Michael F. Holick, Ph.D., M.D., the author of “The Vitamin D Solution” and the director of the Vitamin D, Skin, and Bone Research Laboratory at Boston University School of Medicine, D-codes the vitamin.

By Julia Edelstein
Updated March 27, 2013
Yasu + Junko

Why Does Vitamin D Matter?

Along with calcium, it helps children develop healthy bones and adults maintain them. But the reason vitamin D has become such a hot topic in the last few years is that we recently discovered that many cells in the body have a vitamin D receptor. We believe they’re there because vitamin D also plays an important role in modulating the immune system. If you have sufficient levels, you are better able to fight infection and are less likely to acquire autoimmune diseases, heart disease, and some common cancers.

Why Are So Many of Us Lacking Vitamin D?

For thousands of years, human beings depended on the sun for their required vitamin D. But for the past three decades dermatologists have been telling people to avoid direct sun exposure because of skin cancer. Add an increase in children playing indoors and you get a worldwide deficiency pandemic.

How Much Vitamin Do I Need?

The minimum is 600 IU [international units] a day for the average healthy adult. But I prefer that my patients get 1,500 to 2,000. You can buy supplements (D2 and D3 are both effective) made by any national brand in any form, from gummies to pills.

Do I Need to Get a Blood Test First?

No. Assume that you are deficient. Close to a billion of our health-care dollars are spent on that test every year, and few of us need it. But if you’re obese, if you take antiseizure medicines, if you have a gastrointestinal or intestinal-malabsorption problem or a granuloma disorder, like sarcoidosis, consult your doctor first.

Should I Get Vitamin D From the Sun?

The vitamin D our body makes when it absorbs sunlight is a good source, so I suggest sensible sun exposure in addition to taking supplements and eating foods with vitamin D (see a shopping list of foods rich in vitamin D). Ask yourself how long it takes you to develop a mild sunburn, then go outside for half that time two to three times a week, between 10 a.m. and 3 p.m., during warm months. Expose your arms and legs, but always protect your face. Your skin pigment is a natural sunscreen, so those who are dark-skinned should stay out longer than fair-skinned people to absorb sunshine. For instance, Indian people tend to need two to four times more exposure; African Americans may need 5 to 10 times more.