(NEW YORK) -- Many Americans spend billions of dollars each year on multivitamins, assuming that taking one daily will help protect them against certain diseases, even though there's been no conclusive studies on the benefits of taking multivitamins.
New findings from a long-term study published Monday in the Journal of the American Medical Association, for example, suggest that multivitamins do not protect against heart disease in men. The study, which included nearly 15,000 male physicians over age 50, found that those who took a daily multivitamin for more than 10 years did not reduce their risk for heart attack, stroke or death.
The findings are part of the large-scale Physicians' Health Study II, which has been tracking the long-term effects of multivitamins on the risk of heart-related diseases and cancer.
While this larger trial is the only one to research the effects of multivitamins as opposed to single types of vitamins, the findings add to the debate of what benefits, if any, daily multivitamin use may hold for men.
"Just because [a multivitamin] doesn't do everything for all things, doesn't mean you can't consider all of its effects," said Dr. Michael Gaziano, chief of the Division of Aging at Brigham and Women's Hospital, and senior author of the study.
Gaziano also co-wrote another study published in JAMA in October that used the same group of participants. That study found daily multivitamin use for an average of 11 years had a modest effect on preventing cancer in men over age 50. Older men who took a multivitamin long-term had an 8 percent lower risk of cancer compared to those who took a placebo.
"While I can't say definitively, most of the things that seem to work in men, may work in women too," said Gaziano, who added that additional studies should look at multivitamin effects on women.
These results also contrasted with findings of previous research that found taking some types of vitamins showed either no reduction in cancer risk, or a small increase.
Forty-two percent of American adults take multivitamins regularly, according to the Council for Responsible Nutrition, a trade association representing dietary supplement manufacturers and ingredient suppliers.
In general, studies on multivitamins have left many men and their physicians with more questions than answers, some experts said.
"It's not clear if some men would benefit more than others, and if so, who those men might be," said Keith Ayoob, a registered dietitian and associate professor at the Albert Einstein College of Medicine in New York. "Also, not all multivitamins are the same."
According to Dr. Eva Lonn, professor in the division of cardiology at McMaster University and Hamilton General Hospital in Ontario, Canada, the conflicting findings on vitamin combinations for disease prevention only suggest that vitamins alone should not be used as a single method to prevent any disease.
Many with heart problems avoid tried and true methods of prevention such as healthy eating, exercising, and avoiding smoking, and instead rely solely on vitamins, she said.
"This distraction from effective CVD [cardiovascular disease] prevention is the main hazard of using vitamins and other unproven supplements," Lonn wrote in an accompanying editorial published Monday in JAMA.
For both men and women, multivitamins should be considered an added benefit to be used with other healthy lifestyle habits, said Dr. David Katz, director of the Prevention Research Center at the Yale School of Medicine.
Both findings from the Physician Health Study II are "not a reason to banish the multivitamin, but it certainly is another reason not to bank on one, either," said Katz.
Copyright 2012 ABC News Radio