Does vitamin D improve muscle strength in adults? A randomized, double-blind, placebo-controlled trial among ethnic minorities in Norway.

Randomized, double-blind, placebo-controlled trial in healthy adults aged 18 to 50 years with non-Western immigrant backgrounds living in Oslo, Norway. Participants were born in or had parents born in the Middle East, Africa, or South Asia, and had low baseline vitamin D status.

Daily oral vitamin D3 tablets were given for 16 weeks at 25 g (1000 IU) per day or 10 g (400 IU) per day, compared with placebo.

Vitamin D3 supplementation did not improve muscle strength or power over 16 weeks. The combined intervention groups showed no significant effect on percentage change in jump height versus placebo, -1.4% (95% CI, -4.9% to 2.2%; P = .44), and there were no meaningful benefits for handgrip strength or chair-rising time. For jump height, the 25 g group was -0.4 (95% CI, -5.1 to 4.4; P = .85) and the 10 g group was -2.4 (95% CI, -7.2 to 2.4; P = .24) versus placebo. Serum 25-hydroxyvitamin D increased, but reported adverse events were few, mild, and similar across groups.

The intervention lasted only 16 weeks, which limits assessment of longer-term functional effects. The population was narrowly defined as healthy immigrant adults in Oslo with low vitamin D status, so generalizability is limited. Physical activity was not reported, and baseline dietary/nutritional assessment was limited.