The effect of very low levels of dietary long-chain n−3 fatty acids on Δ6 desaturation of linoleic acid (18∶2n−6) and α-linolenic acid (18∶3n−3), and on Δ5 desaturation of dihomo-γ-linolenic acid (20∶3n−6), in liver microsomes and its influence on tissue fatty acids were examined in obese and lean Zucker rats and in Wistar rats. ANIMALS fed for 12 wk a balanced diet containing ca. 200 mg of long-chain polyunsaturated n−3 fatty acids per 100 G of diet were compared to those fed the same AMOUNT of α-linoleic acid. Low amounts of long-chain n−3 fatty acids greatly inhibited Δ6 desaturation of 18∶2n−6 and Δ5 desaturation of 20∶3n−6, while Δ6 desaturation of 18∶3n−3 was not inhibited in Zucker rats and was even stimulated in Wistar rats. INHIBITION of the biosynthesis of long-chain n−6 fatty acids was reflected in a decrease in arachidonic acid (20∶4n−6) content of serum lipids when fasting, and also in the phospholipid fatty acids of liver microsomes. On the contrary, heart and kidney phospholipids did not develop any decrease in 20∶4n−6 during fish OIL ingestion. Docosahexaenoic acid (22∶6n−3), present in the dietary fish oil, was increased in serum lipids and in liver microsome, heart, and kidney phospholipids.

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