What makes vitamin D unique is that it is a vitamin and also a hormone your body can make from the sun. Despite the ability to get vitamin D from food and the sun, an estimated 40%-75% of people are deficient.
Why? Vitamin D is not abundant in our food choices and the sun is not a reliable source for everyone.
Many factors affect the skin's ability to produce vitamin D, including season, time of day, latitude, air pollution, cloud cover, sunscreen, body parts exposed, color, and age. Dermatologists recommend using sunscreen and getting vitamin D from food and supplements rather than risk the harmful rays of the sun.
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Role of Vitamin D
Vitamin D is naturally present in few foods. Since 1930, virtually all cow's milk in the U.S. has been voluntarily fortified with 100 IU of vitamin D per cup. Food manufacturers are fortifying other foods, such as yogurt, cereal, and orange juice, to help consumers fill the nutrient gap in their diets.
Ideally, vitamin D is added to a food or beverage that contains calcium. Vitamin D is needed for maximum absorption of calcium from the intestine, helping to build strong bones and teeth.
Together with calcium, vitamin D can help prevent osteoporosis in older adults. Without enough vitamin D, bones can become brittle and prone to fracture. It is estimated that more than 40 million adults in the U.S. have or are at risk of developing osteoporosis.
"Vitamin D deficiency is associated with low bone mass and osteoporosis, which is estimated to affect 10 million adults over the age of 50 in the U.S.," says Atlanta rheumatologist Eduardo Baetti, MD. Even in Atlanta, where the sunshine is adequate all year long, Baetti says many of his patients -- especially elderly and dark-skinned people -- have low levels of vitamin D because the sun is not a reliable source.