THE INFLUENCE OF DIETARY SATURATED AND UNSATURATED FAT ON HEPATIC CHOLESTEROL-METABOLISM AND THE BILIARY-EXCRETION OF CHYLOMICRON CHOLESTEROL IN THE RAT

The biliary excretion of [H-3] cholesterol carried in chylomicrons der ived from palm oil (rich in long chain saturated fatty acids), olive o il (rich in monounsaturated fatty acids) or corn oil (rich in n-6 poly unsaturated fatty acids was studied in vivo in rats fed the correspond ing oil in the diet for 21 days. The secretion of radioactivity into b ile as both bile acids and unesterified cholesterol was significantly slower in the animals fed palm oil as compared to those given olive or corn oil, indicating that dietary saturated fat retards the excretion of cholesterol from the diet as compared to mono-or n-6 polyunsaturat ed fat. In order to investigate the mechanisms underlying these differ ences, the influence of the three high fat diets on cholesterol esteri fication, cholesteryl ester hydrolysis and bile acid synthesis in the liver and on biliary lipid output were also measured. The ratio of cho lesterol esterification to cholesteryl ester hydrolysis was markedly r aised in the olive and corn oil-fed as compared to palm oil-fed animal s. Biliary cholesterol secretion was higher in corn oil-fed rats than in those fed olive or palm oil or a low fat diet, and this was associa ted with a markedly increased lithogenic index in these animals. The a ctivity of cholesterol 7 alpha hydroxylase was higher in the olive and corn oil-fed than in the palm oil-fed animals, although the expressio n of mRNA for the enzyme was increased only in the olive oil diet grou p. After 20 h biliary drainage, the rate of bile acid secretion into b ile was increased in the rats fed olive and corn oil rather than to pa lm oil. These findings indicate that feeding rats mono-or n-6 polyunsa turated as compared to saturated fat in the diet promotes the storage of cholesteryl ester in the liver and leads to increased bile acid syn thesis, resulting in the more rapid excretion of cholesterol originati ng from the diet via the bile. (C) 1998 Elsevier Science B.V.