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Most vitamin and mineral supplements, alone or in combination, have not been proved to help or hurt when it comes to preventing cancer or heart disease, says a new report from the high-profile U.S. Preventive Services Task Force.

In most cases, more research is needed, but there are two exceptions, the panel says: Consumers should not take beta-carotene or vitamin E to prevent heart disease or cancer because vitamin E doesn't work and beta-carotene increases the risk for lung cancer in people at high risk for the disease.

The report, published Monday in the Annals of Internal Medicine, finalizes conclusions the group made public in an earlier draft statement. But it does not settle a long list of questions – including whether most consumers are wasting the $28 billion they spend each year on multivitamins and other dietary supplements. The report does not address what the vitamin industry says is the main reason consumers take these pills: to supplement inadequate diets and enhance overall health.

The findings also don't apply to people who take vitamins for other specific reasons, such as pregnancy or a diagnosed vitamin deficiency.

"But there are many people out there who are taking supplements specifically because they want to prevent heart disease and cancer," says Michael LeFevre, a specialist in family medicine at the University of Missouri and co-vice chair of the task force.

For them, he says, the message is that studies have failed to produce convincing evidence most supplements do any good or any harm. "I don't think we should assume these supplements are harmless," LeFevre says.

In the case of beta-carotene, harm has been found: Studies show it can increase the risk of lung cancer in smokers and others at high risk. It's also known that vitamin A can be toxic at high doses. Such findings have led researchers to closely scrutinize other supplements. But most vitamins, taken at usual doses, have "few known risks," the task force report says.

Benefits for heart health and cancer prevention may yet be found, LeFevre says. Indeed, the report calls for more research – in contrast to an editorial from several leading doctors that ran in the Annals in December and said the case was closed on using vitamins for prevention of chronic diseases. That editorial said most consumers were wasting their money on supplements.

The task force notes that two studies have found a possible cancer reduction in men, but not women, who take multivitamins.

Future trials should aim to find if there are real differences among men and women, LeFevre says. The task force also says trials might be "more productive" if they focus on people at "high risk for nutrient deficiency rather than the general population."

Duffy MacKay, senior vice president for scientific and regulatory affairs at the Council for Responsible Nutrition, a supplement industry group, says the call for more research is appropriate: "If you read the actual studies, there are a lot of unanswered questions and a lot of potential for benefit."

But for now, LeFevre says, "I would recommend doing what my mother told me: Eat your vegetables... Eat a good diet rich in fruits, vegetables, whole grains, fat free and low fat dairy products and seafood." That's in line with recommendations from several expert groups, including the American Heart Association.

As the task force notes, 49% of adults used at least one dietary supplement and 32% used a multivitamin in 2007 to 2010. Supplement use is most common among women and older adults.

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