Patients with cancer should avoid fish oil supplements and remove herring and mackerel from the menu during chemotherapy, according to authors of a new study.

Their research shows that consumption of these fish or fish oil supplements significantly increases healthy human volunteers’ plasma levels of a platinum-induced fatty acid known as 16:4[n-3], which has been implicated in cisplatin chemoresistance in preclinical animal studies.

The report is the latest indication that popular dietary supplements might impact cancer treatments.

One in five patients with cancer consume omega-3 fatty acid supplements, usually in the form of fish oil pills.1 But patients undergoing chemotherapy should avoid consuming fish oil, caution authors of a new study published in JAMA Oncology.

The researchers found that eating herring, mackerel, or fish oil supplements increased healthy human volunteers’ circulating plasma levels of the fatty acid (16:4[n-3]), which has been shown to induce chemoresistance in mice.1

“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,” reported lead study author Laura G.M. Daenen, MD, PhD, of the University Medical Center Utrecht in Utrecht, the Netherlands, and coauthors.1 “Ingestion of fish oil and even fish might be harmful during chemotherapy.”

Previously published preclinical studies by the same research team demonstrated that in mice, “minute quantities” of the platinum-induced fatty acid (PIFA) 16:4(n-3), found in fish oils, can induce immune-modulated chemotherapy resistance, and that fish oil can “neutralize” the antitumor activity of cisplatin chemotherapy.2,3

Those findings prompted the team’s analysis of PIFA content in fish oil supplements; after finding “relevant” concentrations of 16:4(n-3) in all fish oil samples tested, they undertook a study of plasma 16:4(n-3) levels in healthy volunteers who consumed the recommended daily amount (10 mL) of fish oil.1

Circulating 16:4(n-3) levels in those volunteers’ plasma reached up to 20 times their baseline levels.1 “Herring and mackerel contained high levels of 16:4(n-3) in contrast to salmon and tuna,” and also resulted in increased plasma 16:4(n-3) levels, the authors reported.1

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“We show that fish oil contains substantial levels of 16:4(n-3), a fatty acid with potent chemotherapy-negating effects in preclinical models, and that intake of low doses of fish oil interferes with chemotherapy activity in mice,” they reported.1

“Ingestion of the recommended daily amount of fish oil by healthy volunteers rapidly increased 16:4(n-3) plasma levels. Since low concentrations of 16:4(n-3) were still active in mice, and since 11% of patients undergoing cancer therapy in our center used omega-3 supplements (and reports in the literature indicate even more frequent use [by cancer patients generally]), these findings may have important clinical implications.”