Effect of vitamin D 3 vs. calcifediol on VDR concentration and fiber size in skeletal muscle

This study sought to examine the effect of vitamin D (VD ) 3200 IU/d, calcifediol (HyD) 20mcg/d, or placebo on intramyonuclear vitamin D receptor (VDR) concentration, muscle fiber cross-sectional area (FCSA), and muscle satellite cell activation. It was conducted on a subset of the VD (n = 12), HyD (n = 11), and placebo (n = 13) groups who participated in the 6-month randomized controlled HyD Osteopenia Study in postmenopausal women. Baseline and 6-month vastus lateralis muscle cross sections were probed for VDR, fiber type I and II, and PAX7 (satellite cell marker) using immunofluorescence. Baseline mean ± SD age was 61 ± 4 years and serum 25-hydroxyvitamin D (25OHD) level was 55.1 ± 22.8 nmol/L. Baseline characteristics did not differ significantly by group. Six-month mean ± SD 25OHD levels were 138.7 ± 22.2 nmol/L (VD ), 206.8 ± 68.8 nmol/L (HyD), and 82.7 ± 36.1 nmol/L (placebo), ANOVA P 0.358). This study demonstrated no significant change in intramyonuclear VDR in response to either form of vitamin D vs. placebo. Type I FCSA significantly increased with VD , but not with HyD at 6 months. As type I fibers are more fatigue resistant than type II, enlargement in type I suggests potential for improved muscle endurance. Although HyD resulted in the highest 25OHD levels, no skeletal muscle benefits were noted at these high levels. NCT02527668.