What is the role of micellar solubilization for intestinal lipid absorption

The work of Hofmann and Borgstrbm [45] showed that lipids in luminal contents during digestion of a fatty meal could be separated into an oil phase and an aqueous micellar phase. It became evident that micellar solubilization of lipolytic products is a significant process, and its possible effect on transport of lipids across the brush border membrane has been an area of intense research since. 

The efficiency of lipid transport of micellar compared to monomer solutions [69] 

A detailed investigation of the effect of micellar solubilization on the transport of lipids has been made by Westergaard and Dietschy [59]. They studied in vitro uptake of lipids into rabbit intestinal disks by varying the proportions of lipid and bile salts in mixed micelles in 3 different ways. Either lipid concentration was increased with bile salts kept at a constant level, lipid concentration was unchanged while bile salt concentration was varied, or both lipid and bile salt concentration was increased with the molar ratio kept constant. Theoretical calculations of how the mass of the lipid probe was distributed between the aqueous and the micellar compartment showed that there was a good correlation between calculated aqueous monomer concentration and experimentally obtained values for lipid uptake. The rate of uptake is thus proportional to the aqueous monomer concentration of a particular lipid. The conclusion drawn was that diffusion of the lipid molecules in monomeric form through the aqueous phase is an obligatory step before uptake into the plasma membrane, and that the role of bile salt is therefore to overcome the resistance of the unstirred water layer by micellar solubilization. 

By comparing the relative rate of uptake of two different lipid probe molecules, another piece of evidence for a monomeric mechanism in lipid uptake by membranes has been found. Hoffman [70] and Hoffman and Yeoh [71] have determined the relationship between micellar concentration and uptake by rat small intestine in vitro for oleic acid and a monoglyceride analog, a 1-monoether. They found that the rate of uptake for both of the two micellar solutes was linearly dependent on the micellar concentration, but that the ratio of rate of uptake was different from the molar ratio of the two lipids in the micellar phase. Oleic acid was absorbed more rapidly than the monoether, probably due to a higher monomer concentration in the 

Different small intestinal sites of absorption for different micellar constituents have also been an argument for uptake of monomers instead of micelles. Whereas the major part of the lipolytic products are absorbed in the proximal part of jejunum, the site of bile salt uptake has been said to be the distal ileum. Recent investigations of the quantitative role of different parts of the small intestine for bile salt absorption suggest that the role of the distal ileum has been overestimated. Sklan et al. [73] found that about 50% of bile salts of endogenous origin were absorbed in the proximal half of the rat small intestine in vivo. A similar investigation was performed by McCUntock and Shiau [74], who injected a bolus dose of bile salts into the jejunum of rats with a bile fistula. They found that 60% of taurocholate was absorbed before the bolus reached distal ileum. 

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What is the role of micellar solubilization for intestinal lipid absorption ... 

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