Cancer patients undergoing chemotherapy should refrain from talking fish oil supplements or eating fatty fish while they are being treated with chemotherapy. A few years ago, the research group of Emile Voest from the Netherlands Cancer Institute discovered that fatty acids which are also present in fish oil can make tumors resistant to chemotherapy. The same research group now shows that these fatty acids are indeed released in the blood when people take fish oil supplements or eat certain types of fatty fish. These new results were published in the journal JAMA Oncology on April 2nd.

Cancer patients often use supplements next to their cancer treatment. But supplements that promise to improve your health might not always be beneficial. In 2011 the research group of cancer researcher and physician Emile Voest, who then worked at the University Medical Center Utrecht, published a notable study in the journal Cancer Cell. The scientists discovered that certain fatty acids can induce chemoresistance in cancer cells, even in low concentrations. These specific fatty acids were also found in fish oil supplements. At the time, a warning was sent out to cancer patients: please be careful with fish oil supplements during chemotherapy treatment. 

Fish oil supplements

In their 2011 study, the researchers did not check whether fatty acids from ingested fish oil are taken up by the bloodstream. This is necessary for the fatty acids to reach the tumor. Therefore, a follow-up study was performed by the team of Voest at the UMC Utrecht. Most of the work was done by Laura Daenen, who is also first author of the paper that is now published in JAMA Oncology. With the help of questionnaires the researchers discovered that eleven percent of patients undergoing chemotherapy took fish oil supplements. When they analyzed the contents of six commercially available fish oils, they discovered that all of these supplements contained fatty acid 16:4(n-3), which was previously found to cause chemoresistance in mouse models.

Experiments performed in mice showed that feeding the animals fish oil did indeed render their tumors chemoresistant. After ingesting fish oil, their tumors no longer responded to the treatment. When translated to the human situation, just 3 milliliters of fish oil would be enough to induce chemoresistance in human patients.

Fresh fish 

Next Daenen administered fish oil supplements to healthy human volunteers in the recommended daily dose. With 10 milliliters, this recommended daily dose is well above the dose necessary to induce chemoresistance according to the mouse experiments. The researchers saw that ingestion of the supplements indeed rapidly led to elevated blood levels of the fatty acid 16:4(n-3), above the concentration that resulted in chemoresistance in mice. Normalization of the blood levels was seen after approximately eight hours.

The researchers also gave the healthy volunteers fresh fish, to see whether this also had an effect on 16:4(n-3) levels in their blood. The subjects each ate a hundred grams of herring, mackerel, tuna or salmon. Herring and mackerel increased the blood levels of the fatty acids; salmon resulted in a short-lived peak, and eating tuna had no effect on 16:4(n-3) blood levels.


These new results add to the idea that it would be wise for cancer patients to avoid fish oil supplements and fatty fish, from the day before they start chemotherapy treatment until a day after. "Patients often take supplements with the best intentions. They believe there is no harm in using them. But our research shows that taking fish oil could do harm. So I would like to advice everyone: if you plan on taking supplements next to your cancer treatment, please always consult your oncologist.", Voest comments.

This study received funding from the Dutch Cancer Society.

Source: Netherlands Cancer Institute


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