DIFFERENTIAL-EFFECTS OF FISH OIL, SAFFLOWER OIL AND PALM OIL ON FATTY-ACID OXIDATION AND GLYCEROLIPID SYNTHESIS IN RAT-LIVER

Studies were conducted to explore the mechanisms by which dietary fish oil decreases hepatic triglyceride secretion. Forty-five rats (15/group) were fed purified diets containing 10% fat as either fish oil, safflower oil or palm oil for 10 d. Plasma triglyceride concentration was lowest in the fish oil-fed group followed by the groups fed safflower oil and palm oil. The liver's capacity to oxidize fatty acids was assessed by assays of mitochondrial and peroxisomal beta-oxidation pathways in whole homogenates. Additionally, key enzymatic activities in the biosynthesis of triglyceride (diacylglycerol acyltransferase, phosphatidate hydrolysis) and phosphatidylcholine (CTP:phosphocholine cytidylyltransferase) were assayed. Compared with those fed palm oil the fish oil-fed animals showed 25% greater mitochondrial beta-oxidation but this difference was not statistically significant (P = 0.1). Fish oil feeding led to 45% greater (P < 0.05) peroxisomal beta-oxidation. Diacylglycerol acyltransferase activity was unaffected by the type of dietary fat and slightly (13%) but significantly (P < 0.02) lower cytidylyltransferase activity due to fish oil feeding was observed. More strikingly, both fish oil and safflower oil diets significantly lowered phosphatidate hydrolysis by 37 and 22%, respectively, compared with the palm oil diet. This activity directly correlated (r = 0.68; P < 0.001) with plasma triglyceride concentration. Thus, dietary fish oil might suppress triglyceride secretion by decreasing glycerolipid synthesis, an effect mediated by changes in one or more enzymes involved in phosphatidate catabolism.