Reversibility of the changes induced by n-3 fatty acids in mouse plasma, liver and blood cell lipids

The changes induced by dietary n-3 fatty acids (FA) in the lipids and FA of plasma, liver and blood cells, and their reversibility, was studied in mice given a diet containing 9% fish oil (FO) for 2 weeks and then returned to, and kept for another 2 weeks on, the usual standard lab chow diet. In plasma, the concentrations of phospholipids (PL), mostly phosphatidylcholine (PC), triacylglycerols (TG), cholesterol and cholesterol esters (CE) decreased rapidly after starting the FO diet, and remained low from day 3 onwards. This decrease was concomitant with a remarkable reduction in the n-6 FA, especially 18:2n-6, not compensated for by the relative enrichment in n-3 FA induced by FO. In liver, TG and CE decreased and PL slightly increased, all of them showing reduced n-6/n-3 ratios. Sphingomyelin, which lacks polyunsaturated FA other than small amounts of 18:2 and 24:2n-6, showed altered ratios between its very long chain monoenes and saturates. In the washout phase, the most rapid event was an immediate increase in 18:2n-6 and after a few days in 20:4n-6 in plasma and liver, where most of the lipid and FA changes were reversed completely in about 10 days. In the case of blood cells even 2 weeks were insufficient for a reversal to the initial n-6/n-3 ratios. The lipid class responsible for this lack of reversibility was phosphatidylethanolamine, PC having returned to the initial fatty acid composition during the stated period.