Lipids in Fish Fillet and Liver - A Comparison of Fatty Acid Compositions

1. "Non-polar" (triglyceride and steryl esters) and "polar" (phospholipids) lipid fractions were prepared from liver and fillet tissues from four fish species. The four species - catfish, plaice, redfish and wrasse - were all known to have fat depots in both liver and fillet. The livers had from 10 to 28 % fat, whereas the fillet had from 1.5 – 4 % fat. The last values are somewhat low. 2. Fatty acid methyl esters were prepared from the 16 lipid fractions and analysed by gas liquid chromatography. Hydrogenated esters were also analysed to ensure good chain length determinations. Identification of minor polyunsaturated fatty acids was not attempted. The results were compared and commented on. 3. All values for the acids 16:0, 20:5 and 22:6 were higher in the phospholipid fractions than in the triglyceride fractions. The average sums for the three acids were 57 % in the phospholipids and 33 % triglycerides. Correspondingly all values for the monoenoic acids 16:1, 18:1, 20:1 and 22:1 were lower in the phospholipid fractions than in the triglyceride fractions. The average totals for these acids were 21 % in the phospholipids and 44 % in the triglycerides. 4. The average fatty acid composition of the liver samples and the fillet samples were similar, and this study did not point to a typical "liver fat" as opposed to a "fillet fat" in fish. The small differences noted may relate to the much higher deposition of fat in livers than in fillets in the analysed samples. Generally, the present analyses showed a better similarity to the fatty acid composition of cod liver oil, than to herring oils and other industrial body oils. It is suggested that this points more to influences of feeding habits, pelagic versus bottom feeders, that is, different diets, than to species or organ differences of the fish. 5. Some minor differences in fatty acid compositions between the four fish species were noted. Of particular interest was the presence of up to 10 % arachidonic acid in some of the samples, probably derived from dietary linoleic acid from algae taken in by these bottom feeding fish species