Consuming herring, mackerel, or certain dietary supplements containing fish oils can induce resistance to chemotherapy and limit the impact treatment has on tumor growth, according to a study published online in JAMA Oncology.

The study authors, including Emile E. Voest, MD, PhD, of the Netherlands Cancer Institute, Amsterdam, and colleagues, had conducted prior preclinical studies that demonstrated the effects of the fatty acid 16:4(n-3). In picomolar quantities, this fatty acid can activate splenic macrophages that lead to chemotherapy resistance, according to the authors.

The current study had three objectives: survey the use of fish oil supplements among cancer treatment patients; evaluate the levels of 16:4(n-3) in three brands of fish oil and four species of fish; and study plasma levels of 16:4(n-3) in a group of healthy volunteers after consuming fish oil or fish.

A total of 118 cancer patients at the Netherlands Cancer Institute responded to the survey about nutritional supplement use. Results showed 30 percent regularly used supplements and 11 percent specifically used supplements containing omega-3 fatty acids.

All of the fish oils tested contained relevant amounts of 16:4(n-3). In the study of whole fish, herring and mackerel were shown to contain high levels of the fatty acid compared with salmon and tuna, which had minimal or no effect on blood levels of fatty acid, according to the authors.

The study of healthy volunteers—30 of whom were given fish oil and 20 of whom were given whole fish to eat—showed increased blood levels of 16:4(n-3) after consumption. For those who had the recommended daily amount of 10 mL of fish oil, the blood levels normalized after eight hours, while 100 grams of herring and mackerel were enough to demonstrate increased levels of the fatty acid.

“Our results add to the growing awareness that not all dietary supplements are beneficial or harmless: some may interfere with treatment outcome,” wore Voest and colleagues.

They advised patients to follow the lead of the Dutch Cancer Society, which recommends temporarily avoiding fish oil from the day prior to chemotherapy till the day after. “Although further evidence on the relation between fish consumption and chemotherapy activity is desired, we would currently also recommend to avoid herring and mackerel in the 48 hours surrounding chemotherapy exposure.”