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

Summary Vitamin D may affect skeletal muscle function. In a double-blind, randomised, placebo-controlled trial, we found that vitamin D 3 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 D 3 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 D 3 (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 D 3 and placebo groups, respectively ( P = .10, ANOVA)) or falls ( P = .65, chi-squared test). Biochemical responses were similar across BMI categories (