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Vitamin Supplements May Increase Cancer Risk, Report Warns

Latest warning about the documented risks of supplements

February 29, 2008

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If you take vitamin supplements in hopes of warding off cancer and other illnesses, you may not be doing yourself any favors. In fact, a new study suggests you might even be increasing your cancer risk.

"Our study of supplemental multivitamins, vitamin C, vitamin E and folate did not show any evidence for a decreased risk of lung cancer," wrote the study's author, Christopher G. Slatore, M.D., of the University of Washington, in Seattle. "Indeed, increasing intake of supplemental vitamin E was associated with a slightly increased risk of lung cancer."

Findings of the study of 77,000 vitamin users were published in the first issue for March of the American Thoracic Society's American Journal of Respiratory and Critical Care Medicine.

Similar warnings have been issued before but have received little attention.

In 2004, a British study found that not only do vitamin supplements not protect against gastrointestinal cancer, they may slightly increase the risk of cancer. If the findings are correct, 9,000 in every million users of such vitamin supplements will die prematurely as a result of taking something they think is good for them.

Last year, the Center for Science in the Public Interest called on the Food and Drug Administration to require manufacturers of dietary supplements that contain large doses of synthetic beta-carotene to warn smokers and people exposed to asbestos of an increased risk of lung cancer if they take the supplements.

"Though there is a lot of wishful thinking about antioxidants preventing cancer, the evidence connecting high-dosage beta-carotene supplements to increased rates of lung cancer in smokers is compelling," said CSPI senior nutritionist David Schardt.

CSPI says supplements with more than 5,000 IU, or 3 mg, should bear warning notices and that FDA should take enforcement action against companies that market the pills without the warnings.

CSPI noted a that a major report on diet and cancer by the World Cancer Research Foundation and the American Institute for Cancer Research found that the evidence linking beta-carotene to cancer in smokers is “convincing.”

Latest study

In the latest study, Slatore and colleagues selected a prospective group of men and women between 50 and 76 years of age in the Washington state VITAL study, and determined their rate of developing lung cancer over four years with respect to their current and past vitamin usage, smoking, and other demographic and medical characteristics.

Of the original group, 521 developed lung cancer, the expected rate for a low-risk cohort such as VITAL. But among those who developed lung cancer, in addition to the unsurprising associations with smoking history, family history, and age, there was a slight but significant association between use of supplemental vitamin E and lung cancer.

"In contrast to the often assumed benefits or at least lack of harm, supplemental vitamin E was associated with a small increased risk of lung cancer," said Slatore.

When modeled continuously, the increased risk was equivalent to a seven percent rise for every 100 mg/day. "This risk translates into a 28 percent increased risk of lung cancer at a dose of 400 mg/day for ten years," wrote Slatore. The increased risk was most prominent in current smokers.

The idea that vitamin supplements are healthy, or at the very least, do no harm, comes from the desire of many people to mimic the benefits of a healthy diet with a convenient pill, says Tim Byers, M.D., M.P.H., of the University of Colorado School of Medicine in an editorial in the same issue of the journal. However, he points out, "fruits contain not only vitamins but also many hundreds of other phytochemical compounds whose functions are not well understood."

The World Cancer Research Fund and the American Cancer Society recommend two servings of fruit each day, based on a study that previously found a 20 percent increase in cancer risk among people who ate the least amount of fruit.

This recommendation "would likely lead to a reduced risk for lung cancer, as well as reduced risk of several other cancers and cardiovascular disease," writes Byers. "However, any benefit to the population of smokers from increasing fruit intake to reduce cancer risk by 20 percent would be more than offset if even a small proportion of smokers decided to continue tobacco use in favor of such a diet change."

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