By Austin Chiang, M.D.

The millions of Americans who take daily supplements may be doing nothing to cut their risk of cancer and heart disease, according to updated guidelines released Monday by the U.S. Preventive Services Task Force USPSTF.

Nearly half of American adults take at least one dietary supplement, according to the U.S. Centers for Disease Control and Prevention - a daily ritual that costs an estimated $12.4 billion. In light of this, researchers have been striving to determine whether vitamin and mineral supplements have any benefit in preventing cardiovascular disease and cancer - two of the greatest killers in America accounting for nearly half of all deaths in the U.S.

The USPSTF, which sets the standard for the country's doctors, reviewed all of this research to come to the conclusion that there is inadequate evidence to support or discourage the use of multivitamins and individual vitamin or mineral supplements in reducing risk of cardiovascular disease or cancer.

While these findings may benefit some wallets, Dr. Michael LeFevre, co-vice chair of the USPSTF and practicing family physician, said he finds these results "disappointing."

"You would think that for something done so commonly as taking a vitamin or multivitamin supplements that science would have progressed," he said. "But we have still don't have enough science to support that there is any benefit or harm for these supplements. We just don't know."

There was enough science, however, for the Task Force to recommend against taking beta-carotene or vitamin E as supplements. Beta-carotene, when taken in excess, appears to increase risk of lung cancer for those at higher risk, including smokers and individuals exposed to asbestos in the past. And while vitamin E does not seem to come attached with any real risks, there seems to be no apparent reason for those in the general population to go out of their way to take supplements to boost their dietary levels of it.

The new USPSTF guidelines build on previous recommendations the group issued in 2003 by adding other supplements such as selenium, calcium, vitamin D, and folic acid to a list of supplements that those in good health probably do not need more of to prevent cancer and heart disease - a list that already included vitamins A, C and E.

Doctor's Take

It's likely that many Americans who do take daily vitamin and mineral supplements do so as an "insurance policy" against disease. The picture that is emerging, however, is that this strategy does not do much - specifically, as Monday's report suggests, when it comes to heart disease and cancer.

Fortunately, there are things that research suggests we can do to lower our risk. And while exercising and adhering to a healthy diet may not be as simple as taking a daily supplement, they are a far better bet for good overall health.

"Over and over again high hopes for an easy quick fix have not lived up to expectations," said Alice Lichtenstein, professor of nutrition and science policy at Tufts University in an e-mail to ABC News. "Popping a pill is no substitute for eating sensibly and moving more."