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Dutch researchers found herring, mackerel and three other fish oils increased blood levels of the fatty acid called 16:4(n-3) in cancer patients. Experiments in mice have suggested this fatty acid makes cancer cells resistant to chemotherapy, the researchers said. But, it's not certain that what was found to happen in mice would happen with human cancer patients.
"Dietary supplements are not necessarily benign," said Dwight Kloth, director of pharmacy at Fox Chase Cancer Center in Philadelphia, who had no role in the new study. "There are numerous cases in the literature where nutritional supplements and herbal drugs have had deleterious interactions with chemotherapy."
Many patients begin taking supplements after they receive a cancer diagnosis, but concern is growing that supplements might interfere with anti-cancer treatments, according to background information with the study.
Christine Metz, director of the Laboratory of Medicinal Biochemistry at the Feinstein Institute for Medical Research in Manhasset, N.Y., is among those with concerns.
"Our research shows that when you affect the membrane of cancer cells by altering the fats in the outer covering of the cell, you can make the membrane stiffer or more fluid," she explained.
"These fats can make it more difficult for chemotherapy to enter the cell or make the cell better at pushing the chemotherapy out of the cell," added Metz, who wasn't involved in the study.
The study authors and other experts said the new report, published online April 2 in the journal JAMA Oncology, doesn't prove that fish oil blunts chemotherapy's effects in humans.
And no clinical trial could be done to test the theory. The reason: it would be unethical to evaluate chemotherapy's effectiveness by giving some patients fish oil that could inactivate the drug treatment, said Dr. Len Lichtenfeld, deputy chief medical officer at the American Cancer Society.
For the study, a team led by Dr. Emile Voest, of the Netherlands Cancer Institute in Amsterdam, looked at fish oil use among 118 cancer patients undergoing chemotherapy. The researchers also assessed the levels of fatty acid in 50 healthy volunteers after they consumed either fish or fish oil supplements.
Among the cancer patients, 35 said they took fish oil supplements regularly, and 13 used supplements containing omega-3 fatty acids.
Levels of the fatty acid 16:4(n-3) increased in the volunteers after they consumed 10 milliliters (ml), the recommended amount of fish oil, Voest's group found. Blood levels returned to normal after eight hours, but it took longer when the volunteers consumed 50 ml of fish oil.
In addition, eating 3.5 ounces of herring and mackerel also increased blood levels of 16:4(n-3). Tuna, however, did not alter blood levels of the fatty acid, and salmon showed only a short spike, the researchers found.
"Taken together, our findings are in line with a growing awareness of the biological activity of various fatty acids and their receptors and raise concern about the simultaneous use of chemotherapy and fish oil," the researchers wrote.
However, Voest said he wouldn't want patients who had unsuccessful chemotherapy to blame it on fish oil.
"Until further data become available, we advise patients to temporarily avoid fish oil from the day before chemotherapy until the day thereafter," the researchers added.
Dr. Nagashree Seetharamu, an oncologist at North Shore-LIJ Cancer Institute in Lake Success, N.Y., said research on whether fish oil is harmful or beneficial for cancer patients has yielded conflicting results.
"It's all over the place," she said, noting several studies have shown that fish oil boosted chemotherapy's effectiveness.
"At this time, there is no strong evidence to support or refute the use of fish oil supplements during chemotherapy," Seetharamu said.
Until more is known, Seetharamu said she'd recommend patients stay away from the supplements. "But if they feel strongly about taking them, I would not stop them," she added.
Lichtenfeld said the new study doesn't settle the confusion about fish oil's potential impact on chemotherapy.
However, "while this study is not a definitive answer to the question about fish oil, it is an important piece of evidence to address the question," he said.
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