Vitamin D is sometimes called the "sunshine vitamin," since this essential nutrient comes primarily from the skin's reaction to sun exposure. People who liberally use sunscreen, which is helpful in preventing sunburns, skin cancer and sun-related signs of aging, may be blocking the specific ultraviolet B-type (UVB) rays needed by the body to create vitamin D.
Vitamin D Synthesis
UVB rays from the sun enter the skin and react with a precursor of cholesterol called 7-dehydrocholesterol. Together, they synthesize a substance called previtamin D3, which exists in a form that cannot be used by the body until further conversions occur. The liver converts previtamin D3 to calcidiol, which goes to the kidneys, where it becomes calcitriol. Now in a physiologically active form, this steroid hormone plays many roles, maintaining mineralization of bones, keeping the immune system working and reducing inflammation.
Food Sources
Vitamin D doesn't exist in great quantities in food. Sockeye salmon and cod liver oil contain the most as a natural component, while dairy products such as milk, yogurt and cheese usually are fortified with amounts meant to satisfy a portion of a body's daily need. Some types of mushrooms also contain fortified vitamin D, providing vegans a dietary option.
Supplement Forms
Supplementation offers vitamin D in two forms, D2 (ergocalciferol) and D3 (cholecalciferol). Ergocalciferol usually comes from vegetarian sources such as irradiated yeast. According to the National Institutes of Health's (NIH) Office of Dietary Supplements, cholecalciferol absorbs about three times as well and has been associated with a lower risk of vitamin D overdose (hypercalcemia) than ergocalciferol.
Sun Exposure
Considering the expense of supplements and the amount of fortified food required to meet daily adequate intakes, obtaining UVB rays from the sun seems the best route to ensuring sufficient vitamin D. The NIH states, "It has been suggested by some vitamin D researchers, for example, that approximately 5-30 minutes of sun exposure between 10 a.m. and 3 p.m. at least twice a week to the face, arms, legs, or back without sunscreen usually lead to sufficient vitamin D synthesis and that the moderate use of commercial tanning beds that emit 2%-6% UVB radiation is also effective."
Sunscreen Issues
Sunscreen of less than 15 SPF may still allow some of the UVB rays through for vitamin D synthesis. However, use of most sunblocks can result in the same vitamin D deficiency experienced by people who avoid the sun and don't take supplements. Some skincare companies now market sunscreen products including vitamin D that sound promising; however, scientific studies have not yet been completed indicating the body can absorb vitamin D from lotion.
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