This study, published in the Journal of Clinical Oncology, confirms previous medical guidance advising cautious use of any supplements, other than a multivitamin, for cancer patients undergoing chemotherapy.
Some evidence suggests antioxidants could interfere with the cancer-killing effects of chemotherapy because these chemical treatments cause oxidative stress, a chemically-triggered reaction in the body, which in turn kills cancer cells but antioxidants fight oxidative stress. While other researchers have found quite the opposite and suggested antioxidants could help the chemotherapy process.
What makes the recent study unique is that it is the first investigation of the effects of supplement use during breast cancer treatment, and only the second to investigate the effects of supplement use during any kind of cancer treatment. The first was conducted by Charles Fuchs, MD, MPH, the director of Yale Cancer Center, who found that vitamin C may be helpful for people undergoing chemotherapy treatment for colorectal cancer.
Lead researcher Christine Ambrosone, Ph.D., chair of the department of cancer prevention and control at Roswell Park, says: "Although this is an observational study and the number of users of supplements was fairly small, the results are compelling.
"Patients using any antioxidant before and during chemotherapy had an increased risk of their breast cancer returning and, to a lesser degree, had an increased risk of death. Vitamin B12, iron, and omega-3 fatty acid use was also associated with poorer outcomes."
SWOG is a cancer clinical trials network funded by the National Cancer Institute (NCI) through the National Institutes of Health.
Ambrosone conducted her study as part of a randomised phase III SWOG trial determining the best dose and schedule for using three chemotherapy drugs — doxorubicin, cyclophosphamide, and paclitaxel — as adjuvant therapy for high-risk, early-stage breast cancer.
In its first iteration, the trial enrolled 2,716 patients between 2003 and 2010. Patients were followed for a median of six years to identify any side effects from the chemotherapy combinations tested and to measure how long, if ever, it took for their breast cancer to return.
To better understand the role supplements might play in chemotherapy response, Ambrosone and her team asked every woman and man on the trial whether they would answer detailed questionnaires about their use of dietary supplements - first at the time they were assigned to a treatment group, then again six months after their chemotherapy was complete. Of the 2,014 patients eligible for this part of the study, 1,607 (80%) agreed.
In the end, 1,134 patients completed both surveys, and of these, 18% used at least one antioxidant daily, while 44% took multivitamins.
The results report that patients who took any antioxidant (vitamins A, C, E and carotenoids and Coenyzme Q10) were 41% more likely to have their breast cancer return when they took the supplements both before and during chemotherapy treatment
Patients had a similar, but weaker, increased risk of death when taking those antioxidants and patients taking vitamin B12, iron, and omega-3 fatty acid supplements were at significantly greater risk of breast cancer recurrence and death.
Patients taking multivitamins showed no signs of poorer or better outcomes after chemotherapy.
Ambrosone cautions that her study results are not definitive enough to influence how doctors treat cancer patients. To do that, she notes, the research community would need to run a larger, randomised trial testing groups who do and do not take supplements to get a clear and strong connection. However, she said the results do support the current cautious approach to supplement use for people undergoing chemotherapy.
"People diagnosed with any cancer should talk with their doctors about whether they should be taking vitamins or other supplements.
"I'd recommend that they try to get their vitamins and minerals—including antioxidants—from food. With a healthy and balanced diet, you can get all the nutrients your body needs, even while undergoing chemo."
If changes in taste or loss of appetite related to the effects of cancer treatment are making whole vegetables, fruits, and grains unappealing or difficult to eat, Ambrosone advises, patients should seek out guidance from their medical team or a dietitian to find out ways to incorporate these foods into their diets.
Source: Journal of Clinical Oncology
Ambrose. C. B., et al
"Dietary Supplement Use During Chemotherapy and Survival Outcomes of Patients With Breast Cancer Enrolled in a Cooperative Group Clinical Trial"