Influence of vitamin D supplementation on muscle strength and exercise capacity in South African schoolchildren: 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 bronchoconstriciton (EIB) in South African schoolchildren. Methods: Sub-study (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 (VO2peak, determined using 20-metre multi-stage shuttle run tests) and the proportion of children with EIB, all measured at end-study. Results: 64.7% of participants had serum 25(OH)D3 concentrations <75 nmol/L at baseline. At 3-year follow-up, children randomised to vitamin D vs. placebo had higher mean serum 25(OH)D3 concentrations (97.6 vs. 58.8 nmol/L respectively; adjusted mean difference [aMD] 39.9 nmol/L, 95% CI 36.1 to 43.6) and long jump distance (128.3 vs. 122.1 cm; aMD 3.6 cm, 95% CI 0.0 to 7.2). No end-study differences in grip strength, VO2peak, or spirometric lung volumes were seen, but administration of vitamin D vs. placebo was associated with a borderline-significant increased risk of EIB (14.5% vs. 8.6%; adjusted odds ratio 1.92, 95% CI 0.99 to 3.73). 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 and induced a small increase in long jump distance, but had no effect on grip strength or VO2 peak. Potential effects of vitamin D on risk of EIB require further research.