Is there a role for vitamins in cancer prevention?

Issue 21, Summer/Fall 2013

bottle of pills

According to recent national surveys, approximately 40 percent of US adults are using multivitamins/multiminerals, and the specific use of calcium, folic acid, vitamin D and other vitamin and mineral supplements is common.

People take dietary supplements for multiple reasons: to prevent diseases, manage diseases, extend life and promote general health. Some individuals take supplements with the goal of preventing cancer occurrence or recurrence and prolonging life after a cancer diagnosis.

The question that people want answered is: Do these supplements work?

Historically, there has been a fascination with finding an elixir or "magic bullet" that will stave off disease and promote long life, and in times when nutrient deficiencies were common, vitamin supplementation may have been a valuable public health strategy. However, now that the U.S. food supply is largely fortified with folic acid, iodine, niacin and vitamin D, this rationale may be less appropriate.

Nonetheless, many people take vitamins and minerals daily, sometimes with large doses, with the goal of optimizing their health, and often for non-specific reasons.

Many healthy individuals hope that these vitamins and minerals might prevent cancer, while people with cancer hope that these supplements might extend survival or prevent a recurrence.

If either answer is yes, we want to know which vitamins might do so, what is the appropriate/ necessary dose, and how long you have to take the vitamin to see an effect.

Unfortunately for vitamin/mineral proponents, if we look at data from human studies, we see limited support for the use of vitamins/minerals to broadly prevent cancer.

No Clear Benefit, Some Evidence of Harm

Numerous observational studies (studies in which the study participants were not asked to change their behavior) and clinical trials have shown limited or no benefits from vitamins and minerals.

In the past, these results provided evidence to say that at least the supplements were not harmful, even if they were not providing any specific benefit. However, more recent reports have suggested that some forms of vitamins and minerals may be harmful.

Investigators from the Iowa Women's Health Study, an observational study of over 38,000 older women, reported that use of several commonly used vitamin and mineral supplements (multivitamins, vitamin B6, folic acid, iron, magnesium, zinc and copper) was associated with increased total mortality. This was surprising as there was little prior evidence suggesting harm.

Our group recently reported on data from the observational Life After Cancer Epidemiology (LACE) study, which suggested that among women with breast cancer who were primarily recruited at Kaiser-Permanente of Northern California, the use of dietary supplements with carotenoids may increase overall mortality, whereas the use of other forms of antioxidants (vitamin C, vitamin E) may be associated with a protective effect (though this may not be real and could be explained by the "healthy user" bias). What this study most importantly illustrated was that different types of antioxidant supplements may have different directions of effects.

All of these results need to be replicated in other studies, and none of these results can stand on their own. We need to interpret study results and infer causation in light of other data. If, for example, we examine the literature on the use of carotenoid supplements for cancer prevention, we can see that there are other trials that have shown harm, and that few, if any, have shown benefit related to cancer prevention.


No Magic Bullet

Clinical trials are our gold standard for testing agents of interest. The SELECT trial was a randomized controlled trial of vitamin E and selenium for prostate cancer prevention among healthy men. While neither supplement prevented prostate cancer, study results suggested that vitamin E increased the risk of prostate cancer.

The most recent vitamin trial to receive attention was the Physicians' Health Study II randomized controlled trial, which was conducted among over 14,000 male U.S. physicians aged 50 years or older. In this study, men were randomized to take either a daily multivitamin or a placebo. This study showed that for men in the multivitamin arm, there was an 8% reduction in total cancer incidence, excluding nonmelanoma skin cancer.

However, there was no difference in overall deaths between the two groups. These results raise the question: From the public health perspective, if multivitamin use decreased cancer incidence, but did not decrease overall deaths, what would be the benefit to encourage men to take multivitamins if they don't affect their overall mortality?

These studies are just a few among the many observational and clinical trials that have failed to reveal the magic bullet that will prevent cancer and extend life among the general population. These studies illustrate that there is the potential for harm with these agents. Additional studies in vitamin D, fish oils, and other agents are ongoing and will reveal whether these other supplements will be beneficial on the population level.

lady stretching

Focus on Diet, Exercise, Not Supplements

A current approach to cancer prevention trials is to examine whether specific populations may benefit from dietary supplements. For example, vitamin D may be of benefit to individuals with low levels of vitamin D due to darker skin pigmentation or low sun exposure. From a global perspective, individuals with environmental exposures, such as arsenic exposure in groundwater wells in Bangladesh, may benefit from folic acid supplementation to decrease the carcinogenic effects of arsenic. But these are targeted populations that may benefit -- not broadly applied to all people.

So -- where does that leave us? Both the American Cancer Society (ACS) and the American Institute for Cancer Research (AICR) clearly state in their clinical guidelines that current evidence does not support the use of any dietary supplements for primary cancer prevention or for the prevention of cancer recurrence.

What should we do? Until we have evidence to suggest otherwise, we can follow the other cancer prevention recommendations from both the ACS and AICR. 1) Eat a diet rich in vegetables and fruits, high in whole grains, and low in processed and energy-dense foods -- if your diet is replete, you probably don't need a multivitamin; 2) Be physically active every day; and 3) Maintain a lean body.

And if you do feel compelled to take dietary supplements, understand what it is you are taking, why you think it may be of benefit and the risks that may be involved. Finally, understand that a supplement should never be viewed as a sole means of cancer prevention.

Heather Greenlee, ND, PhD

Heather Greenlee, ND, PhD
Assistant Professor of Epidemiology
Mailman School of Public Health
Columbia University Medical Center
New York, NY