Effects of Vitamin D3 Supplementation on Muscle Strength, Mass, and Physical Performance in Women with Vitamin D Insufficiency: A Randomized Placebo-Controlled Trial.

Vitamin D insufficiency and hyperparathyroidism have been associated with reduced muscle strength, physical performance, postural stability, well-being, and quality of life. In a double-blinded, randomized placebo-controlled trial, we aimed to investigate effects of vitamin D3 supplementation on above-mentioned outcomes in healthy community-dwelling postmenopausal women with plasma levels of 25-hydroxyvitamin D (25(OH)D) below < 50 nmol/l and high parathyroid hormone (PTH) levels. Participants (N = 81) were 1:1 treated with vitamin D3, 70 µg (2800 IU)/day or identical placebo for three months during wintertime (56°N). Vitamin D3 supplementation increased levels of 25(OH)D and 1,25(OH)2 D by 230% (95% CI 189 to 272)%, p < 0.001 and 58% (190 to 271%), p < 0.001, respectively, and reduced PTH by 17% (- 23 to - 11%), p < 0.001. Compared with placebo, vitamin D3 significantly reduced maximal handgrip strength by 9% (- 15 to - 3%; p < 0.01) and knee flexion strength by 13% (- 24 to - 2%; p = 0.02) and increased the time spent on performing the Timed Up and Go test by 4.4%; (0.1-8.6%; p < 0.05). Levels of physical activity, total lean body mass, appendicular lean mass index, postural stability, well-being, and quality of life did not change in response to treatment. Compared with placebo, a daily supplement with a relatively high dose of vitamin D3 had no beneficial effects on any outcomes. In some measures of muscle strength and physical performance, we even saw a small unfavorable effect. Our data call for caution on use of relatively high daily doses of vitamin D3 in the treatment of vitamin D insufficiency.