BILE ACIDS & LIPIDS ABSORPTION

Bile Acids & Lipids Absorption



Abstract

Bile acids originate from the liver and are transported via bile to the intestines where they perform an important role in the absorption of lipids and lipid-soluble nutrients. Most of the bile acids are reclaimed from the terminal ileum and returned to the liver via portal blood for reuse. The transport of bile acids is vectorial in both liver and intestinal cells, originating and terminating at opposite poles. Bile acids enter through the basolateral pole in liver cells, and through the apical pole in intestinal cells. During the past decade, much has been learned about the mechanisms by which bile acids enter and exit liver and intestinal cells. By contrast, the mechanisms by which bile acids are transported across cells remain poorly understood. The current body of evidence suggests that bile acids do not traverse the cell by vesicular transport. Although a carrier-mediated mechanism is a likely alternative, only a handful of intracellular proteins capable of binding bile acids have been described. The significance of these proteins in the intracellular transport of bile acids remains to be tested.

Bile Acids & Lipids Absorption

Introduction

Bile acids are amphipathic molecules allowing them to interact with both lipids and the aqueous environment. Synthesis of bile acids from cholesterol involves hydroxylation and saturation of the sterol ring, epimerization of the 3ß-hydroxyl group, trimming of the cholesterol side chain and conjugation to either taurine or glycine. There is structural diversity among the bile acids found in the enterohepatic circulation. The major species of bile acid produced by the liver (termed primary bile acids) are taurine and glycine conjugates of cholic acid (a 3a, 7a-dihydroxy bile acid) and chenodeoxycholic acid (a 3a,7a, 12a-trihydroxy bile acid). These primary bile acids undergo further structural modifications characterized by deconjugation, dehydroxylation, sulfation, and glucuronidation. The resulting bile acid metabolites (known as secondary and tertiary bile acids) are eventually excreted out of the body.

A specific sodium-dependent active transport system for conjugated and unconjugated bile salts is located in the distal half of the small intestine. Considerations of physical-chemical factors suggest that passive diffusion of unconjugated bile salts and possibly of glycine-conjugated bile salts also occurs throughout the small bowel and colon. Studies in animals designed to assess the extent of this passive absorption have been complicated by species variation.

These considerations have prompted questions in normal man, dealing with the physiologic significance of the ileal transport system in the maintenance of enterohepatic bile salt circulation, and the relative importance of other intestinal sites and mechanisms for bile salt absorption. (Axelson, 1998)

Discussion

Hydrolysis of conjugated bile salts occurs regularly during their enterohepatic circulation in normal man, resulting in unconjugated primary and secondary bile salts that can be absorbed by passive processes in regions other than the ileum. The site and extent of intestinal absorption of unconjugated bile salts is dependent upon the presence of micoorganisms capable of deconjugating bile salts and a luminal milieu compatible with these enzymatic ...