Feature Archive

The Truth About Vitamin D

Get answers to commonly asked questions about vitamin D.

By Daniel J. DeNoon
WebMD Feature

Reviewed by Laura J. Martin, MD

The hottest topic in medicine isn't the newest drug or the latest surgical device. It's vitamin D.

What brought the simmering debate to a boil was a 2007 study showing that people taking normal vitamin D supplements were 7% less likely to die than those who didn't take the daily supplements.

A year later, a major study found that when women with low vitamin D levels get breast cancer, they have a much higher chance of dying from their cancer than women with normal vitamin D levels.

That was surprising news. But just as surprising are assertions that many men, women, and children have insufficient blood levels of this important vitamin.

How many? Data suggest many of us don't get the vitamin D we need. For example, a 2007 study of childbearing women in the Northern U.S. found insufficient vitamin D levels in 54% of black women and in 42% of white women.

These findings led the American Academy of Pediatrics to double the recommended amount of vitamin D a child should take -- and have led many doctors to advise their adult patients to up their vitamin D intake.

Not so fast, says an expert panel convened by the prestigious Institute of Medicine. In it's long-awaited November 2010 report, the IOM committee expressed horror at the idea that many North Americans are vitamin D deficient.

"Of great concern recently have been the reports of widespread vitamin D deficiency in the North American population," the committee wrote. "The concern is not well founded. In fact, the cut-point values used to define deficiency, or as some have suggested, 'insufficiency,' have not been established systematically using data from studies of good quality."

The IOM committee put its emphasis on what science has proved, not on what studies may suggest. Using this conservative approach, the committee found no proof that vitamin D has health effects beyond building strong bones.

"While the current interest in vitamin D as a nutrient with broad and expanded benefits is understandable, it is not supported by the available evidence," the IOM committee concluded.

Why do I need vitamin D?

Your body must have vitamin D to absorb calcium and promote bone growth. Too little vitamin D results in soft bones in children (rickets) and fragile, misshapen bones in adults (osteomalacia). You also need vitamin D for other important body functions.

Vitamin D deficiency has now been linked to breast cancer, colon cancer, prostate cancer, heart disease, depression, weight gain, and other maladies. These studies show that people with higher levels of vitamin D have a lower risk of disease, although they do not definitively prove that lack of vitamin D causes disease -- or that vitamin D supplements would lower risk.

The Vitamin D Council -- a scientist-led group promoting vitamin D deficiency awareness -- suggests vitamin D treatment might be found helpful in treating or preventing autism, autoimmune disease, cancer, chronic pain, depression, diabetes, heart disease, high blood pressure, flu, neuromuscular diseases, and osteoporosis. However, there have been no definitive clinical trials.

That's why the Institute of Medicine expert committee's November 2010 review found no conclusive evidence that vitamin D, by itself, offers wide-ranging health benefits.

"Despite the many claims of benefit surrounding vitamin D in particular, the evidence did not support a basis for a causal relationship between vitamin D and many of the numerous health outcomes purported to be affected by vitamin D intake," the IOM committee concluded.

The only proven benefit of vitamin D is its role in helping calcium build strong bones. But that's far from the whole story. Vitamin D helps regulate the immune system and the neuromuscular system. Vitamin D also plays major roles in the life cycle of human cells.