With benefits unproven, why do millions of Americans take multivitamins?

"Most people were using supplements because they believe it will improve their health, but we really don't know whether that's true," said study lead author Regan Bailey, a nutritional epidemiologist in the Office of Dietary Supplements at the U.S. National Institutes of Health.

"Moreover, the vast majority of supplements used in the U.S. are based on personal choice, not because they are recommended by health care professionals," she added.

Nearly half of U.S. adults use dietary supplements, Bailey noted, and supplements are a $30-billion-a-year business.

"People have very strong beliefs about these products and I don't know where they are getting their information," Bailey said. "It's not from the doctors. The majority of scientific data available do not support the role of dietary supplements for improving health or preventing of disease."

Another expert said supplements can be expensive.

"A multivitamin might cost $20 a month. Why not spend that on more fresh produce?" said Marian Neuhouser, of the cancer prevention program at the Fred Hutchinson Cancer Research Center, in Seattle. "If someone is eating a healthy diet with plenty of fruits and vegetables and whole grains—a wide variety of foods—they should be getting all the nutrition they need."

The new report was published online Feb. 4 in the journal JAMA Internal Medicine.

To examine why Americans take multivitamins, Bailey's team collected data on nearly 12,000 adults who took part in the 2007 to 2010 U.S. National Health and Nutrition Examination Survey.

The researchers found that 45 percent of those taking a multivitamin did so because they believed it would improve their health, and 33 percent did so because they thought it would maintain their health.

Only 23 percent said their decision was based on advice from their doctor. When they recommend supplements, doctors are most likely to recommend calcium for bone health (24 percent) or to improve overall health (18 percent), or fish oil for heart health (12 percent) or to supplement diet (11 percent), Bailey said.

It's hard to tell whether vitamins actually improve health, because "adults who use dietary supplements tend to report more healthy lifestyles," Bailey said. "They report better overall health, more exercise, moderate alcohol consumption and are more likely to [have never smoked] or be former smokers."

A clear role exists for some dietary supplements—such as folic acid to reduce the risk of birth defects. Calcium and vitamin D play an important role in bone health, Bailey said.

Duffy MacKay, a spokesman for the supplement industry, said taking a multivitamin and other supplements is part of a healthy lifestyle.

"People who take a multivitamin in combination with a healthy diet, exercising regularly and practicing stress management are people who live long and prosperous," said MacKay, vice president for scientific and regulatory affairs at the Council for Responsible Nutrition.

Two recent studies highlight the ongoing debate on the value of multivitamins.

One, published in the Nov. 7 issue of the Journal of the American Medical Association, found that multivitamins will do nothing to help stave off heart disease, heart attack or stroke.

Yet another study published in the Oct. 17 issue of the same journal found that men who take multivitamins every day for several years may lower their risk of cancer by a small amount.

Another expert weighed in on the discussion.

Although vitamin and mineral supplements may be of benefit in certain instances, they cannot take the place of eating a variety of healthy foods every day, said Samantha Heller, an exercise physiologist and clinical nutrition coordinator at the Center for Cancer Care at Griffin Hospital in Derby, Conn.

"I wish that a portion of the $30 billion spent on dietary supplements was spent on healthy foods and gym memberships," Heller said.

More information: For more about multivitamins, visit the U.S. National Library of Medicine.

Journal reference: JAMA Internal Medicine Journal of the American Medical Association

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