Vitamin D is produced naturally in the skin as part of a reaction involving sunlight, but is also consumed in the diet. Many people are vitamin D deficient because they shun vitamin D-fortified dairy products and don’t get much outdoor activity. Vitamin D is critical for normal calcium metabolism and plays a vital role in maintaining bone and muscle health. A review of literature by Bruce Hamilton, and a study led by Vicente Gilsanz from the University of Southern California (USC) concluded that adequate vitamin D levels were important for promoting muscle mass and strength. The USC study showed that women with low intakes of vitamin D had increased levels of muscle fat. Dr. Hamilton concluded that vitamin D plays a critical role in muscle protein synthesis and suggested that vitamin D-deficient athletes might benefit from ultraviolet B radiation (i.e., tanning beds). Dermatologists would undoubtedly disagree, because of the link between tanning beds and skin cancer.
Source: Scandinavian Journal Medicine Science in Sports, 20: 182-190, 2010