Micelles lipid digestion

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). 

Abnormalities of Lipid Digestion Due to Impaired Micelle Formation Leading to Decreased Lipolysis... 

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."... 

Dietary retinyl esters must be hydrolyzed in the lumen of the small intestine before retinol is absorbed, while carotenoids must be absorbed into the intestinal mucosa before being cleaved intracel-lularly. Several enzymes with retinyl ester hydrolase (REH) activity are present in pancreatic juice or on the brush border of duodenal and jujenal enterocytes (Figure 3). Retinol and carotenoids must be solubilized in the lumen in mixed micelles composed of bile acids and products of lipid digestion prior to their uptake into enterocytes. These processes require the release of an adequate amount of bile... 

With few exceptions, small particles of vegetable foods are generally stripped of their more accessible nutrients during digestion in the GI tract. In this way starch, protein, fat and water-soluble small components (sugars, minerals) are usually well absorbed. This is not always the case, however, for larger food particles or for molecules that cannot diffuse out of the celF tissue. Neither is it the case for the lipid-soluble components. These need to be dissolved in lipid before they can be physically removed from the cell to the absorptive surface, since the cell wall is unlikely to be permeable to lipid emulsions or micelles, and the presence of lipases will strip away the solvating lipid. 

The bioaccessibility of a compound can be defined as the result of complex processes occurring in the lumen of the gut to transfer the compound from a non-digested form into a potentially absorbable form. For carotenoids, these different processes include the disruption of the food matrix, the disruption of molecular linkage, the uptake in lipid droplets, and finally the formation and uptake in micelles. Thus, the bioaccessibility of carotenoids and other lipophilic pigments from foods can be characterized by the efficiency of their incorporation into the micellar fraction in the gut. The fate of a compound from its presence in food to its absorbable form is affected by many factors that must be known in order to understand and predict the efficiency of a compound s bioaccessibility and bioavailability from a certain meal. ... 

A and B are in cis position relative to each other (see p. 54). One to three hydroxyl groups (in a position) are found in the steroid core at positions 3, 7, and 12. Bile acids keep bile cholesterol in a soluble state as micelles and promote the digestion of lipids in the intestine (see p.270). Cholic add and cheno-deoxychoMc acid are primary bile acids that are formed by the liver. Their dehydroxylation at C-7 by microorganisms from the intestinal flora gives rise to the secondary bile acids lithocholic acid and deoxycholic acid. 

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