Lipid metabolism and FA composition in tissues of Eurasian perch Perca fluviatilis as influenced by dietary fats

Eurasian perch, Perca fluviatilis, were fed a semipurified fat-free diet for 4 wk, followed by a 16% feeding supplementation of either olive oil (OO), safflower oil (SO), linseed oil (LO), or cod liver oil (CLO) as the only lipid source in each diet for 10 wk. Significant reductions in total lipid of tissues were observed (31.4% in viscera, 66.7% in muscle, and 74.1% in liver) after feeding the fat-free diet. The SO-, LO-, and CLO-fed fish significantly increased lipid deposition in liver and viscera compared to fish fed the OO diet; however, muscle lipid levels were not significantly affected. Large amounts of dietary 18∶1n−9 were incorporated directly into tissue lipids when fish were fed the OO diet. The LO diet significantly elevated 18∶4n−3, 20∶5n−3, 22∶5n−3, and 22∶6n−3 in the liver compared to fish fed OO or SO diets, and the n−3/n−6 ratio was 16 times that of the SO group, with significantly high desaturation and elongation products of 18∶3n−3. These results suggest that Δ6 and Δ5 desaturases are highly active in Eurasian perch, and that the enzymes at this dietary n−3/n−6 ratio favor 18∶3n−3 over 18∶2n−6 as substrate. The SO diet significantly increased 18∶3n−6, 20∶3n−6, and 22∶5n−6 in the liver and significantly decreased EPA and DHA. This indicates that desaturation enzymes were not specifically favoring n−3 over n−6 acids in perch lipid metabolism, and that these elongation and desaturation enzymes were influenced by n−3 and n−6 FA content in the diet. The present study indicates that high tissue content of DHA in the muscle of Eurasian perch was attributable to the greater ability for n−3 acid bioconversion.