Background Vitamin D deficiency is common in the general public and athletic populations and may impair skeletal muscle function. We therefore assessed the effects of vitamin D₃ supplementation on serum 25[OH]D concentrations and physical performance.

Methods 30 club-level athletes were block randomised (using baseline 25[OH]D concentrations) into one of three groups receiving either a placebo (PLB), 20 000 or 40 000 IU/week oral vitamin D₃ for 12 weeks. Serum 25[OH]D and muscle function (1-RM bench press and leg press and vertical jump height) were measured presupplementation, 6 and 12 weeks postsupplementation. Vitamin D deficiency was defined in accordance with the US Institute of Medicine guideline (<50 nmol/l).

Results 57% of the subject population were vitamin D deficient at baseline (mean±SD value 51±24 nmol/l). Following 6 and 12 weeks supplementation with 20 000 IU (79±14 and 85±10 nmol/l, respectively) or 40 000 IU vitamin D₃ (98±14 and 91±24 nmol/l, respectively), serum vitamin D concentrations increased in all participants, with every individual achieving concentrations greater than 50 nmol/l. In contrast, vitamin D concentration in the PLB group decreased at 6 and 12 weeks (37±18 and 41±22 nmol/l, respectively). Increasing serum 25[OH]D had no significant effect on any physical performance parameter (p>0.05).

Conclusions Both 20 000 and 40 000 IU vitamin D₃ supplementation over a 6-week period elevates serum 25[OH]D concentrations above 50 nmol/l, but neither dose given for 12 weeks improved our chosen measures of physical performance.