What vitamin D is: Vitamin D is a hormone manufactured in the body when skin is exposed to sunlight. Because vitamin D is fat-soluble, the body can store it for the days or even months when you don’t get any sun or consume any vitamin D. Besides fortified milk and some fish, few foods supply the vitamin.
Many people are deficient in vitamin D, especially those who are over 60, live at northern latitudes, have darker skin or are rarely outdoors. In the northern United States and in Canada, blood levels drop markedly in the winter, when days are shorter, the sun is weaker, and we wear more clothes. Obesity is associated with reduced blood levels of vitamin D.
Claims, purported benefits: Works with calcium to keep bones strong. May reduce the risk of everything from heart disease, multiple sclerosis and some cancers to diabetes, infections, hypertension and falls caused by age-related muscle weakness.
What the studies show: Studies on vitamin D (mostly observational) are published nearly every month, most suggesting potential benefits. In 2010, the Institute of Medicine (IOM) came out with guidelines for recommended intakes of vitamin D. These pertain only to bone health and fracture reduction, since the IOM panel concluded that the evidence for all other benefits was “inconsistent and/or inconclusive or did not demonstrate causality.” The recommended intake is 600 International Units (IU) a day through age 70. For those over 70, the goal is 800 IU. The panel also doubled the safe “upper level intake” to 4,000 IU a day. It said that the great majority of people do not need supplements.
Many prominent vitamin D experts were disappointed by the guidelines, which they consider too low, and typically recommend supplements of at least 1,000 to 2,000 IU a day. And they point to evidence that it’s safe to take as much as 10,000 IU a day.
In 2012, the U.S. Preventive Services Task Force (USPSTF) concluded with “moderate certainty” that standard doses of calcium (1,000 milligrams) and low doses of vitamin D (400 IU or less) do not prevent fractures in postmenopausal women without osteoporosis. Thus it advised such women not to take the supplements. Because the evidence is inadequate, the panel did not pass judgment on calcium and higher doses of vitamin D for fracture risk, nor did it discuss younger women, men, people with osteoporosis or those living in nursing homes.
Many researchers raised objections to the draft guidelines, saying the Task Force misinterpreted the research or at least put too negative a spin on it. In particular, critics questioned the focus on low-dose vitamin D, since there’s a growing consensus that higher doses are needed for bone health. The Task Force itself stated that 400 IU “would not be considered sufficient today.”
Also in 2012, another USPSTF panel reviewed research on fall prevention and recommended that older people take vitamin D supplements to prevent falls, which are the leading cause of fractures.