Unstirred Water Layers in Intestine: Rate Determinant of Fatty Acid Absorption from Micellar Solutions

Abstract

Bile acid and fatty acid uptake from micellar solutions by intestinal cells fails to reflect the incremental free energy changes expected for permeation that is rate limited by cell membranes. However, altering the size of the diffulsing particle or the thickness of tle unstirred water layer does change uptake. These observations show that the unstirred water layer is rate limiting for intestinal absorption of lipids from micellar solutions.

Spontaneous lipid transfer between organized lipid assemblies

“…The majority of studies involving lipid uptake in the intestine have focused on fatty acid transfer from bile salt micelles to enterocytes [86][87][88][89][90]. However, the recent discovery that non-micellar phases such as unilamellar vesicles may also be formed during fat digestion may prompt further investigation [91][92].…”

“…However, the recent discovery that non-micellar phases such as unilamellar vesicles may also be formed during fat digestion may prompt further investigation [91][92]. In any case, Dietschy has argued that bile salt micelles enhance lipid uptake by acting as large capacity vehicles for moving fatty acids and other lipids across the unstirred water layers adjacent to enterocytes [89,90,93]. The micelles, by carrying large amounts of lipid, increase the effective lipid concentration to levels much higher than those achieved by aqueous solubilization of individual lipid monomers in the unstirred water layers.…”

“…The micelles, by carrying large amounts of lipid, increase the effective lipid concentration to levels much higher than those achieved by aqueous solubilization of individual lipid monomers in the unstirred water layers. Once positioned next to the enterocyte, uptake is thought to occur by desorption of lipid molecules from the bile salt micelles [89,90,[93][94][95]. More recent insights into how bile salts may greatly facilitate the rapid and complete transfer of phospholipid monomers from mixed micelles to the plasma membranes of enteroeytes in the intestinal muscosa have been reported by Nichols and colleagues [96][97][98] (see sections IV-A and IV-B for further details).…”

Intestinal transport and metabolism of bile acids

“…In the lumen of the small intestine, bile acids are present at micellar concentrations and form mixed micelles with dietary lipids and their digestion products such as monoacylglycerols and fatty acids ( 30-32 ). Bile acids also solubilize nonpolar lipids such as cholesterol and fat-soluble vitamins, increasing their water-solubility and promoting their diffusion across the unstirred water layer for delivery to the intestinal epithelium ( 33 ).…”

Untitled

“…The explanation of this apparent discrepancy may be found in the importance of the so-called unstirred layer in lipid absorption. The absorptive surface of the intestinal mucosa cell is considered to be covered by a layer of unstirred water through which the molecules to be absorbed have to pass by diffusion (69). I t is furthermore considered that the diffusion rate through this unstirred layer is related to the concentration of the lipid in the molecular dispersed phase and that transport of lipids in the absence of bile salt therefore is extremely slow.…”