had 25(OH)D below 32 ng/mL (about 80 nmol/L). Risk higher in winter and spring (RR 1.85) and in indoor sports (RR 1.19), and higher in the UK and Middle East. The inadequacy threshold is generous, the athletes young and mostly male, and no triathlete studies were included.
In a UK winter, 62% of 61 professional athletes and 73% of 30 non-athletes had 25(OH)D below 50 nmol/L. In a placebo-controlled trial (group sizes not in abstract), 5,000 IU/day of vitamin D3 for 8 weeks raised levels from 29 to 103 nmol/L and improved 10 m sprint and vertical jump; placebo (53 to 74 nmol/L) did not. The supplemented group started deficient; sports and sex not reported in the abstract.
5 RCTs, 163 athletes (86 vitamin D3, 77 placebo). Supplementation raised 25(OH)D to sufficiency but did not significantly improve 1RM bench press, maximal quadriceps strength or overall strength (pooled SMD -0.75, 95% CI -1.82 to 0.32). Small evidence base; sports, sex and ages not reported in the abstract; strength, not endurance outcomes.
Narrative review: vitamin D's bone role is established in non-athletes but athlete studies have failed to link 25(OH)D with bone health; possible roles in muscle recovery and infection; the authors' data suggest high-dose supplementation may be problematic; Black athletes often have low measured 25(OH)D without pathology, so routine screening may lead to inappropriate supplementation.
UK guidance: everyone should consider a daily 10 microgram (400 IU) vitamin D supplement in autumn and winter, high-risk groups all year; between October and early March sunlight does not provide enough; do not take more than 100 micrograms (4,000 IU) a day, as long-term excess can cause calcium build-up that weakens bones and damages kidneys and heart. Page last reviewed 3 August 2020.