Time for more vitamin D

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Missing out on the "sunshine vitamin" has consequences for more than just bone health.

September brings the end of summer in the northern hemisphere and, for many of us, that means less time in the sun. The sun's rays provide ultraviolet B (UVB) energy, and the skin uses it to start making vitamin D. (The skin actually produces a precursor that is converted into the active form of the vitamin by the liver and kidneys.) Vitamin D is best known for its vital role in bone health. Without this "sunshine vitamin," the body can't absorb the calcium it ingests, so it steals calcium from bones, increasing the risk of osteoporosis and fractures. Vitamin D also helps maintain normal blood levels of phosphorus, another bone-building mineral.

Vitamin D would be essential if it did nothing else. But researchers have discovered that it's active in many tissues and cells besides bone and controls an enormous number of genes, including some associated with cancers, autoimmune disease, and infection. Hardly a month goes by without news about the risks of vitamin D deficiency or about a potential role for the vitamin in warding off diseases, including breast cancer, multiple sclerosis, and even schizophrenia. In June 2008, a study published in the Archives of Internal Medicine found that low blood levels of vitamin D were associated with a doubled risk of death overall and from cardiovascular causes in women and men (average age 62) referred to a cardiac center for coronary angiography. At a scientific meeting in May 2008, Canadian researchers reported that vitamin D deficiency was linked to poorer outcomes in women with breast cancer. And a large study of aging in the Netherlands published in the May 2008 issue of Archives of General Psychiatry found a relationship between vitamin D deficiency and depression in women and men ages 65 to 95.

The picture of vitamin D's health benefits beyond bones has been drawn mainly from epidemiologic and observational investigations. The findings of such studies can suggest correlations between disease risk and certain factors — sun exposure or blood levels of vitamin D, for example — but they don't prove cause and effect. Promising findings from observational studies don't always pan out when put to the test in clinical trials. However, in one of the few randomized trials testing the effect of vitamin D supplements on cancer outcomes, postmenopausal women who took 1,100 international units (IU) of vitamin D plus 1,400 to 1,500 milligrams of calcium per day reduced their risk of developing non-skin cancers by 77% after four years, compared with a placebo and the same dose of calcium. The 1,100 IU dose — nearly three times the 400 IU per day recommended in federal and other expert guidelines — was correlated with a 35% higher blood level of vitamin D, on average. In the only other randomized trial of vitamin D and cancer — part of the Women's Health Initiative — researchers found no effect on colorectal cancer. Critics say that the dose, 400 IU per day, was too small to make a difference.

More trials are needed to elucidate vitamin D's benefits and risks at different doses and in different populations. In fact, a large-scale randomized trial that would include 20,000 U.S. women and men has been proposed by Harvard researchers and will be considered for funding by the National Institutes of Health. In the meantime, the evidence is so compelling that some experts already recommend at least 800 to 1,000 IU of vitamin D per day for adults.

In search of vitamin D

Under the right circumstances, 10 to 15 minutes of sun on the arms and legs a few times a week can generate nearly all the vitamin D we need. Unfortunately, the "right circumstances" are elusive: the season, the time of day, where you live, cloud cover, and even pollution affect the amount of UVB that reaches your skin. What's more, your skin's production of vitamin D is influenced by age (people ages 65 and over generate only one-fourth as much as people in their 20s do), skin color (African Americans have, on average, about half as much vitamin D in their blood as white Americans), and sunscreen use (though experts don't all agree on the extent to which sunscreen interferes with sun-related vitamin D production).

Lack of sun exposure would be less of a problem if diet provided adequate vitamin D. But there aren't many vitamin D–rich foods (see chart, below), and you need to eat a lot of them to get 800 to 1,000 IU per day. People who have trouble absorbing dietary fat — such as those with Crohn's disease or celiac disease — can't get enough vitamin D from diet no matter how much they eat (vitamin D requires some dietary fat in the gut for absorption). And people with liver and kidney disease are often deficient in vitamin D, because these organs are required to make the active form of the vitamin, whether it comes from the sun or from food.

For these and other reasons, a surprising number of Americans — more than 50% of women and men ages 65 and older in North America — are vitamin D–deficient, according to a consensus workshop held in 2006. Growing awareness of vitamin D's benefits coupled with the risk of vitamin D deficiency has led some experts to recommend a blood test that assesses the amount of vitamin D in the body. The test measures the concentration of 25-hydroxyvitamin D₃, or 25(OH)D, the precursor produced by the skin and converted in the body to vitamin D. If you're over age 70, have darker skin, or live at a northern latitude, you might want to ask your clinician about the test. People who have malabsorption problems or take medications that interfere with vitamin D activity (for example, glucocorticoids) should consider it as well. However, some experts think testing is unnecessary as long as you get 800 to 1,000 IU of vitamin D a day.

Although there's no agreement on an optimal level of 25(OH)D, deficiency is generally defined as a blood level less than 20 nanograms per milliliter, or 20 ng/mL (see chart). Levels that low have been linked to poor bone density, falls, fractures, cancer, immune dysfunction, cardiovascular disease, and hypertension. Many experts recommend a level of at least 32 and suggest that 800 to 1,000 IU of vitamin D per day is required to maintain that level.

How to reach 1,000 IU

Unless you live in the South and spend a fair amount of time outdoors, or you like eating lots of fatty fish and vitamin D–fortified foods, supplements are the best way to make sure you're getting 800 to 1,000 IU per day. (Higher doses may be prescribed if you've been diagnosed with vitamin D deficiency.) Most multivitamins contain only 400 IU. But don't just take two, because getting double doses of other vitamins and minerals can be unsafe (for example, too much vitamin A as retinol can cause hair loss and diarrhea and is associated with hip and other bone fractures, possibly due to an adverse interaction with vitamin D). Many calcium pills contain about 200 IU of vitamin D, so one multivitamin and two or three calcium pills should suffice. Or you can take a vitamin D pill to round out your daily needs. Until we know more, make sure your intake from supplemental sources doesn't exceed 2,000 IU per day, the current upper limit set by the National Academy of Sciences.