Questions About Supplements and Cancer Risk

Why a well-balanced diet may be a better approach to cutting risk.
by Karen Collins, R.D., American Institute for Cancer Research

The American Institute for Cancer Research's (AICR) latest recommendation—not to rely on supplements for cancer prevention—took many people by surprise. Why, many wonder, does a report that links vegetables, fruits and whole grains with lower cancer risk not attribute that same protective influence to the supplements that contain the very nutrients and antioxidants found in the whole foods? In addition, they ask, what harm can really be done by buying some additional "nutritional insurance" from nutritional supplements?

For starters, taking a dietary supplement does not provide the same benefits as healthful eating. The disease-fighting properties ascribed to a diet that features antioxidant-rich plant foods cannot be packaged into a pill or powder. Experts believe that it is the interaction of the vitamins, minerals and phytochemicals in these foods—a process called synergy—that provides cancer protection. Adding just a handful of these compounds to a supplement will not garner the same effect as eating a varied plant-based diet.

Although supplements can play an important role in filling specific nutritional gaps, they don't provide the protection that some people may expect—especially when it comes to disease prevention. If these supplements worked, we would see lower incidence of cancer among those who take them, but we don't. In fact, a report issued by the National Institutes of Health (NIH) on multivitamin/mineral supplements and chronic disease prevention recently concluded: "The present evidence is insufficient to recommend either for or against the use [of multivitamins/minerals] by the American public."

Those still convinced that they should pop a pill "just in case" should note that a study published last year in the Journal of the National Cancer Institute advises caution. In the study, although no effect (positive or negative) was seen between multivitamin intake and risk of localized prostate cancer, researchers saw a 32 percent increase in risk of advanced prostate cancer among men who used more than seven multivitamins per week. This increase was also evident among men who took a multivitamin along with an individual nutrient supplement such as selenium, beta-carotene or zinc.

The notion that supplements may cause more harm than good is not new. A recently published analysis of 68 large trials involving more than 232,000 adults given antioxidant supplements showed either no benefit or even increased mortality with some antioxidants. Similarly, AICR's landmark report, Food, Nutrition, Physical Activity, and Cancer Prevention: A Global Perspective, concluded that there is strong and convincing evidence that beta-carotene supplements cause lung cancer in smokers.

The AICR report also urges caution even among those supplements that have shown a positive effect in reducing cancer risk. Selenium supplements, for example, can probably lower risk of prostate cancer, according to AICR experts, but the mineral can be toxic at high doses. In addition, although the AICR report concludes that calcium supplements can probably protect against colorectal cancer, they also warn that calcium intake beyond amounts recommended for healthy bones (1,200-1,500 milligrams per day) probably increases risk of prostate cancer. A recent study linking calcium supplements with risk of heart attack in older women also raises important questions.

Is the AICR report saying, "Don't use supplements?" No, the report is saying not to turn to supplements for cancer protection at this time. Certain people may benefit from supplements in ways unrelated to cancer risk, including:

• People at risk for B-12 deficiency, including men and women over age 50 and vegans who consume no animal foods at all
• Pregnant women with increased folic acid needs
• Those at risk for vitamin D deficiency, namely individuals who are dark-skinned or live in northern latitudes
• The elderly
• Breastfeeding women
• Exclusively breastfed infants
  

People at risk for osteoporosis may require calcium and vitamin D supplements, too.