Effect of vitamin D3 vs. calcifediol on VDR concentration and fiber size in skeletal muscle

Introduction This study sought to examine the effect of vitamin D-3 (VD3) 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. Materials and methods It was conducted on a subset of the VD3 (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. Results 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 (VD3), 206.8 +/- 68.8 nmol/L (HyD), and 82.7 +/- 36.1 nmol/L (placebo), ANOVA P < 0.001. There were no significant group differences in 6-month change in VDR concentration (ANOVA P = 0.227). Mean +/- SD percent 6-month changes in type I FCSA were 20.5 +/- 32.7% (VD3), - 6.6 +/- 20.4% (HyD), and - 0.3 +/- 14.0% (placebo, ANOVA P = 0.022). Type II FCSA or PAX7 concentration did not change significantly by group (all P > 0.358). Conclusion 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 VD3, 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.

Automated summary

This study compared the effect of different forms of vitamin D on intramyonuclear vitamin D receptor concentration, muscle fiber cross-sectional area, and muscle satellite cell activation. The results showed that although calcifediol resulted in the highest serum 25-hydroxyvitamin D levels, no skeletal muscle benefits were observed at these high levels.