What My Patients Are Asking: Should I Avoid Fish Oil While I’m Getting Chemotherapy?

By on April 27th, 2015 Categories: Treatment & Side Effects

Several news stories, including one in Time magazine, are saying that people diagnosed with cancer should avoid fish oil supplements, as well as eating mackerel and herring, while being treated with chemotherapy.

The study these news stories are based on, “Increased Plasma Levels of Chemoresistance-Inducing Fatty Acid 16:4(n-3) After Consumption of Fish and Fish Oil,” was published online on April 2, 2015 by the journal JAMA Oncology.

While this research is early and was done mainly in mice, I’m going to talk to my patients about avoiding fish oil and certain fish while they’re being treated with chemotherapy. Let me tell you why.

Fish and fish oil supplements contain fatty acids such as omega-3 and omega-6 fatty acids. These polyunsaturated fats have been shown to lower LDL (“bad”) cholesterol and also may help keep triglyceride levels low. Triglycerides are a form of fat in your bloodstream. People with high triglyceride levels often have high total cholesterol, high LDL cholesterol, and low HDL (“good”) cholesterol. Studies have linked high triglyceride levels to increased risk of stroke and heart disease. So many people take fish oil supplements to keep their risk of heart disease and stroke as low as it can be. I also know that many people diagnosed with cancer who are being treated with chemotherapy take fish oil supplements to ease chemotherapy side effects.

Still, in 2011, the same researchers who did the study in JAMA Oncology found that certain stem cells in mice with cancer also produced fatty acids called PIFA 16:4(n-3). These fatty acids cancelled out the effects of chemotherapy. So the researchers wondered if the same thing was happening in people with cancer who took fish oil supplement or ate certain fatty fish because this same fatty acid is in fish.

In the JAMA Oncology study, the researchers first looked at how much PIFA 16:4(n-3) there was in common fish oil supplements. The researchers said the amount was “relevant,” which means there was likely enough to have an effect on the level of PIFA 16:4(n-3) in people’s blood.

So the researchers asked 30 healthy volunteers to take the recommended dose of fish oil (10 ml per day — about 2 teaspoons) for 8 months. They also asked 20 healthy volunteers to eat either salmon, tuna, herring, or mackerel.

The results showed that the volunteers had higher blood levels of PIFA 16:4(n-3) — about double the normal level — for several hours after taking the supplements. There were similar rises in PIFA 16:4(n-3) blood levels for the volunteers who ate mackerel and herring. Salmon and tuna didn’t really seem to have an effect on the fatty acid levels.

So we know that fish oil supplements and mackerel and herring raise blood levels of PIFA 116:4(n-3). But does this have an effect on how chemotherapy works in people? That’s what we don’t know absolutely for sure. We can make an educated guess based on what happened in the mice, but people aren’t mice and not everything that happens in a mouse study happened in people.

So it’s important to know that the researchers haven’t tested the effects of fish and fish oil on chemotherapy in people diagnosed with cancer. Because the studies in mice showed that the fish oil could dramatically reduce the benefits of chemotherapy, the researchers felt strongly that it would be unethical to conduct an experiment on people; it would mean that patients would experience risk without justification.

While this evidence could be stronger, I’m going to do what the lead author of the study, Emile Voest, M.D., Ph.D., is doing: advising my patients to avoid fish oil, mackerel, and herring the day before, the day of, and the day after receiving chemotherapy. That schedule seems to allow people to get the benefits of fish oil but reduce the risk of it having any effect on chemotherapy.

Has anyone else talked to their doctors about fish oil and chemotherapy?

Brian S. Wojciechowski, M.D. joined the Breastcancer.org team as medical adviser in July 2012. He specializes in the care of patients with cancer and practices medical oncology in Delaware County, Pennsylvania at Riddle, Taylor, and Crozer Hospitals. A proud native of South Philadelphia, he trained at Temple University School of Medicine and Lankenau Medical Center. His research has been presented at the San Antonio Breast Cancer Symposium, the world's largest scientific meeting on breast cancer. Dr. Wojciechowski is a sought-after speaker on the topics of medical ethics and the biology of cancer. Beyond medicine, he is devoted to his faith, his family, and his guitar. He sees cancer as a scientifically complex disease with unique psychological, social, and spiritual dimensions, a perspective he takes into relationships with patients.

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