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 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 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)D ₃ concentrations <75 nmol/L at baseline. At 3-year follow-up, children randomised to vitamin D versus placebo had higher mean serum 25(OH)D ₃ 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 D ₃ elevated serum 25(OH)D ₃ concentrations in South African schoolchildren but did not influence muscle strength, exercise capacity or risk of EIB.
Competing Interests: ARM declares receipt of funding in the last 36 months to support vitamin D research from the following companies who manufacture or sell vitamin D supplements: Pharma Nord, DSM Nutritional Products, Thornton & Ross and Hyphens Pharma. ARM also declares receipt of vitamin D capsules for clinical trial use from Pharma Nord, Synergy Biologics and Cytoplan; support for attending meetings from Pharma Nord Ltd and Abiogen Pharma; receipt of consultancy fees from DSM Nutritional Products and Qiagen; receipt of a speaker fee from the Linus Pauling Institute; participation on Data and Safety Monitoring Boards for the VITALITY trial (Vitamin D for Adolescents with HIV to reduce musculoskeletal morbidity and immunopathology, Pan African Clinical Trials Registry ref PACTR20200989766029) and the Trial of Vitamin D and Zinc Supplementation for Improving Treatment Outcomes Among COVID-19 Patients in India (ClinicalTrials.gov ref NCT04641195) and unpaid work as a Programme Committee member for the Vitamin D Workshop.
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