"Dietary supplements are used by more than half of all Americans, who spend more than $20 billion on these products each year. However, scientific data are lacking on the long-term health benefits of supplements," said lead author Marian L. Neuhouser, Ph.D., an associate member of the Public Health Sciences Division at the Hutchinson Center.

The study focused the effects of multivitamins because they are the most commonly used supplement. "To our surprise, we found that multivitamins did not lower the risk of the most common cancers and also had no impact on heart disease," she said.

The study assessed multivitamin use among nearly 162,000 women enrolled in the Women's Health Initiative, one of the largest U.S. prevention studies of its kind designed to address the most common causes of death, disability and impaired quality of life in postmenopausal women. The women were followed for about eight years.

Nearly half of the study participants – 41.5 percent – reported using multivitamins on a regular basis. Multivitamin users were more likely to be white, live in the western United States, have a lower body-mass index, be more physically active and have a college degree or higher as compared to non-users. Multivitamin users also were more likely to drink alcohol and less likely to smoke than non-users, and they reported eating more fruits and vegetables and consuming less fat than non-users.

During the eight-year study period, 9,619 cases of breast, colorectal, endometrial, renal, bladder, stomach, lung or ovarian cancer were reported, as well as 8,751 cardiovascular events and 9,865 deaths. The study found no significant differences in risk of cancer, heart disease or death between the multivitamin users and non-users.

These findings are consistent with most previously published results regarding the lack of health benefits of multivitamins, Neuhouser said, but this study provides definitive evidence. "The Women's Health Initiative is one of the largest studies ever done on diet and health. Thus, because we have such a large and diverse sample size, including women from 40 sites across the nation, our results can be generalized to a healthy population." Since the study did not include men, Neuhouser cautions that the results may not apply to them.

So what advice do Neuhouser and colleagues offer to women who want to make sure they're getting optimal nutrition? "Get nutrients from food," she said. "Whole foods are better than dietary supplements. Getting a wide variety of fruits, vegetables and whole grains is particularly important."

Note: If no author is given, the source is cited instead.

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