According to recent national surveys, approximately 40 percent of U.S. adults take multivitamins/multiminerals; many also take calcium, folic acid, vitamin D, and other specific vitamin and mineral supplements.
People take dietary supplements for multiple reasons: to prevent diseases, manage diseases, extend life, and promote general health. Some take supplements with the goal of preventing cancer occurrence or recurrence and of prolonging life after a cancer diagnosis.
The question that people want answered is: Do these supplements work?
Historically, there has always been a quest to find 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.
If some of these supplements are effective, which ones are they? What is the appropriate/ necessary dose? And how long must you 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 supplemental vitamins and minerals.
In the past, these results provided sufficient evidence to say that at least the supplements were not harmful, even if they provided no 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 more than 38,000 older women, found that several commonly used vitamin and mineral supplements (multivitamins, vitamin B6, folic acid, iron, magnesium, zinc, and copper) were 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). The most important implication of these results was that different types of antioxidant supplements may have effects.
None of these results stands on its own; all of them need to be replicated in other studies. We must interpret study results and infer causation in light of other data. If, for example, we look at the literature on the use of carotenoid supplements for cancer prevention, we see that there are other trials that have shown harm and that few, if any, have shown any 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, the 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, a randomized controlled trial that included more than 14,000 male U.S. physicians 50 years or older. The men were randomized to take either a daily multivitamin or a placebo. The study found that for men taking the multivitamin, there was an 8 percent 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?
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. The studies also show the potential for harm with these agents. Additional ongoing studies of supplemental vitamin D, fish oils, and other agents will reveal whether they would be beneficial on the population level.
Focus on Diet, Exercise, Not Supplements
A current approach to cancer prevention trials is to ask 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 in groundwater wells in Bangladesh, may benefit from folic acid supplementation to decrease the carcinogenic effects of arsenic. But such supplementation is beneficial to targeted populations—not broadly applicable to all people.
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 nutritionally balanced, you probably don’t need a multivitamin. 2) Be physically active every day. 3) Maintain a lean body.
If you do feel compelled to take dietary supplements, understand what you are taking, why you think it may be beneficial, and what risks may be involved. Finally, understand that a supplement should never be viewed as a sole means of cancer prevention.