A new study suggests that patients with breast cancer who take additional vitamins during chemotherapy treatment may face increased risks.

Researchers said the use of dietary supplements that increase levels of antioxidants, iron, vitamin B12 and omega-3 fatty acids appeared to lower the effectiveness of chemotherapy. Researchers reported their findings in the Journal of Clinical Oncology.

Christine Ambrosone is the head of cancer prevention and control at the Roswell Park Comprehensive Cancer Center in Buffalo, New York. She said, "From this study and others in the literature, it seems that it may not be wise to take supplements during chemotherapy."

"It's thought that antioxidants might interfere with the ability of chemotherapy to kill cancer cells," Ambrosone said.

Some doctors have been advising patients for a number of years not to take antioxidants during chemotherapy. "But there was no strong empirical data for that recommendation," Ambrosone said.

So, Ambrosone and other researchers decided to study whether supplement use might affect chemotherapy's effectiveness. They looked for evidence in an earlier study on diet, exercise, lifestyle and cancer results.

In the earlier research, people who took part were asked about their use of supplements at the beginning of and during treatment, and about their lifestyle, diet and exercise.

The researchers studied 1,134 patients who filled out the surveys and followed them for a median of six years. Their supplement use was much lower than usual, Ambrosone said. About 20 percent of patients were taking supplements before starting chemotherapy and 13 percent during the treatments.

The researchers searched for other possibilities that might increase the risk of the disease reappearing or of death. They found that patients who took any supplements at the beginning of and during chemotherapy were 41 percent more likely to have their breast cancer return than those who did not. In addition, the supplement takers were 40 percent more likely to die later on compared to patients using no supplements. The supplements included vitamin A, C and E.

Vitamin B12 and iron

Those taking vitamin B12 and iron supplements were at greater risk of cancer returning, the researchers said. Women taking vitamin B12 were 83 percent more likely to experience a return of their disease and 22 percent more likely to die from it than those not taking those supplements. Those taking omega-3 supplements were 67 percent more likely to have the disease return. That percentage rose to 79 for those taking iron supplements.

Amy Tiersten is a professor of medicine at the Icahn School of Medicine at Mount Sinai in New York City. She said she was pleased with the research.

"For years we have been cautioning patients about the use of vitamins, in particular antioxidants, during chemotherapy for breast cancer," Tiersten said.

In an email to Reuters news service, she said that she "always told patients on chemotherapy that the best way to get their vitamins is through a well-balanced diet, and will continue to do so given these data."

I'm Mario Ritter, Jr.

Mario Ritter Jr. adapted this Reuters story for VOA Learning English. Caty Weaver was the editor.

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supplement –n. an additional amount of something (like vitamins or minerals)

literature –n. books and articles about a particular subject

empirical –adj. based on testing or experience

recommendation –n. a suggestion for someone to do something

habit –n. a usual way of behaving, something done in a repeated way

survey –n. an activity in which people are asked questions in order to gather information about what most people do or think

median –adj. the middle value in a series of numbers

recurrence –n. when something happens again or returns

caution –v. to warn or tell (someone) about a possible danger, problem, etc.