Vitamin D supplementation and increased dairy protein intake do not affect muscle strength or physical function in healthy 6–8-year-old children: the D-pro randomized trial

Purpose To investigate separate and combined effects of vitamin D supplementation during the extended winter and increased dairy protein intake on muscle strength and physical function in children, and furthermore to explore potential sex differences. Methods In a 2 × 2-factorial, randomized winter trial, 183 healthy, 6–8-year-old children received blinded tablets with 20 µg/day vitamin D 3 or placebo, and substituted 260 g/day dairy with yogurts with high (HP, 10 g protein/100 g) or normal protein content (NP, 3.5 g protein/100 g) for 24 weeks during winter at 55° N. We measured maximal isometric handgrip and leg press strength, and physical function by jump tests and a 30 s sit-to-stand test. Physical activity was measured by 7-day accelerometry. Results Baseline (mean ± SD) serum 25-hydroxyvitamin D was 80.8 ± 17.2 nmol/L, which increased to 88.7 ± 17.6 nmol/L with vitamin D supplementation and decreased to 48.4 ± 19.2 nmol/L with placebo. Baseline protein intake was 15.5 ± 2.4 E %, which increased to 18.4 ± 3.4 E % with HP and was unchanged with NP. We found no separate or combined effects of vitamin D supplementation and/or increased dairy protein intake on muscle strength or physical function (all P > 0.20). There was an interaction on the sit-to-stand test ( P vitamin×yogurt = 0.02), which however disappeared after adjusting for physical activity ( P = 0.16). Further, vitamin D supplementation increased leg press strength relatively more in girls compared to boys (mean [95% CI] 158 [17, 299] N; P vitamin×sex = 0.047). Conclusion Overall, vitamin D and dairy protein supplementation during the extended winter did not affect muscle strength or physical function in healthy children. Potential sex differences of vitamin D supplementation should be investigated further. Registered at clinicaltrials.gov NCT0395673.