Are the benefits of vitamin D overhyped?

Another day, and another vitamin has failed to live up to all of its hype. This time it's vitamin D.

The reality check is coming from two new research reviews published in the Annals of Internal Medicine.

The reviews, which looked at hundreds of previous studies of the "sunshine vitamin," conclude that there's little evidence that vitamin D protects against cancer or heart disease.

They also show that vitamin D doesn't prevent fractures when it's taken alone. Pairing vitamin D with extra calcium does appear to help prevent broken bones in the elderly, however.

"For many years, the enthusiasm for vitamin D has outpaced the evidence," says JoAnn Manson, MD, DrPH, who heads the division of preventive medicine at Brigham and Women's Hospital in Boston.

"The evidence is actually fairly thin," especially for any benefits beyond bone health, she says.

To help fill the knowledge gap, Manson is mounting a nationwide trial that will test vitamin D and fish oil for the prevention of heart attacks and cancer. She was not involved in the reviews.

Is Vitamin D a Dud?

In recent years, vitamin D has been touted as a nutritional superstar.

Beyond its well-known role as a bone builder, studies have suggested that high levels of D, usually achieved by taking supplements, may do everything from reducing chronic pain to preventing the common cold.

At the same time, other reports have found that as many as half of all adults have less than ideal blood levels of D.

That has sent sales of vitamin D blood tests and supplements soaring.

But experts say science doesn't yet support the use of the high doses that many people may be taking.

Last year, the Institute of Medicine increased the recommended dietary allowance (RDA) of vitamin D for children and adults ages 1-70 to 600 international units (IU), and adults over 70 to 800 IU. But the agency also said many people already get that much from sun exposure and from foods like fish and fortified dairy products.

But the promise of D is so powerful that many policy makers continue to consider emerging evidence.

The latest is the U.S. Preventive Services Task Force, which is set to update its recommendations on vitamin D for cancer and fracture prevention in January. They commissioned one of these two new evidence reviews.

Vitamin D, Cancer, and Fractures

For that review, researchers at Tufts University reanalyzed data from more than 40 studies on vitamin D.

They set out to answer several key questions:

• Does vitamin D, taken with or without calcium, affect the risk for cancer or broken bones?
• Are high or low blood levels of vitamin D linked to a person's risk for cancer or broken bones?
• Are there harms linked to taking extra D?

Among several studies of vitamin D taken alone or with calcium, researchers say a high degree of statistical uncertainty made it impossible to tell whether taking supplements increased or decreased the risk of cancer.

With respect to fractures, data from five other studies showed that vitamin D supplementation alone, in doses ranging from 400 to 1,370 IU daily, did not appear to reduce the risk of breaking a bone.

That picture changed when calcium was combined with vitamin D.

Across 11 studies of more than 52,000 people who were followed from one to seven years, those taking 300 to 1,000 IU of vitamin D each day, along with 500 to 1,200 milligrams of daily calcium, saw their risk of breaking a bone drop by an average of 12% compared with those taking a placebo.

To answer the second question, researchers relied on studies that looked at the relationship between blood levels of vitamin D and the risk for breast, prostate, colorectal, or any kind of cancer.

There was some evidence that higher blood levels of vitamin D might protect against colon cancer. But there was no evidence that having a higher vitamin D level could protect a person against breast or prostate cancer.

In fact, some studies suggested that men who had higher levels of D had an increased risk for cancer death. The same did not appear to be true for women.

Other possible harms included an increased risk for kidney stones and bladder stones seen in one study among women taking vitamin D supplements.

Vitamin D and Heart Disease

In the second review, Irish researchers looked at the connection between vitamin D and heart health.

They found biological evidence that vitamin D is linked with heart and blood vessel health. For example, vitamin D regulates hormones that affect blood pressure. It also controls blood calcium levels. Calcium helps keep muscle cells working smoothly.

But trials that have put vitamin D supplementation to the test for heart disease prevention haven't panned out, they say.

The studies that look at the effect of vitamin D supplementation on heart disease and stroke risk "have been inconclusive or contradictory," says Cora McGreevy, MBBCh, a clinical lecturer at the Royal College of Surgeons in Dublin, Ireland, in an email.

McGreevy says vitamin D may simply turn out to be a sign of other health problems that put a person at risk for heart attacks and strokes.

Until more is known, "vitamin D cannot be recommended as a treatment for [heart disease and stroke]," she says.

Bottom line: Even though there are some promising signs for vitamin D, there is not enough scientific evidence yet to show that taking vitamin D supplements will "prevent" heart attacks and strokes, Manson says.