Men who take a daily multivitamin over the long term may help reduce their risk of cancer, a large new U.S. study concludes.

The study, led by Boston researcher Dr. J. Michael Gaziano, follows other studies that have had mixed conclusions on whether multivitamins help cut cancer risk.

The study released in Wednesday's online issue of the Journal of the American Medical Association looked at 14,641 male physicians with an average age of 64 who were randomly assigned to take a multivitamin or placebo.

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The study of 14,641 men taking multivitamins or placebos found an overall cancer risk reduction that was relatively small. (David Gray/Reuters)

"In this large-scale, randomized, placebo-controlled trial among middle-aged and older men, long-term daily multivitamin use had a modest but statistically significant reduction in the primary end point of total cancer after more than a decade of treatment and followup," Gaziano, of Brigham and Women’s Hospital and Harvard Medical School in Boston, and his co-authors conclude.

"Although the main reason to take multivitamins is to prevent nutritional deficiency, these data provide support for the potential use of multivitamin supplements in the prevention of cancer in middle-aged and older men."

The 2010 Dietary Guidelines for Americans states that for the general population of healthy people, "there is no evidence to support a recommendation for the use of multivitamin/mineral supplements in the primary prevention of chronic disease," a conclusion supported by the U.S. National Institutes of Health's conference on the state of the science.

In the new study, multivitamins reduced the chance of developing cancer by eight per cent — less than a good diet, exercise and not smoking, each of which can lower cancer risk by 20 to 30 per cent, cancer experts say.  

"This is a small, one could almost use the word tiny push in the right direction," said Dr. Michael Pollak, director of cancer prevention at the Segal Cancer Centre at the Jewish General Hospital in Montreal.

"It's probably that many people get no benefit but a subset of people get a big benefit," he said of the eight per cent risk reduction.

Cancer rates and deaths

The researchers said that while total cancer rates in the study were likely influenced by increased use of the prostate-specific antigen (PSA) test starting in the late 1990s.

About half of the confirmed cancers in the study were earlier stage cancers affecting the prostate. The researchers did not find a reduction in risk of prostate cancer, colorectal cancer or other "site-specific" cancers such as lung or bladder, but Gaziano told reporters that the study wasn't large enough to look for those.

There was no reduction in cancer death rates at a statistically significant level, that is, beyond chance alone.

The reduction in risk only appeared when total cancers were counted. During the course of the study, there were 2,669 cancer cases, including 1,373 prostate cases and 210 colorectal cases.

It’s not clear what components of the multivitamin made a difference. While the multivitamin used in the study stayed the same, vitamin makers have changed their formulations over time.

'Mild effect'

The researchers can't say whether the findings would apply to women, younger men, or those who were less healthy than the physicians participating in the study. About 40 per cent of them were former smokers, three per cent were current smokers and many were taking Aspirin as part of another study.

The Canadian Cancer Society cautiously welcomed the new research.

"Although the results from this study seem promising, more research about the potential benefits of multivitamins on reducing cancer risk is necessary since the findings of this study only show overall reduction in risk to be relatively small," the society said in an email.

"The society believes multivitamins should not replace other known ways of reducing one's risk of cancer, such as eating a diet that includes lots of fruits and vegetables (which are excellent sources of vitamins, minerals and phytochemicals), maintaining a healthy body weight, avoiding smoking and limiting your alcohol intake."

Dr. Ernest Hawk, vice-president of cancer prevention at the University of Texas MD Anderson Cancer Center and formerly of the U.S. National Cancer Institute also called it a "very mild effect."

"Personally, I'm not sure it's significant enough to recommend to anyone" although multivitamins are promising, added Hawk,, who reviewed the research for the American Association for Cancer Research.

The research was also presented on Wednesday at the U.S. group's Research Frontiers in Cancer Prevention Research meeting in Anaheim, Calif.

Dr. Howard Sesso, one of the study's authors and an associate epidemiologist at Brigham and Women's Hospital in Boston, cautioned consumers that studies have found that the contents of vitamins don't always match what it says on the label.

Side-effects were about the same in both groups although there were more rashes among those taking vitamins, the researchers said.

Pfizer provided the pills. BASF and DSM Nutritional Products provided packaging for the study that was funded by the U.S. National Institutes of Health and BASF Foundation.

With files from The Associated Press and CBC's Amina Zafar