Vitamin D in the Spotlight

By Meir J. Stampfer, M.D., DR. P.H.
Stampfer is professor of medicine at Harvard Medical School and professor and chair of epidemiology at the Harvard School of Public Health. For more on vitamin D from Harvard, go to health.harvard.edu/NEWSWEEK.

For many years, vitamin D was boring--even to doctors. Because it was considered good for bones and not much else, multitaskers like vitamin A, B vitamins and vitamin E hogged all the press. But recent studies have thrust this long-neglected nutrient into the spotlight. Scientists now think vitamin D may affect everything from diabetes to cancer. They're also finding that many people don't have enough of it.

When vitamin D was discovered a century ago, it solved a major public-health problem: rickets, a disease caused by vitamin D deficiency in which bone development is delayed and deformed. When a synthetic version of vitamin D was added to milk, rickets virtually disappeared, as did any concern about vitamin D deficiency. For most of the 20th century, scientists defined a person's daily requirement of vitamin D--called the recommended dietary allowance, or RDA--as the level needed to prevent rickets. Nearly everyone in the developed world was thought to be taking in a healthy amount.

But new research suggests that the RDA may not be sufficient to protect people against several diseases other than rickets. Studies link low blood levels of vitamin D to type 1 diabetes, multiple sclerosis, tuberculosis, colon cancer and even the flu. High levels of the vitamin may help fight HIV infection. And the vitamin's role in bones extends way beyond preventing rickets. Levels higher than the RDA offer older adults protection against fractures, through strengthening muscles as well as bones.

Many people living in this country and northern Europe have levels of vitamin D that are low, based on the latest evidence. Why is that? Unlike most other vitamins, vitamin D is found in only a few foods. Instead, we get most of it from the action of sunlight on our skin. In retrospect, we recognize that rickets became a problem in the early 20th century when increasing urbanization and air pollution in cities caused less sunlight to strike the skin.

A century later, almost every aspect of modern life seems designed to lower our ability to produce vitamin D. Compared with our ancestors, we spend a lot more time indoors, wear more clothes and use sunscreen. If applied adequately to protect against sun-induced skin damage and to reduce the risk of skin cancer, sunscreen lowers the skin's ability to form vitamin D by more than 95 percent. More of us are older and fatter; age and obesity also reduce the amount of vitamin D we produce. An average 70-year-old can produce only about a quarter of the vitamin D of a 20-year-old. Obese people generally have substantially lower blood levels of vitamin D.