Bile duct ligation was performed in germ-free and conventional rats in order to study the effects of bile deprivation on the absorption of dietary lipids and the excretion of faecal lipids in the presence or the absence of gastrointestinal flora. The main consequence of bile duct ligation in conventional rats was decrease of about 50% in the apparent absorption of dietary lipids (peanut oil). In germ-free rats, absorption decreased by only about 25%.In conventional as well as in germ-free controls, faecal lipids were mainly excreted as compounds directly soluble in organic solvents that is, free fatty acids, triglycerides, partial glycerides, cholesterol, cholesterol esters. Deprivation of bile secretion significantly increased the faecal excretion of 'insoluble' compounds-that is, calcium soaps-both in germ-free and conventional rats. Free fatty acids and sterol esters were the two main class of soluble faecal lipids both in germ-free and conventional rats deprived of bile secretion. Faecal excretion of triglycerides remained low in germ-free as well as in conventional animals. No significant difference of fatty acid absorption was observed between germ-free and conventional controls. Deprivation of bile secretion resulted in a significant decrease in the absorption of all fatty acids in germ-free as well as in conventional animals. However, the decrease was larger for saturated fatty acids-that is, 16:0 or 18:0- than for unsaturated fatty acids-that is, 18:1 or 18:2. The absorption of all fatty acids, except linoleic acid (18:2), was significantly lower in conventional rats artificially deprived of bile secretion than in their germ-free counterparts. Evidence was given for a negative digestive balance of stearic acid (18:0) in bile deprived conventional animals. This observation was correlated with a very efficient biohydrogenation of dietary unsaturated fatty acids as revealed by radio gas chromatography of faecal acids in bile deprived conventional rats fed a diet containing 1-14C oleic acid (18:1) as homogeneous triglycerides. Nevertheless, biohydrogenation of unsaturated dietary fatty acids by the gastrointestinal flora was not considered to be the only factor involved in the origin of the difference of fat absorption between bile deprived germ-free and conventional animals.
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