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: I try to make things clear, but the value of vitamin D supplements is complicated. Here it is in a nutshell.

We get most of the vitamins we need in our diet. However, vitamin D is found naturally in only a few foods. Fatty fish is the main food source. Milk doesn't naturally contain vitamin D, but it's fortified with it. So are many juices and breakfast cereals. (I've put a table of food sources of vitamin D on my website, AskDoctorK.com.)

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. Most experts agree that if your blood levels are lower than 20 ng/ml, you tend to develop thinning of the bones, and that taking vitamin D supplements can help protect you. But the value of vitamin D supplements for protecting you against other diseases is uncertain.

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.

Some of my colleagues disagree with this advice. Nevertheless, this is what I do myself. Studies are under way that will tell me if I'm right or wrong. I'll keep you informed of new developments.

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