Mixed micelles lipid digestion

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. 

C. These products of lipid digestion combine to form mixed micelles, which are taken up efficiently by enterocytes. 

The products of lipid digestion—free fatty acids, 2-monoacylglycerol, and cholesterol—plus bile salts, form mixed micelles that are able to cross the unstirred water layer on the surface of the brush border membrane. Individual lipids enter the intestinal mucosal cell cytosol. 

Solubilization of lipid digestion products in intestinal mixed micelles enhances their dissolution and dramatically increases the GI lumen-enterocyte concentration gradient that drives absorption by means of passive diffusion. Micelles, however, are not absorbed intact [8, 9], and lipids are thought to be absorbed from a monomolecular intermicellar phase in equilibrium with the intestinal micellar phase [10], The dissociation of monomolecular lipid from the micellar phase appears to be stimulated by the presence of an acidic microclimate associated with the enterocyte surface [11,12], In addition to passive diffusion, growing evidence suggests that active uptake processes mediated by transport systems located in the enterocyte membrane are also involved in the absorption of (in particular) fatty acids into the enterocyte [4],... 

This could include retinol, cholecalciferol/ergocalciferol, a-tocopherol, vitamin K from food, all of which are more readily absorbed when they can be part of a normal mixed micelle process that occurs with lipid digestion and absorption... 

The formed mixed micelles can diffuse to the unstirred water layer that lines the epithelium, where the micelles disintegrate and lipid amphiphiles are ahsorhed. Bile salts are recycled hack into the lumen and continue to interact with lipid digestion. Thus at any given time during lipid digestion, a complex mixture of different colloid phases is present in the intestinal lumen (Rigler et al., 1986). 

Fig. I. Overall scheme for cholesterol balance across the intestinal epithelial cell. After digestion, lipids in the intestinal lumen combine with bile acids to form mixed micelles (MM) that promote uptake into the intestinal epithelial cell. Within the intestinal cell, triglyceride and cholesterol, along with specific apoproteins, are synthesized into the chylomicron (CM) which, ultimately, delivers much of the triglyceride to peripheral organs and most of the cholesterol to the liver. Also shown in this diagram are the 3 major sources for epithelial cell cholesterol including (1) uptake from the lumen, (2) synthesis from acetyl-CoA and (3) uptake of low-density lipoproteins (LDL) by both receptor-dependent and receptor-indqjendent mechanisms.

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... 

Stages of lipid digestion in the intestinal tract. Step 1 is the emulsification of fat droplets by bile salts. Step 2 is the hydrolysis of triglycerides in emulsified fat droplets into fatty acids and monoglycerides. Step 3 involves dissolving fatty acids and monoglycerides into micelles to produce "mixed micelles."... 

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