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

ABSTRACT 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 D 3 (n=228) or placebo (n=222) for 3 years. Outcome measures were serum 25-hydroxyvitamin D 3 (25[OH]D 3 ) concentrations, grip strength, standing long jump distance, peak oxygen uptake (VO 2peak , 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)D 3 concentrations <75 nmol/L at baseline. At 3-year follow-up, children randomised to vitamin D vs. placebo had higher mean serum 25(OH)D 3 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, VO 2peak , 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 D 3 elevated serum 25(OH)D 3 concentrations in South African schoolchildren and induced a small increase in long jump distance, but had no effect on grip strength or V O2 peak . Potential effects of vitamin D on risk of EIB require further research. KEY MESSAGES What is already known on this topic? Observational studies have reported that vitamin D deficiency associates with reduced muscle strength and peak oxygen uptake and increased risk of exercise-induced bronchoconstriction (EIB) in children. Randomised controlled trials (RCT) of vitamin D supplementation to improve children’s muscle strength and cardiorespiratory fitness have yielded conflicting results. What this study adds This RCT, conducted in South African schoolchildren aged 8-11 years at baseline, found that a 3-year course of weekly oral supplementation with 10,000 IU vitamin D 3 improved vitamin D status and resulted in a small (3.6 cm), borderline-significant increase in long jump distance, but had no effect on grip strength or peak oxygen uptake. Administration of vitamin D was associated with a borderline-significant increased risk of EIB. How this study might affect research, practice or policy Taken together with null results from another Phase 3 randomised controlled trial of vitamin D supplementation conducted in Mongolian children, our findings do not suggest that weekly oral vitamin D supplementation exerts clinically significant effects on muscle strength or peak oxygen uptake in schoolchildren in whom rickets has been excluded. Further research into potential effects of vitamin D supplementation on risk of EIB is needed.