The researchers analyzed data from the Life After Cancer Epidemiology (LACE) prospective cohort study of 2,300 women with early-stage breast cancer. They found that 81% of the women said they took antioxidant supplements such as vitamin C, E and carotenoids. Those who regularly took either vitamins C or E supplements had a lower risk of cancer recurrence over five years compared to those who didn’t take them.
However, women who regularly consumed a mix of carotenoids like vitamin A, beta-carotene, and lutein, were at greater risk of dying from breast cancer, and at risk of dying from any cause, when compared with those who didn’t take them. Moreover, much of the benefit associated with vitamins C and E could be explained by “healthy user bias,” which means that women who use dietary supplements generally have healthier lifestyles, according to Dr. Greenlee. Healthy user bias did not explain the findings on mixed carotenoids.
“Our paper adds to the growing body of literature suggesting that dietary supplements containing high doses of carotenoids may be harmful, and people should think twice before taking them,” says Dr. Greenlee, who is also on an Assistant Professor of Medical Oncology at Columbia’s College of Physicians and Surgeons. “Dietary supplement use is high among cancer patients and they need specific guidance on what is safe and effective.”
Concerns have been raised that taking supplements with antioxidant properties, like carotenoids, vitamin E, vitamin C, selenium and zinc, during chemotherapy or radiation can interfere with treatments. This could occur because the supplements may protect tumor cells from the “pro-oxidant” effects of cancer treatments. However, antioxidants cannot be assumed to uniformly act in a similar function, since they have different physiological functions in the body, Dr. Greenlee emphasizes.
Although the study could not prove that carotenoids are harmful to cancer patients, the results heighten concern about carotenoids’ health effects, following on previous randomized controlled trials that found beta-carotene to increase the risk of lung cancer among smokers.