A parallel group double-blind RCT of vitamin D3 assessing physical function: is the biochemical response to treatment affected by overweight and obesity?

UNLABELLED: Vitamin D may affect skeletal muscle function. In a double-blind, randomised, placebo-controlled trial, we found that vitamin D3 supplementation (400 or 1,000 I.U. vs. placebo daily for 1 year with bimonthly study visits) does not improve grip strength or reduce falls.

INTRODUCTION: This study aimed to test the supplementation effects of vitamin D3 on physical function and examine associations between overweight/obesity and the biochemical response to treatment.

METHODS: In a parallel group double-blind RCT, healthy postmenopausal women from North East Scotland (latitude-57° N) aged 60-70 years (body mass index (BMI), 18-45 kg/m(2)) were assigned (computer randomisation) to daily vitamin D3 (400 I.U. (n = 102)/1,000 I.U. (n = 101)) or matching placebo (n = 102) (97, 96 and 100 participants analysed for outcomes, respectively) from identical coded containers for 1 year. Grip strength (primary outcome), falls, diet, physical activity and ultraviolet B radiation exposure were measured bimonthly, as were serum 25(OH)D, adjusted calcium (ACa) and phosphate. Fat/lean mass (dual energy X-ray absorptiometry), anthropometry, 1,25-dihydroxyvitamin D and parathyroid hormone were measured at baseline and 12 months. Participants and researchers were blinded throughout intervention and analysis.

RESULTS: Treatment had no effect on grip strength (mean change (SD)/year = -0.5 (2.5), -0.9 (2.7) and -0.4 (3.3) kg force for 400/1,000 I.U. vitamin D3 and placebo groups, respectively (P = .10, ANOVA)) or falls (P = .65, chi-squared test). Biochemical responses were similar across BMI categories (<25.25-29.99, ≥30 kg/m(2)) with the exception of a small change at 12-months in serum ACa in overweight compared to non-overweight participants (P = .01, ANOVA; 1,000 I.U. group). In the placebo group, 25(OH)D peak concentration change (winter to summer) was negatively associated with weight (r = -.268), BMI (r = -.198), total (r = -.278) and trunk fat mass (r = -.251), with total and trunk fat mass predictive of winter to summer 25(OH)D change (P = .01/.004 respectively, linear regression).

CONCLUSION: We found no evidence of an improvement in physical function following vitamin D3 supplementation for 1 year.