In addition to suspected cardiovascular benefits, fish oil supplementation has been proposed as a potential nutritional intervention against cachexia in patients with cancer.4

However, the new study suggests that during chemotherapy, such potential benefits should take a back seat to precaution over the potential for induced tumor chemotherapy resistance.

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The study authors caution patients to avoid fish oil during chemotherapy: “Based on our findings, and until further data become available, we advise patients to temporarily avoid fish oil from the day before chemotherapy until the day thereafter.”

The authors also point out the inconsistent chemical composition of fish oil supplements, which “may vary between supplements and sometimes even from batch to batch.”1

While labels typically make claims about fish oil supplements’ concentrations of total omega-3 fatty acid, eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA), they do not list levels of 16:4(n-3) or other fatty acids.

RELATED: Do Fish Oils Contribute to Prostate Cancer?

“It remains unclear which other biologically active molecules are present in the mixtures,” the coauthors noted.1

Researchers have long known that cancer diagnoses frequently prompt the use of dietary supplements believed by patients to improve their health and even to help fight tumor growth; women are three times more likely to use dietary supplements after a breast cancer diagnosis, for example.5

The new fish oil study is just the latest in a growing list of reports bolstering concerns that some of these dietary supplements might affect anticancer drug pharmacodynamics—and potentially, effective treatment.6-8

The study was funded by the Dutch Cancer Society.


  1. Daenen LGM, Cirkel GA, Houthuijzen JM, et al. Increased plasma levels of chemoresistance-inducing fatty acid 16:4(n-3) after consumption of fish and fish oil. JAMA Oncology. April 2, 2014. [Epub ahead of print] doi:10.1001/jamaoncol.2015.0388.
  2. Roodhart JML, Daenen LG, Stigter ECA, et al. Mesenchymal stem cells induce resistance to chemotherapy through the release of platinum-induced fatty acids. Cancer Cell. 2011;20:370-383.
  3. Houthuijzen JM, Daenen LG, Roodhart JML, et al. Lysophospholipids secreted by splenic macrophages induce chemotherapy resistance via interference with the DNA damage response. Nat Commun. 2014;5:5275.
  4. Giacosa A, Rondanelli M. Fish oil and treatment of cancer cachexia. Genes Nutr. 2008;3(1):25-28.
  5. Burnstein HJ, Gelber S, Guadagnoli E, Weeks JC. Use of alternative medicine by women with early-stage breast cancer. N Engl J Med. 1999;340(22):1733-1739.
  6. Alsanad SM, Williamson EM, Howard RL. Cancer patients at risk of herb/food supplement-drug interactions: a systematic review. Phytother Res. 2014;28(12):1749-1755.
  7. Haefeli WE, Carls A. Drug interactions with phytotherapeutics in oncology. Expert Opin Drug Metab Toxicol. 2014;10(3):359-377.
  8. Furlow B. Oncology Nurse Advisor. Supplement use with cancer treatment: helpful or harmful? Published February 26, 2014. Accessed April 1, 2015.