The Accepted Mechanism of Cholesterol-Lowering Effects
Previous studies, both in vitro and in vivo, have indicated that the underlying mechanisms for inhibition of absorption of dietary cholesterol in the small intestine can be attributed to two processes:
Binding and incorporation of cholesterol by bacterial cells
Suppression of bile acid reabsorption mediated by bacterial bile salt hydrolysis[25]
It is known that cholesterol levels in plasma and the consumption of diets rich in cholesterol are closely related to each other. It is important to determine, therefore, if organisms located in the intestine can assimilate some of the cholesterol ingested in the diet and make it unavailable for absorption into the blood. The cholesterol-lowering effect of probiotics has been partly attributed to their ability to bind cholesterol in the small intestine. In 1985, Gilliland et al. reported that certain Lactobacillus acidophilus strains could remove cholesterol when grown in culture medium under conditions similar to those found in the intestine.[26] Not only dividing cells, but nongrowing and dead Lactobacillius could also remove cholesterol from the medium.[27,28] The ability to grow in the presence of bile and to remove cholesterol from laboratory medium, however, varied considerably depending upon the Lactobacillus strain used.[29] These reports suggested that cholesterol could be removed by binding to the Lactobacillus surface and consequently, dietary cholesterol would be less available for absorption in the intestine. Liong and Shah indicated that this phenomenon in Lactobacilli may be caused by the incorporation of cholesterol into the cellular envelope and subsequent changes in the cellular membrane of the organism.[30]
Several studies have indicated that the mechanism for in vitro removal of cholesterol is linked to the bile salt hydrolase activity of probiotic strains.[27,31,32] Bile salt hydrolase is an enzyme that catalyses the deconjugation of bile salts to liberate free primary bile acids. Deconjugated bile salts are less soluble and less efficiently reabsorbed from the intestinal lumen than conjugated bile salts, therefore, increased amounts of free bile acids are excreted in the faeces.[31] Xiao et al. indicated that the administration of yoghurt drinks fermented by Bifidobacterium longum strain BL1 improved blood lipids in subjects who had total cholesterol concentrations of more than 240 mg/dl, and attributed this fact to the ability of this strain to deconjugate bile salts via strong bile salt hydrolase activities.[17]
Clin Lipidology. 2012;7(5):501-507. © 2012 Future Medicine Ltd.