A recent randomized controlled trial examining the effects of vitamin D on muscle strength in young women showed no effect. The researchers, based out of the India Institute of Medical Sciences, used 60,000 IU/week for 8 weeks followed by 60,000 every two weeks for four more months. Mean 25(OH)D levels in the treatment group went from 9 to 30 ng/ml over the six months of the trial.
Goswami R, Vatsa M, Sreenivas V, Singh U, Gupta N, Lakshmy R, Aggarwal S, Ganapathy A, Joshi P, Bhatia H. Skeletal Muscle Strength in Young Asian Indian Females after Vitamin D and Calcium Supplementation: A Double-Blind Randomized Controlled Clinical Trial. J Clin Endocrinol Metab. 2012 Aug 17. [Epub ahead of print]
These results stand in contrast to the group’s first trial, where improvements in handgrip strength were noted in young men and women given 60,000 IU/week for 8 weeks followed by 60,000 IU/month for 4 months.
Gupta R, Sharma U, Gupta N, Kalaivani M, Singh U, Guleria R, Jagannathan NR, Goswami R. Effect of cholecalciferol and calcium supplementation on muscle strength and energy metabolism in vitamin D-deficient Asian Indians: a randomized, controlled trial. Clin Endocrinol (Oxf). 2010 Oct;73(4):445-51. doi: 10.1111/j.1365-2265.2010.03816.x.
The authors note that numerous studies show vitamin D helps muscle strength in the elderly with baseline levels less than 10 ng/ml and wondered why their two trials in young people had such different results. They noted that the first trial included men and women, while their second trial was all women. When they went back and reanalyzed their first trial by sex, they noted all the improvement in handgrip strength in their first trial occurred in the men, not in the women.
Dr. Ravinder Goswami, the senior author on the 2010 study and the lead author on the recent study, reasoned that vitamin D might improve muscle strength in males via raising testosterone levels. Remember that 3,300 IU/day of vitamin D for a year raised testosterone levels in healthy men by about 30% in a randomized controlled trial.
Pilz S, Frisch S, Koertke H, Kuhn J, Dreier J, Obermayer-Pietsch B, Wehr E, Zittermann A. Effect of vitamin D supplementation on testosterone levels in men. Horm Metab Res. 2011 Mar;43(3):223-5. Epub 2010 Dec 10.
I have two comments. The first is that all the athletic performance enhancement studies done by the Germans and Russians in the 1950s, which I wrote about in my book, used sunbeds, not vitamin D supplements. If sunlight or sunbeds induce this effect in athletes, but not supplements, this would explain why such strong associations were seen in muscle strength and 25(OH)D levels in a previous English trial, where 25(OH)D levels were ostensibly achieved by sunlight, not supplements.
Ward KA, Das G, Berry JL, Roberts SA, Rawer R, Adams JE, Mughal Z. Vitamin D status and muscle function in post-menarchal adolescent girls. J Clin Endocrinol Metab. 2009 Feb;94(2):559-63. Epub 2008 Nov 25.
Second, natural 25(OH)D levels were not achieved in either of Dr. Goswami’s trials, so we still don’t know if muscle strength in women would improve if levels of 50 ng/ml were achieved. However, cross-sectional studies of the elderly indicate most of the improvement in muscle strength occurs in raising levels from below 10 ng/ml to above 20 ng/ml, so if vitamin D supplements improve muscle strength in young women, I would have expected to see an effect.
We need a trial of sunshine or sunbeds to see if that improves muscle strength in young women.