Does a threshold for the effect of dietary omega-3 fatty acids on the fatty acid composition of nuclear envelope phospholipids exist?

Existence of a dietary maximal level or threshold for incorporation of omega 3 fatty acids into membrane phospholipids is of interest as it may further define understanding of the dietary requirement for omega 3 fatty acids. To test whether feeding increasing levels of dietary omega 3 fatty acids continues to increase membrane omega 3 fatty acid content, weanling rats were fed a nutritionally adequate semipurified diet which provided increasing amounts of C20 and C22 omega 3 fatty acids, such as 20:5 omega 3 and 22:6 omega 3. Dietary 20:5 omega 3 and 22:6 omega 3 were provided by substituting a purified shark oil concentrate of high 22:6 omega 3 content for safflower oil high in 18:2 omega 6. After four weeks of feeding, nuclear envelopes from four animals in each diet group were prepared, lipid was extracted and phospholipids separated. Arachidonic acid content in membrane phosphatidylcholine, phosphatidylethanolamine, phosphatidylinositol and phosphatidylserine was significantly reduced by feeding increased dietary levels of omega 3 fatty acids. Decline of 20:4 omega 6 level in phospholipid tended to stabilize when the dietary content of total omega 3 fatty acids reached 4-5% of total fatty acids. Above this level, dietary omega 3 fatty acids did not result in a further decrease in membrane content of 20:4 omega 6. Increase in membrane phospholipid content of 20:5 omega 3 occurred as the dietary intake of omega 3 fatty acids increased from 1.1% to 5% of total fatty acids.(ABSTRACT TRUNCATED AT 250 WORDS)