Dietary linolenic acid and longer-chain n-3 fatty acids: comparison of effects on arachidonic acid metabolism in rats.

Rats were fed graded amounts of purified 18:3n-3 or fish oil concentrate in the presence of a constant amount of 18:2n-6 to evaluate the ability of 18:3n-3 compared with longer-chain n-3 fatty acids to inhibit 20:4n-6 metabolism in platelets and lungs. Dietary 18:3n-3 at a ratio of 0.28 (n-3 to n-6 fatty acids) suppressed levels of 20:4n-6 in lung and plasma phospholipids and the capacity of the tissues to synthesize cyclooxygenase-derived products in a dose-dependent fashion. At similar ratios of n-3 to n-6 dietary fatty acids, longer-chain n-3 fatty acids, which are abundant in fish oil, appear to be more effective than 18:3n-3 in suppressing 20:4n-6 levels and the capacity of the tissues to synthesize cyclooxygenase-derived products. Much greater amounts of 12-hydroxyeicosapentaenoic acid (12-HEPE) and 5-HEPE than of 12-hydroxyeicosatetraenoic acid (12-HETE) and 5-HETE appeared to be formed in tissues of the group receiving the highest amount of fish oil. These results suggest that ingestion of fish oil leads to increased formation of lipoxygenase-derived products of longer-chain n-3 fatty acids.