Winter vitamin D3 supplementation does not increase muscle strength, but modulates the IGF-axis in young children

PurposeTo explore whether muscle strength, the insulin-like growth factor axis (IGF-axis), height, and body composition were associated with serum 25-hydroxyvitamin D [25(OH)D] and affected by winter vitamin D supplementation in healthy children, and furthermore to explore potential sex differences.MethodsWe performed a double-blind, placebo-controlled, dose-response winter trial at 55oN. A total of 117 children aged 4-8years were randomly assigned to either placebo, 10, or 20 mu g/day of vitamin D-3 for 20 weeks. At baseline and endpoint, we measured muscle strength with handgrip dynamometer, fat mass index (FMI), fat free mass index (FFMI), height, plasma IGF-1, IGF-binding protein 3 (IGFBP-3), and serum 25(OH)D.ResultsAt baseline, serum 25(OH)D was positively associated with muscle strength, FFMI, and IGFBP-3 in girls only (all p<0.01). At endpoint, baseline-adjusted muscle strength, FMI and FFMI did not differ between intervention groups. However, baseline-adjusted IGF-1 and IGFBP-3 were higher after 20 mu g/day compared to placebo (p=0.043 and p=0.006, respectively) and IGFBP-3 was also higher after 20 mu g/day compared to 10 mu g/day (p=0.011). Children tended to be taller after 20 mu g/day compared to placebo (p=0.064). No sex interactions were seen at endpoint.ConclusionsAvoiding the winter-related decline in serum 25(OH)D may influence IGF-1 and IGFBP-3 in children. Larger trials are required to confirm these effects, and the long-term implication for linear growth.