June 8, 2015

Dietary Supplements: Friend or Foe?

By cancerconnect

With research pointing to pros and cons of vitamin and mineral supplements, these dietary decisions become increasingly complex.

By Eleanor Mayfield

Here’s a quick quiz:
1. Are you a cancer patient or survivor?
2. Do you take any dietary supplements such as vitamins, minerals, or herbs?
3. Have you discussed with your doctor the pros and cons of using these supplements?

If you answered yes to questions 1 and 2 and no to question 3, you’re not alone. A recent review in the Journal of Clinical Oncology[1]found that supplement use is widespread among cancer patients and longer-term survivors and that most don’t discuss their supplement use with their doctors. In fact, in different studies 64 to 81 percent of respondents reported using vitamin or mineral supplements. Up to 68 percent of doctors were unaware of supplement use by their patients. Cancer patients and survivors tend to use dietary supplements at higher rates than the rest of the U.S. population, the review found. Supplement use was highest among women, breast cancer survivors, and people with higher levels of education.

People who take supplements generally believe that doing so will benefit their health. In studies included in the review, the reported reasons for supplement use included strengthening the immune system, helping cope with stress, improving the chance of a cure, and helping the user feel better.

Users may assume that the supplements they take can’t do them any harm. Research findings suggest, however, that supplements can sometimes be harmful. In cancer patients who are receiving radiation or chemotherapy, some dietary supplements may interfere with treatment effectiveness while others may accelerate cancer growth. This article summarizes what you need to know to make an informed decision about supplement use as a cancer patient or survivor.

What exactly is a dietary supplement?
The Dietary Supplement Health and Education Act, passed by Congress in 1994, defines a dietary supplement as “a product (other than tobacco) that…contains one or more dietary ingredients (including vitamins, minerals, herbs or other botanicals, amino acids, and other substances) or their constituents [and] is intended to be taken by mouth as a pill, capsule, tablet, or liquid.”[2]

Dietary supplements must be safe or they wouldn’t be on the market, right?
Not exactly. Under the 1994 law, manufacturers of dietary supplements are not required to test their products in clinical trials to prove them safe and effective before putting them on the market, as drug manufacturers must do. The law considers supplements safe unless they are found to cause harm. The U.S. Food and Drug Administration (FDA) may stop a company from marketing a dietary supplement only if it proves that the product poses a significant health risk.

Since 1994 the FDA has found problems with some dietary supplements. Some products have been contaminated; others have not contained the ingredients listed on the label or have contained a lower or higher dose of the active ingredient than the label stated.[3]Some supplements have caused damage to the liver, kidneys, or other organs.[4]The supplement ephedra, often sold as a weight-control remedy and an energy booster, was taken off the market in 2003 after being linked to such health problems as high blood pressure and irregular heart rhythm.[5]

Some supplements may pose particular risks for cancer patients. For example, St. John’s wort—an herb marketed as helpful for depression—has been shown to interfere with the body’s metabolism of the cancer drug Camptosar® (irinotecan).[6]Folic acid—a vitamin that reduces the risk of some birth defects—may have biologic effects that promote the progression of colorectal cancer.

“The term dietary supplements may lead people to think that these products can be used as you would use a spice to garnish your food,” says Patrick J. Mansky, MD, a medical oncologist and researcher at the National Center for Complementary and Alternative Medicine, part of the National Institutes of Health in Bethesda, Maryland. “But many products termed ‘supplements’ contain biologically active components that may be beneficial but are also potentially harmful if they interact adversely with medications or have other undesirable biologic effects.”

What about antioxidant supplements?
One category of supplements whose use by cancer patients is particularly controversial is antioxidants, such as beta-carotene, lycopene, selenium, and vitamins A, C, and E. These substances work in the body to slow or prevent damage to cells caused by unstable molecules known as free radicals. Antioxidants are found in many foods, including fruits, vegetables, grains, and nuts as well as some red meats, poultry, and fish.

Some cancer patients take high-dose antioxidant supplements in the hope that these products will help their bodies better tolerate treatment side effects or boost treatment effectiveness. Laboratory and animal studies have shown that antioxidants can protect cells against radiation and chemotherapy and may therefore reduce the adverse effects of radiation treatment and chemotherapy. Some lab studies suggest that antioxidants enhance the effectiveness of chemotherapy and radiation. Other studies, however, suggest that antioxidants may reduce the anticancer activity of these treatments.[7]

“Radiation and many kinds of chemotherapy work by damaging the DNA of both tumor cells and normal cells,” explains Brian D. Lawenda, MD, a radiation oncologist at Naval Medical Center San Diego. “When you put high doses of antioxidants into the cellular environment, it has been shown that they can repair DNA damage and prevent cell death.” The bottom line: high doses of antioxidants may reduce the ability of radiation or chemotherapy to kill tumor cells, potentially reducing the effectiveness of treatment.

Dr. Lawenda is the first author of a recent review article published in the Journal of the National Cancer Institute,[8]which concludes, “the use of antioxidant supplements during chemotherapy and radiation treatment should be discouraged because of the possibility of tumor protection and reduced survival.” The researchers base this conclusion on a review of all the randomized clinical trials they could identify in which patients received antioxidant supplements concurrently with radiation or chemotherapy.

Many of the trials they found enrolled fewer than 50 patients, Dr. Lawenda says, and therefore lack the statistical power to support conclusions about the benefits or risks of antioxidant supplements. In the largest trial they identified, 540 patients with head and neck cancer received radiation therapy plus either antioxidant supplements or placebo pills. Compared with those in the placebo group, patients who received the antioxidant supplements had fewer severe side effects from treatment, but their tumors were more likely to come back and their overall survival was poorer.[9],[10]

The results of this trial don’t prove that it’s unsafe for cancer patients to take antioxidant supplements, says Dr. Lawenda, but they suggest it may be unsafe. “The studies done so far don’t provide a definitive answer,” he says. “What we need are more large clinical trials designed specifically to look at survival and tumor control endpoints in cancer patients who receive high-dose antioxidant supplements. We recommend that patients not take high-dose supplemental antioxidants during treatment until we have better research that shows this is safe.”

This advice is consistent with that issued in 2006 by an expert panel convened by the American Cancer Society. “Whether antioxidants or any other dietary supplements are beneficial or harmful is…without a clear scientific answer at this time,” the panel wrote. “Given this uncertainty, until more evidence is available that suggests more benefit than harm, it is prudent for cancer survivors receiving chemotherapy or radiation therapy to avoid exceeding more than 100 percent of the daily value for antioxidant-type vitamins such as vitamins C and E during the treatment phase.”[11]

Antioxidant supplements may offer no health benefits to those who are cancer-free, either. In a recent review of 67 randomized trials, participants who took high doses of beta-carotene, vitamin A, or vitamin E had a higher risk of death than those who took either no supplements or a placebo. Vitamin C supplements had no effect on death rates. In some studies selenium supplements seemed to reduce death rates, but the evidence was not conclusive. Some participants were healthy while others had various diseases, but this made no difference to death rates.[12]

So what about antioxidants in food?
Many fruits and vegetables—especially berries, tomatoes, and dark green leafy vegetables—are rich in antioxidants. If antioxidants might interfere with cancer treatment, should patients avoid eating these foods? Absolutely not, says Dr. Lawenda. So why are antioxidant-rich foods okay if antioxidant supplements are potentially harmful?

The answer is that the body makes better use of antioxidants that are “packaged” along with many other nutrients in a fruit or vegetable than it does of isolated antioxidants consumed as supplements, says Karen Collins, MS, RD, nutrition adviser to the American Institute for Cancer Research (AICR). “When antioxidants are consumed in food, you get a wide variety of other nutrients as well and they work synergistically—their healthful effects are stronger together than alone; plus you don’t get too much of any single nutrient,” she explains. “More is not always better—you can have too much of a good thing.”

Why should I tell my doctor what supplements I take?
Much has been learned over the past few years about how supplements—particularly herbs and other supplements derived from plants—can interact with prescription medications, says Dr. Mansky: “Not just cancer drugs, but blood pressure medications, antibiotics, antiviral drugs, blood thinners, and cholesterol-lowering agents. As a physician you want to be aware of a patient’s whole situation so that you can provide the best care. Patients should be as informed and aware as possible about supplements they are taking or considering taking. You want an informed patient talking to an informed healthcare provider.”

Are any supplements recommended during cancer treatment?
A regular multivitamin (not a “megavitamin”) may be helpful and is unlikely to be harmful, those interviewed for this article agree. If a patient has lost weight or has no appetite, a protein or calorie supplement might be appropriate, says Collins. If someone is deficient in a specific nutrient, supplementation of that nutrient may help eliminate the deficiency. “Recommendations for supplementation should be individualized based on what type of treatment a patient is receiving and how well they’re able to get the nutrients they need through food,” she adds.

Can supplements prevent cancer or stop it from coming back?
In a comprehensive report published by AICR in November 2007, scientists concluded—after reviewing more than 7,000 studies on diet and cancer prevention—that no dietary supplement can currently be recommended to prevent cancer.[13]

Some supplements may reduce the risk of certain types of cancer but increase it for others, says Collins. “Calcium supplements probably lower colon cancer risk, but in excessive doses they may increase the risk of prostate cancer. Selenium supplements may help reduce prostate cancer risk but may increase the risk of skin cancer. Right now we can’t identify who is likely to be helped by these supplements and who might be harmed. Until we can do that, it’s premature to recommend any supplements for cancer prevention.”

How can I be a savvy supplement user?
Be aware that supplements may be helpful in some circumstances but possibly harmful in others. If you’re receiving treatment for cancer, inform yourself about the pros and cons of using supplements. If you’re hoping to prevent cancer, don’t count on supplements to do it. Don’t use supplements as a substitute for a healthy diet. Finally, always talk with your doctor about supplements you’re taking or considering taking.


[1] Velicer CM, Ulrich CM. Vitamin and mineral supplement use among US adults after cancer diagnosis: A systematic review. Journal of Clinical Oncology. 2008;26(4):665-73.

[2] Center for Food Safety and Applied Nutrition, U.S. Food and Drug Administration. Dietary Supplement Health and Education Act of 1994. Available at: http://www.cfsan.fda.gov/~dms/dietsupp.html. Accessed September 19, 2008.

[3] Dietary Supplements: How to Know What Is Safe. American Cancer Society Web site. Available at: http://www.cancer.org/docroot/ETO/content/ETO_5_3x_How_to_Know_What_Is_Safe_Choosing_and_Using_Dietary_Supplements.asp. Accessed September 19, 2008.

[4] Marwick C. Adverse reactions to dietary supplements under investigation by FDA. BMJ. 2002;325(7359):298.

[5] Meadows M. Ensuring the Safety of Dietary Supplements. FDA Consumer. July-August 2004. Available at: http://www.fda.gov/fdac/features/2004/404_supp.html. Accessed September 19, 2008.

[6] Mathijssen RH, Verweij J, de Bruijn, Loos WJ, Sparreboom A. Effects of St. John’s wort on irinotecan metabolism. Journal of the National Cancer Institute. 2002;94(16):1247-49.

[7] D’Andrea GM. Use of antioxidants during chemotherapy and radiotherapy should be avoided. CA: A Cancer Journal for Clinicians. 2005;55(5):319-21.

[8] Lawenda BD, Kelly KM, Ladas EJ, Sagar SM, Vickers A, Blumberg JB. Should supplemental antioxidant administration be avoided during chemotherapy and radiation therapy? Journal of the National Cancer Institute. 2008;100(11):773-83.

[9] Bairati I, Meyer F, Gélinas M, et al. Randomized trial of antioxidant vitamins to prevent acute adverse effects of radiation therapy in head and neck cancer patients. Journal of Clinical Oncology. 2005;23(24):5805-13.

[10]Bairati I, Meyer F, Jobin E, et al. Antioxidant vitamins supplementation and mortality: A randomized trial in head and neck cancer patients. International Journal of Cancer. 2006;119(9):2221-24.

[11] Doyle C, Kushi LH, Byers T, et al. Nutrition and physical activity during and after cancer treatment: An American Cancer Society guide for informed choices. CA: A Cancer Journal for Clinicians. 2006;56(6):323-53.

[12]Bjelakovic G, Nikolova D, Gluud LL, Simonetti RG, Gluud C. Antioxidant supplements for prevention of mortality in healthy participants and patients with various diseases. Cochrane Database of Systematic Reviews. 2008;(2):CD007176.

[13]Food, Nutrition, Physical Activity, and the Prevention of Cancer: A Global Perspective. American Institute for Cancer Research/World Cancer Research Fund Web site. Available at: http://www.dietandcancerreport.org. Accessed September 19, 2008.

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