Sometimes called the "sunshine vitamin," many people think of vitamin D as the nutrient they absorb from the sun. The sun rays themselves don’t actually contain any vitamin D; instead, skin that is penetrated by specific ultraviolet rays works to synthesize this energy into vitamin D, starting a process involving the liver and kidneys to create the essential hormone calcitriol.
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Vitamin D functions as a precursor to the hormone calcitriol, which current research finds plays a major role in how cells are developed and maintained. This makes vitamin D different from other vitamins that fuel bodily processes rather than direct them. As a fat-soluble vitamin, excess vitamin D is stored in the body’s fatty tissues rather than being excreted through the kidneys like water-soluble vitamins, such as vitamin C and the various B vitamins.
Vitamin D plays many roles in the body, primarily promoting the uptake of calcium in the small intestine during the digestive process. Without vitamin D, proper mineralization of the bones cannot occur, including the building and remodeling of bones. Deficiencies can lead to rickets in children and osteomalacia and osteoporosis in adults. Vitamin D helps regulate the amount of calcium and phosphorus in the blood, reduce inflammation, support immune function and modulate the development of proteins involved with gene encoding.
Vitamin D Sources
Specific sunlight kicks off the vitamin D synthesis process. According to the National Institutes of Health’s Office of Dietary Supplements, "Ultraviolet (UV) B radiation with a wavelength of 290-315 nanometers penetrates uncovered skin and converts cutaneous 7-dehydrocholesterol to previtamin D3, which in turn becomes vitamin D3." Other non-sun sources include sockeye salmon, cod liver oil, fortified dairy and mushrooms, as well as supplements, since vitamin D doesn’t naturally occur in significant amounts in other forms.
How to Get It
The best way to receive sufficient UVB rays to facilitate vitamin D synthesis requires a balance of enough sun, but not so much as to increase the risk of skin cancer. Just a few minutes of exposure a day provide enough UVB to kick off the vitamin D process. People with darker skin need more time in the sun than those with fair skin, due to their skin’s melanin, which impedes absorption. Researchers suggest spending five to 30 minutes in the sun between the hours of 10 a.m. and 3 p.m. twice a week, to replenish your vitamin D stores. For people at higher risk of skin cancer, or for people with sedentary, indoor lives, supplements work as well.
What Prevents Absorption
Location, season and physical blocks can prevent UVB absorption. Regions north of latitude 40, running horizontally from northern California to New York City do not receive sufficient sun rays from September through May. In addition, anything physically blocking the sun’s rays impedes vitamin D synthesis, such as clothing, sunscreen with a sun protection factor higher than 15, coatings on windows and clouds in the sky. People with darker skin also require more sunlight, as their higher amount of melanin reduces their UVB exposure.