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Vitamin level a source of controversy

Sat., March 16, 2013

DEAR DOCTOR K: I’ve read a lot about vitamin D deficiency in the news. How much do I need? Where can I get it?

DEAR READER: We get most of our vitamin D from sunlight: When sun strikes the skin, certain cells make vitamin D.

But people get a lot less sunlight than they used to. It’s not just the concern about skin cancer; it’s mainly the fact that most of us spend much less time outdoors than our ancestors did. Throughout most of human history, humans spent much of the daytime outdoors. At the turn of the 20th century, more than 90 percent of U.S. citizens lived and worked on farms.

We didn’t know about vitamin D at the turn of the 20th century and couldn’t measure blood levels. So we don’t know for sure, but most experts think that our blood levels of vitamin D today are likely much lower than our ancestors’.

Is that a problem? We know from epidemiologic studies that the risk of getting many important diseases – autoimmune diseases, heart disease, certain types of cancer – is greater among people whose blood levels of vitamin D are lower. Few people dispute that.

Here’s where the controversy begins. It is clear that taking vitamin D supplements can raise your blood levels. But it’s by no means clear that this is good for your health.

The current recommendation for vitamin D is 600 international units (IU) per day for people up to age 70, and 800 IU per day for those over age 70. Vitamin D comes in two forms: D3 and D2. If you take supplements, some experts recommend choosing one that contains D3.

Here’s the bottom line, at least for me: Get your vitamin D from foods. Avoid too much sun exposure, which can increase your risk of skin cancer. If your blood level of vitamin D is lower than 30 ng/ml, then I recommend you talk to your doctor about taking at least 1,000 IU a day.

Dr. Komaroff is a physician and professor at Harvard Medical School. To send questions, go to, or write: Ask Doctor K, 10 Shattuck St., Second Floor, Boston, MA 02115.


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