Influence of vitamin D supplementation on muscle strength and exercise capacity in South African schoolchildren: secondary outcomes from a randomised controlled trial (ViDiKids).

Objective: To determine whether vitamin D supplementation influences grip strength, explosive leg power, cardiorespiratory fitness and risk of exercise-induced bronchoconstriction (EIB) in South African schoolchildren.

Methods: Substudy (n=450) in Cape Town schoolchildren aged 8-11 years nested within a phase 3 randomised placebo-controlled trial (ViDiKids). The intervention was weekly oral doses of 10 000 IU vitamin D₃ (n=228) or placebo (n=222) for 3 years. Outcome measures were serum 25-hydroxyvitamin D₃ (25(OH)D₃) concentrations, grip strength, standing long jump distance, peak oxygen uptake (VO_2peak, determined using 20 m multistage shuttle run tests) and the proportion of children with EIB, measured at end-study.

Results: 64.7% of participants had serum 25(OH)D₃concentrations <75 nmol/L at baseline. At 3-year follow-up, children randomised to vitamin D versus placebo had higher mean serum 25(OH)D₃ concentrations (97.6 vs 58.8 nmol/L, respectively; adjusted mean difference 39.9 nmol/L, 95% CI 36.1 to 43.6). However, this was not associated with end-study differences in grip strength, standing long jump distance, VO_2peak or risk of EIB.

Conclusion: A 3-year course of weekly oral supplementation with 10 000 IU vitamin D₃ elevated serum 25(OH)D₃ concentrations in South African schoolchildren but did not influence muscle strength, exercise capacity or risk of EIB.