Emerging evidence suggests that vitamin D plays an important role in exercise, health and physical performance. Specifically, it is now recognized that vitamin D is imperative for bone health, immune function, and inflammatory modulation and it may be necessary for optimal muscle function.
Vitamin D is a unique fat-soluble secosteroid synthesized from cholesterol in the skin through exposure to solar ultraviolet-B radiation as previtamin D3 or absorbed from ingested food as vitamin D2 or D3. Once transported to the liver in a protein-bound state, vitamin D is metabolized as 25-hydroxyvitamin D [25-(OH)D1]. This active hormone circulates in the blood stream and acts upon target tissues (intestinal, kidney and bone) to regulate calcium and phosphate homeostasis, thereby ensuring healthy growth and remodeling of bone tissue. Vitamin D has a direct role in calcium homeostasis and has a direct effect on bone health, immune function and muscle function. Vitamin D deficiency may cause muscle weakness inhibition protein synthesis and is linked to several chronic diseases, including cardiovascular disease, cancer and upper respiratory tract infections. According to the U.S. Department of Health and Human Services, vitamin D status in the U.S. population at risk of deficiency by age ranged from 1% to 8% in males and 1% to 12% in females and therisk of inadequacy by age ranged from 9% to 28% in males and 11% to 28% in females.
Seasonal variation: Vitamin D is produced by the biosynthesis of serum 25-(OH) D1 in the skin via sunlight exposure. As the sun exposure varies by season, the role of vitamin D in supporting muscle function is likely to be compromised during the winter months when the exposure to ultraviolet-B radiation may be reduced.
How Much Sun is Needed?
The sun - Anywhere from 5–30 min per day of sun exposure is suggested. The lighter the skin color one has, the less time is needed in the sun to absorb vitamin D; the darker the skin color one required a longer exposure time for optimal conversion.
From food - There are very few dietary sources of vitamin D: These include oily fish, such as salmon, mackerel, tuna and sardines. Some foods, including milk, orange juice, bread, cereals, and yogurts, are fortified with vitamin D in the United States.
Individuals with limited sun exposure may require vitamin D supplementation to maintain sufficient levels. According to the US Institute of Medicine, the overall recommendation for dietary allowance of vitamin D is 600IU per day for children at least 1 year of age and adults up to 70 years, and 800IU per day for older adults.