Antioxidants and cancer risk: the good, the bad, and the unknown

By Nyedra W. Booker, PharmD, MPH and Diana Zuckerman, PhD

Updated 2015

Do you take vitamin pills or dietary supplements? If so, you are not alone! Most adults in the U.S. take at least one dietary supplement,¹ and products with claims of cancer and other disease-fighting benefits are increasingly popular. Vitamins A, C, E, beta carotene and selenium are all considered “antioxidants,” and people take them hoping to prevent disease and improve overall health.  You have probably heard that antioxidants fight cancer, but the latest research suggests that any benefits they have in preventing cancer may be reversed for people who already have cancer (including those who have cancer but haven’t been diagnosed yet).

What are antioxidants?

Antioxidants prevent a chemical process known as “oxidation,” which is a natural part of living and aging. Oxidation damages cells and can lead to the development of disease, including Alzheimer’s, heart disease, and cancer. Antioxidants are found in fruits, vegetables, nuts, beans, grain cereals, and other foods. Even dark chocolate is rich in antioxidants.

The antioxidants made by our body play an important role in finding and repairing damaged cells, as well as boosting our immune system. But do antioxidants in dietary supplements provide the same health benefits as the antioxidants we make on our own? Supplements are often produced using man-made products or by extracting the antioxidants from certain foods. Unfortunately much of the health benefits may be lost during these manufacturing processes.²

The good, the bad and the unknown

A landmark study from the late 1980s was among the first to look at the effect of antioxidants on cancer risk. The study involved almost 30,000 Chinese men and women at high risk for cancer who took daily vitamin and mineral supplements for 5 years. Patients were assigned to receive one of the following combinations of supplements:  (a) retinol and zinc, (b) riboflavin and niacin, (c) vitamin C and molybdenum, or (d) beta carotene, vitamin E and selenium. Men and women taking the antioxidant combination of beta carotene, vitamin E and selenium had significantly lower death rates and a reduced risk for cancer after 1-2 years of taking the supplements.³

However, excitement about antioxidants to prevent cancer was short-lived. In 1994, the New England Journal of Medicine (NEJM) published findings from an 8-year study on antioxidant use to prevent lung cancer in men who smoke. Almost 30,000 male smokers age 50-69 were randomly assigned to one of the following: (a) vitamin E, (b) vitamin E + beta carotene, (c) beta carotene or (d) placebo (a sugar pill).  But rather than reduce the risk for lung cancer, the men taking beta carotene (either alone or in combination with vitamin E) were more likely to develop lung cancer after only 18 months of daily beta carotene use, and the number of lung cancer cases continued to increase disproportionately for the duration of the study.⁴

In 1996, a study of more than 18,000 men and women at high-risk for lung cancer looked at whether antioxidants could reduce the risk of lung cancer in high-risk patients. Study participants included current smokers, former smokers, and workers with prior exposure to asbestos, who were randomized to receive beta carotene, vitamin A or placebo.  The study was stopped early when preliminary results showed a 17% higher death rate in the antioxidant groups.⁵

These two lung cancer studies caused great concern about antioxidants.  Beta carotene was discontinued two years into a study involving 540 patients with head and neck cancer taking a combination of vitamin E + beta carotene supplements to prevent a second cancer. But the researchers found a significant increase in the risk of a second cancer in the patients who continued to take vitamin E. Once this supplement was discontinued, the cancer risk decreased to that of patients taking the placebo.⁶

A 2012 review of almost 80 randomized clinical studies of antioxidant use (vitamin A, C, E, beta-carotene and selenium) again showed cause for concerns. Together the studies included a total of almost 300,000 men and women (described as both “healthy” and with diseases in a “stable phase”). Men and women were more likely to die if they were taking Vitamin E, beta-carotene or doses of vitamin A that exceed the Recommended Dietary Allowance (700µg for women and 900µg for men). The authors concluded that the use of antioxidant supplements could be dangerous for the general population and those diagnosed with various diseases.⁷

What could explain the possible increased cancer risk?

Starting around 2008, there was growing evidence that antioxidants could be dangerous for anyone who already had cancer, and that could explain some of the contradictory results of previous studies. In one study, large doses of vitamin C supplements reduced the effectiveness of several anti-cancer drugs including Methotrexate, Doxorubicin and Imatinib, resulting in 30-70% fewer cancer cells killed. The authors concluded that vitamin C may actually be helping cancer cells survive by protecting the cells’ power source.⁸ In another study, the antioxidants vitamin C and N-acetyl cysteine (often sold under the name “NAC”) significantly reduced the effectiveness of anti-cancer drugs Vinblastine and Cisplatin.⁹

As more recent studies continue to suggest antioxidants could actually help cancer cells grow, research by Zachary Schafer shows that cancer cells’ survival can be aided by antioxidants that protect these cells from free radicals.¹⁰ Free radicals harm cells, and getting rid of free radicals therefore can help cancer cells. “If you are a person who is healthy, meaning no tumors of any kind, antioxidants are probably going to protect against cancer,” Schafer says. But he points out that if a person has cancer cells, antioxidants can help those cancer cells survive.

The bottom line

Dietary supplements are intended to be used when your body is not receiving certain nutrients in the right amounts, but like drugs, they can have unintended side effects so they should only be taken as recommended.

If a person definitely doesn’t have cancer, antioxidants can help them.  However, if they have cancer, even if they have early cancer that hasn’t been diagnosed yet (which is certainly a risk for heavy smokers or former smokers, for example), antioxidants could be harmful.

It may be tempting to go out and buy nutritional products that claim to be all-natural and will make you feel great and stay healthy. Remember, however, that a product labeled as being “natural” or “organic” is not necessarily safe. The Food and Drug Administration (FDA) does not test the safety and effectiveness of nutritional supplements before they are placed on the shelf the way they do for prescription and over-the-counter medications.  Consequently, you may be purchasing a product that is neither safe nor effective!

So what are some proven strategies that can help reduce your risk of cancer?

• Maintain a healthy weight by limiting the high-calorie foods you eat and getting regular physical activity
• Eat plenty of fruits and vegetables every day (fruits and vegetables should cover half of your plate. For more tips, see My Plate: A New Alternative to the Food Pyramid)
• Limit the amount of red meat and processed meat (hot dogs, sausages, bologna, etc.) that you eat
• Eat foods made of whole grains