Micellar phase lipids during digestion

The mechanism for the inhibition of cholesterol absorption is thought to involve competitive transfer to the micellar phase during absorption from the intestinal lumen. Phytosterols in the micellar phase may also act as emulsifying agents that selectively inhibit the transfer of cholesterol and other lipids (e.g., carotenoids and vitamins) and, thereby, limit their absorption. The exact kinetics governing the sterol competition for transfer are not known, but dietary sterols are absorbed differently in the digestive tract 40-50% for cholesterol, 12-16% for campesterol, 4-5% sitosterol, and <0.5% for phytostanols (37). Before absorption, esterified sterols are hydrolysed effectively in the upper intestine (191). Absorbed phytosterols are excreted by the liver into the bile but are hardly converted to bile acids (192). Numerous studies in animals and humans approved the safety of phytosterols and phytostanols (37). 

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. 

It is interesting to note that during digestion of triglyceride oils, all the substances discussed above are present as well as the phases mentioned. This was nicely illustrated by Patton and Carey [10] in an in vitro study where the fate of a drop of soybean oil in simulated intestinal fluid was monitored under the microscope. The cubic phase is formed at the oil/water interface, and due to its bicontinuous structure it is capable of both delivering the water molecules necessary for hydrolysis and taking care of the resulting fatty acids. The role of lamellar and micellar phases is to act as carriers of lipids to the intestinal wall [11,12]. 

Fig. 15. Schematic representation of the physical and chemical states of lipids in intestinal content during fat digestion and absorption. An oil phase composed of higher glyceride and some nonionized fatty acid is in equilibrium with an aqueous micellar phase of bile acid, monoglyceride, nonionized fatty acids, and fatty acid soaps. Nonpolar lipids—e.g., fat-soluble vitamins,...

A. F. Hofmann, B. Borgstrom. Tlie intraluminal phase of fat digestion in man the lipid content of the micellar and oil phases of intestinal content during fat digestion and absorption. J Clin Invest 43 247-257. 1964,... 

A micellar phase is formed in the intestinal lumen when the bile salt concentration exceeds the critical micellar concentration (approximately 3-4 mM). This concentration of bile salts is usually exceeded during normal digestion. Mixed micelles contain bile salts, fatty acids, monoglycerides, cholesterol, and other lipid-soluble molecules (including fat-soluble vitamins) and are considered to be the major route of delivery of the products of fat digestion to the absorptive mucosal cell. Other nonmicellar phases may coexist in the intestinal lumen with the micellar phase these include an oil phase and a viscous isotropic phase. 

Bile is a mixed micellar solution of bile salt-lecithin-cholesterol which on dilution forms aggregates of much larger size than micelles indicating the formation at the phase limits of liposome-like bodies [9]. In intestinal content during lipid digestion in man, saturated mixed micelles and vesicles or liposomes containing the... 

Up till now intestinal lipid absorption from mixed micellar solutions has been studied in detail. The existence of non-micellar dispersions of lipolytic products in intestinal contents during fat digestion has recently been proposed [18,47], but so far only little information regarding the importance of these phases for the lipid... 

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See also in sourсe #XX -- [ ]

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