Sun and muscle are often seen together, compare the sunshine on Muscle Beach to the fog of Ocean Beach or the before and after snapshots found in supplement testimonials, you will find a nearly ubiquitous pallor in the former and the bronze kiss of the sun in the latter. One explanation for these phenomenon is that warmth encourages disrobing which encourage vanity and steroids and the unscrupulous purveyor of MuscleGro 5000 employed the tanning to make the after shots look more muscular than they really were. Many would accept the above as a sufficient explanation, however a second possibility lurks in the shadows, could it be that sunshine actually helps muscles grow? Could vitamin D supplementation improve muscle strength?
To answer this question I found a systematic review and meta-analysis published this month in Osteoporosis International titled “The Effect of Vitamin D Supplementation on Muscle Strength .” The studies included in this review used various forms of vitamin D supplementation, some of which combined it with calcium and only one study using timed sunlight exposure as an intervention for institutionalized patients with Alzheimer’s. Dosing varied from 400 IU of D3 per day to a one time dose of 600,000 IU of D2 by injection. The outcome measures were grip strength and leg strength and not muscle size and appearance such as was assessed in our MuscleGro 5000 testimonial. In total 17 randomized controlled trials involving 5,072 human subjects mostly over the age of 60 that investigated the topic of vitamin D supplementation on muscle strength were included in the review. Blood tests measuring the inactive form of vitamin D, 25-hydroxy vitamin D (25(OH)D), were taken at baseline to determine the nutrient status of the participants.
Meta-analysis showed no significant effect of vitamin D supplementation on grip strength or proximal lower extremity strength for individuals whose pre-intervention levels of 25(OH)D were greater than 25 nmol/L of blood. So if supplementation won’t improve the muscle strength in vitamin D replete individuals then getting a tan certainly won’t help these folks lift the end of their car to retrieve a tub of ice cream (for calcium, ahem) that rolled under it.
That’s all well and good for those swarthy vitamin D replete individuals but what about the estimated one billion people worldwide who are deficient in vitamin D? This includes darker skinned women, older people in residential care and quite possibly a few cave dwelling UCSF students in fog land. In the four studies whose 465 participants began the trial with <25 nmol/L 25(OH)D significant gains in hip muscle strength were found.
These results are consistent with a general rule of supplements that they only work if you are deficient in that nutrient. The art of the salesperson is then to convince everyone that they are indeed deficient. Since very little vitamin D is obtained from diet, the conversion of 7-dehydrocholseterol to pre-vitamin D3 in the skin by solar ultraviolet B radiation is the primary source of vitamin D. An Australian study calculated that when the ultra-violet index (UVI) is above 3 sufficient production can be accomplished in <30 minutes on the arms and face in the fair skinned among us . The UVI can be checked on weather sites and peaks at 7 on a clear day in San Francisco.
While vitamin D receptors have been found in muscle tissue, the exact mechanism of interaction is not yet known. Some interesting but inconclusive tidbits about the topic include a positive correlation between vertical jump velocity in teenage girls and reports of type II muscle fiber atrophy in the vitamin D deficient elderly.
Given that the 16 of the 17 studies included in the review had a population of adults aged 60 and older, the generalizability of the results to the aforementioned cave-dwelling UCSF student is poor. However if you are someone who doesn’t get much sun, particularly if you are a dark-skinned individual or someone who has been around for awhile, it would be prudent to talk to your health care provider about testing your 25(OH)D. For those of who have patients that meet the criteria above, go forth confident that your decision regarding vitamin D supplementation is based on the best current evidence.
1. Stockton K, Mengersen K, Paratz J, Kandiah D, Bennel K. (2011) Effect of vitamin D supplementation on muscle strength: a systematic review and meta-analysis. Osteoporos Int 22:859-871
2. Bilinski S, Kellie K. (2011) Burning daylight: balancing vitamin D requirements with sensible sun exposure. Medical journal of Australia. 194:345-348