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Why taking antioxidants during chemotherapy for breast cancer could be counterproductive


Canadians spend an estimated $3 billion a year on over-the-counter vitamin supplements. And while the medical community has long questioned their usefulness in otherwise healthy people, a new study adds to the small body of research that suggests taking supplements during cancer treatment could be counterproductive.

Researchers at Roswell Park Comprehensive Cancer Center in Buffalo, N.Y., followed 1,134 breast cancer patients. The six-year study was part of a Phase 3 clinical trial to determine the best dose and schedule for three chemotherapy drugs in high-risk, early-stage breast cancer.

As part of the research, participants filled in surveys about their use of supplements before and during chemotherapy, as well as their lifestyle, diet and exercise habits.

Among the 18 per cent who took vitamins like A, C or E, all of which are antioxidants, their risk of the cancer returning was 40 per cent higher than the participants who didn’t take supplements.

Though concerns over taking supplements during cancer treatment have been around for a while, the study’s lead author, Christine Ambrosone, was still surprised by the findings.

“When I started this study, I really didn’t think we’d see anything at all,” she said.

Christine Ambrosone, chair of cancer prevention and control and senior vice-president for population sciences at the Roswell Park Comprehensive Cancer Center in Buffalo, N.Y., says the findings provide some empirical data for physicians to consider when discussing use of dietary supplements with patients undergoing chemotherapy. (Craig Chivers/CBC)

It’s important to note the study is small, from a statistical perspective, and only suggests an association between vitamins and treatment outcomes, not cause and effect.

Chemotherapy uses special drugs to destroy cells, both cancerous and healthy ones. But antioxidants like vitamins A, C, E and coenzyme Q-10 help repair cell damage, which could explain how they might interfere with cancer treatment.

“Many chemotherapy agents work by generating all these reactive oxygen species that are really damaging to cells and damaging to DNA,” Ambrosone said. “The thought is that if you’re taking antioxidants to reduce oxidative stress then you’re going to reduce the capability of the chemotherapy to kill those cancer cells.”

‘I was shocked’

Ingrid Foshay Murphy, 54, of Dartmouth, N.S., took a variety of vitamins as she prepared for surgery, chemotherapy and radiation to treat her breast cancer eight years ago.

“I went to meet with the oncologist and I had to write down exactly all of the vitamins that I was on,” Foshay Murphy recalled. “He said immediately that I had to stop taking all of them. I was shocked.”

She said the oncologist told her he was trying to destroy cancer cells and the vitamins she was taking were making his job more difficult.

She followed his advice.

‘There is validation now’

Dr. Kurian Joseph, a radiation oncologist in Edmonton, said data from his practice finds many patients are taking supplements.

“Now we can clearly tell the patient, ‘Hey, you shouldn’t take these dietary supplements because it will affect your cancer outcome.’ There is validation now,” Joseph said of the U.S. research.

In the study, relationships between individual antioxidants and increased risk of cancer returning were weaker, perhaps due to the small sample size, the researchers said.

A 2019 study on antioxidant supplements and breast cancer prognosis among postmenopausal women in Germany also suggested an association with increased risk of negative outcomes.

“Short of a randomized trial of supplements in patients with cancer, the findings provide some empirical data for consideration when discussing with patients the use of dietary supplements during chemotherapy,” the U.S. researchers wrote.

Ambrosone said patients undergoing chemotherapy should exercise caution when using supplements other than a multivitamin. 

The study was funded by the U.S. National Institutes of Health and the Breast Cancer Research Foundation.



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