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Q: I had some blood work done recently and was surprised to learn that I am deficient in vitamin D. How is this possible? What can I do about it?

A: Vitamin D is an unusual vitamin. We get most vitamins from the foods we eat; our body can't make them. Unfortunately, vitamin D isn't found naturally in many foods. Fatty fish and milk (which is fortified with vitamin D) are the main food sources.

Fortunately, your body can make its own vitamin D. In order to do so, your body needs sunshine - more specifically, ultraviolet B (UVB) rays. But most of us spend the vast majority of our time indoors. Even when we're outside, we often use sunscreen because it protects us against skin cancer. But less exposure to the sun means that our bodies may be making less vitamin D than we need.

What are the health benefits of vitamin D, and the health problems caused by low blood levels of vitamin D? I wish I could give you just a simple answer. Here's the simple part: Adequate blood levels of vitamin D protect us against osteoporosis, a disease that increases the risk of bone fractures. Taking vitamin D supplements if your blood level of vitamin D is low can help prevent osteoporosis. Most doctors agree on that.

Here's where it gets complicated: Many studies have found that people with low blood levels of vitamin D also are at greater risk for high blood pressure, heart disease, various autoimmune diseases (including multiple sclerosis and Type 1 diabetes), depression and colon cancer. So you might think taking vitamin D supplements would protect you against these diseases, like it protects against osteoporosis. But it isn't that simple.

To prove that taking vitamin D supplements protects against these diseases requires a very large study called a randomized clinical trial. Fortunately, such a study is underway (with leadership here at Harvard Medical School). But it will be years before the results are known.

For now, here's what most experts agree on. The recommended dietary allowance of vitamin D is 600 international units for adults up to age 70, and 800 IU from age 71 on. If you're not getting enough vitamin D from foods and modest sun exposure, consider a daily supplement to get 600 to 800 IU per day. Choose a supplement that contains the form of vitamin D called D3. Don't take more than 4,000 IU per day.

I think this is reasonable advice. However, when the results of further studies are analyzed, we may come to a different conclusion. We may discover that we need more vitamin D than these recommendations. Or it could turn out that we need to take vitamin D supplements only when our blood levels of vitamin D are low, and with the only expected health benefit being healthy bones. That's why we do research.
