Sun, supplement, or food — what's the best source? Neal Barnard, MD, offers his insight.
Q: I hear people say getting enough vitamin D is important, but what does it actually do?
A: For starters, vitamin D maintains strong bones. That is, it helps your body absorb calcium from the foods you eat. It also keeps your kidneys from losing too much calcium in the urine. From moment to moment, your body monitors how much calcium is in your blood, and if the level gets too high or too low, your body adjusts it using vitamin D. Having plenty of vitamin D in your bloodstream today can mean fewer bone breaks tomorrow. As part of Harvard's Nurses' Health Study that followed 72,337 postmenopausal women over 18 years, those who had the highest vitamin D intake had 37 percent fewer hip fractures, compared with those getting less vitamin D.
Vitamin D may also help prevent cancer. A few years ago, researchers from the University of California, San Diego examined lung cancer incidence rates from 111 countries and found an astounding pattern. Although lung cancer is mainly caused by smoking, it turns out that the closer you are to the equator, the less likely you are to develop it. So if you live in equatorial East Africa, your risk is just a fraction of that of a person living in Canada, all other things being equal. Scientists believe that vitamin D from the sun helps the cells of the body maintain their "maturity." That is, it keeps lung cells functioning like normal cells, rather than turning into the oddly shaped, proliferating cells that are the hallmark of cancer. And it's not just lung cancer. In 2007, the American Institute for Cancer Research reported that people with more vitamin D in their blood have less risk for colorectal cancer. The same may be true for prostate cancer, breast cancer, and other forms.
Q: How should I be getting my vitamin D?
A: The most natural source is the sun. Getting about 15 minutes of direct sun on your face and arms each day produces a healthful dose of vitamin D. Your body can store the vitamin too. So if you can't get sunlight every day, you can draw on your reserves. Needless to say, many people are indoors much of the day or live at latitudes where sunlight is limited. And cloudy skies, sunscreens, darker skin, and window glass filter out the ultraviolet (UVB) rays that produce vitamin D. So, in the absence of regular sun exposure, the answer is a supplement. Any common multivitamin will do. The recommended dietary allowance (RDA) for adults up to age 50 is 200 international units (IU). If you're between 51 and 70, the RDA is 400 IU per day. If you're over 70, it's 600 IU per day. Here is how supplements compare to sunlight: taking 200 IU of vitamin D daily produces about the same amount of vitamin D as modest daily sun exposure, and 10,000 IU daily equals the amount produced by a sunburn.
Most foods have little or no vitamin D. Among the exceptions are certain fish species (salmon, sardines, herring) and mushrooms (shiitakes and chanterelles). Four dried shiitakes provide about 250 IU of vitamin D. Fortified milks (including fortified soymilk) usually contain about 100 IU per cup. That said, many health authorities—me included—now recommend very high vitamin D intakes, up to 2,000 IU per day—in order to take advantage of the vitamin's cancer-fighting properties. No food gets you anywhere near that level. To get there, you'll need to takea vitamin D supplement.
Q: Are vitamin D supplements vegetarian?
A: Many of them are. Check the label on the bottle to be sure. Vitamin D2 (ergocalciferol) is plant-derived, while vitamin D3, or cholecalciferol, typically comes from lanolin in sheep's wool. Although older studies suggested the D3 form was more potent than D2, newer studies show the two to be equally effective. One common supplement brand, VegLife, provides plant-derived vitamin D at doses up to 2,000 IU, the maximal safe level per day (set by the U.S. government) for adults. Bottom line: the human body is designed for a bit of sunlight every day; that's the natural vitamin D source. If that's not possible, supplements are a perfectly adequate substitute.