In Reply Mazurak et al and Baracos provide a welcome discussion on a topic that is subject to debate: fish oil use in anticancer therapy and—in a broader context—the use of supplements by patients with cancer in general. It is becoming increasingly clear that use of certain supplements can impair the effectiveness of chemotherapy and other anticancer therapies. Examples are St John’s wort and betacarotene.
There is no discussion on the necessity of ω-3 fatty acids as essential parts of a healthful diet. These fatty acids are not produced by the body and have to be ingested from dietary sources. However, it is important to realize that fish oil is a complex and unstandardized mixture of fatty acids. Its production requires no Food and Drug Administration review or approval. The identity and function of a large part of ω-3 fatty acids present in fish oil remains unclear, and it might therefore contain unknown biologically active molecules. We have identified 16:4(n-3) as an example, which induces chemoresistance in preclinical tumor models.1,2 This effect is seen when minor quantities of 16:4(n-3) are administered, which is underscored by the calculations performed by Mazurak et al.
Daenen LGM, Voest EE. Let Them Eat Fish—Reply. JAMA Oncol. 2015;1(6):841. doi:10.1001/jamaoncol.2015.2056