In my 2008 book, Athletes Edge; Faster, Quicker, Stronger with vitamin D, I reviewed studies showing vitamin D affects muscle tissue. Several earlier studies suggest vitamin D increases the size of muscle fibers, especially the type II, fast twitch, type of fiber. Type II fibers are thought to contribute most to muscle strength.
Recently, researchers at Tufts University, led by Doctor Lisa Ceglia, under the supervision of one of the most respected vitamin D researchers, Professor Bess Dawson-Hughes, completed a randomized controlled trial to see if vitamin D affects muscle.
Ceglia L, Niramitmahapanya S, da Silva Morais M, Rivas DA, Harris SS, Bischoff-Ferrari H, Fielding RA, Dawson-Hughes B. A randomized study on the effect of vitamin d3 supplementation on skeletal muscle morphology and vitamin d receptor concentration in older women. J Clin Endocrinol Metab. 2013 Dec;98(12):E1927-35.
They studied 21 elderly women, giving 4,000 IU/day of vitamin D3 to 9 patients and placebo to 12 patients, getting muscle biopsies at baseline and again in 4 months (the length of the study). Mean baseline vitamin D levels were about 18 ng/ml. After 4 months of intervention, levels stayed about the same in the placebo group but only increased to 32 ng/ml in the treated women.
They found a 10% increase in total (type I and II) muscle fiber size in the treated group (p=.012), but most of the increase was in type II fibers. They also found a very interesting 30% increase in vitamin D receptors (VDR) in the treated group (p=.025). This increase was much more robust in type II fibers (p=.002). There was also a very strong correlation coefficient (R=.87) between 25(OH)D levels and expression of VDRs.
It has been known for a while that vitamin D induces expression of the VDR but this was nice confirmatory proof in a RCT of humans. This means when you take vitamin D, your muscle tissue makes more receptors to bind to and signal vitamin D’s hormonal effects.
The authors concluded:
“In conclusion, supplementation with vitamin D3 for 4 months in older mobility-limited women with moderately low vitamin D status increased intramyonuclear VDR concentration and muscle fiber size. Further work is needed to confirm these findings in a larger sample and to determine whether our two main findings are related, that is, whether vitamin D increases muscle fiber size by activating the VDR. Additionally, it will be important to identify the signaling pathways involved.”