Influence of vitamin D supplementation on muscle strength and exercise capacity in Mongolian schoolchildren: a randomised controlled trial

ObjectiveTo determine whether weekly oral vitamin D supplementation influences grip strength, explosive leg power, cardiorespiratory fitness or spirometric lung volumes in Mongolian schoolchildren. MethodsMulticentre, randomised, double-blind, placebo-controlled clinical trial conducted in children aged 6-13 years at baseline attending 18 schools in Ulaanbaatar. The intervention was weekly oral doses of 14,000 IU vitamin D3 (n=4418) or placebo (n=4433) for 3 years. Outcome measures were grip strength, standing long jump distance and serum 25-hydroxyvitamin D (25[OH]D) concentrations (determined in all participants), peak oxygen uptake (VO2peak, determined in a subset of 632 participants using 20-metre multi-stage shuttle run tests) and spirometric outcomes (determined in a subset of 1,343 participants). Results99.8% of participants had serum 25(OH)D concentrations <75 nmol/L at baseline, and mean end-study 25(OH)D concentrations in children randomised to vitamin D vs. placebo were 77.4 vs. 26.7 nmol/L (mean difference 50.7 nmol/L, 95% CI, 49.7 to 51.4). However, vitamin D supplementation did not influence mean grip strength, standing long jump distance, VO2peak, spirometric lung volumes or peak expiratory flow rate, either overall or within sub-groups defined by sex, baseline 25(OH)D concentration <25 vs. [&ge;]25 nmol/L or calcium intake <500 vs. [&ge;]500 mg/day. ConclusionA 3-year course of weekly oral supplementation with 14,000 IU vitamin D3 elevated serum 25(OH)D concentrations in Mongolian schoolchildren with a high baseline prevalence of vitamin D deficiency. However, this intervention did not influence grip strength, explosive leg power, peak oxygen uptake or spirometric lung volumes, either overall or in sub-group analyses. KEY MESSAGESWhat is already known on this topic? Observational studies have reported that vitamin D deficiency associates with reduced muscle strength and peak oxygen uptake in children, but randomised controlled trials (RCT) of vitamin D supplementation to improve grip strength and cardiorespiratory fitness in this age-group have yielded conflicting results. What this study adds This Phase 3 multicentre RCT of vitamin D supplementation, conducted in Mongolian schoolchildren with a high baseline prevalence of asymptomatic vitamin D deficiency, found that a 3-year course of weekly oral supplementation with 14,000 IU vitamin D3 was effective in elevating serum 25-hydroxyvitamin D concentrations. However this intervention did not influence participants grip strength, long jump distance, peak oxygen uptake, spirometric lung volumes or peak expiratory flow rate, either overall or in sub-group analyses. How this study might affect research, practice or policy Taken together with results from another Phase 3 randomised controlled trial of vitamin D supplementation conducted in South African children, our findings do not suggest a role for weekly oral vitamin D supplementation to enhance muscle strength, peak oxygen uptake or respiratory function in schoolchildren in whom rickets has been excluded.