IntroductionFish oil is rich in n-3 polyunsaturated fatty acids PUFA , such as eicosapentaenoic acid EPA and docosahexaenoic acid DHA . The intake of EPA and DHA has beneficial effects on human health, including anti-arrhythmic 1 , antiinflammatory 2 , and anti-hypertriglyceridemic 3 effects. In addition to EPA and DHA, fish oil is composed of uncommon n-3 PUFA such as 6,9,12,15-octadecatetraenoic acid C18:4n-3 and 4,7,10,13-hexadecatetraenoic acid C16:4n-3 4 , which are physiologically relevant. Previous studies reported that C18:4n-3 has inhibitory effects on platelet aggregation 5 and the metabolism of C18:4n-3 in cultured NIH-3T3 cells 6 , while C16:4n-3 and C18:4n-3 have been reported to inhibit the production of leukotriene B4, leukotriene C4, and 5-hydroxyeicosatetraenoic acid in mouse mast cells 7 . A trace amount of C16:4n-3 induced by platinum-based chemotherapy was resistant to antitumor activity in mice, and fish oil containing C16:4n-3 neutralized the antitumor activity of platinum-based chemotherapy in various mouse models 8 . Fish oils derived from sardine and herring contain approximately 1 to 3 of 6,9,12, 15-hexadecatetraenoic acid C16:4n-1, HDTA , an isomer of C16:4n-3 4 . HDTA is uniquely characterized by the pres-

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