Absorption of solubilized fat
The ilial or absorptive phase involves the transit of dietary fats from mixed micelles into the enterocyte. Although originally believed to be a purely passive process, dependent on factors such as the rate of gastric emptying, extent of mixing, and gut motility, the translocation of LCFAs and 2-MAG from the micelle into the enterocyte is now known to be assisted by the presence of FABPs within the cell membrane and the cell. These maintain a diffusion gradient down which LCFAs and MAGs can flow into the cell. FABPs have numerous roles within cells and specific- ity for different types of LCFAs. Thus, the absorption of LCFAs and 2-MAG derived from dietary TAGs occurs by facilitated diffusion via FABP, which increases membrane permeation and promotes cel-lular uptake of LCFAs and monoglycerides. An addi-tional factor that drives the diffusion gradient is the rapid re-esterification of LCFAs into 2-MAG and 2-MAG into TAGs within the enterocyte by the enzyme acyl-CoA-cholesterol acyltransferase (ACAT). The absorption of dietary TAGs in the small intestine is extremely efficient, with up to 90% being absorbed. Dietary cholesterol also associates within mixed micelles and is absorbed in a similar manner by spe-cific sterol-carrying proteins resident in the entero-cyte membrane. Thus, cholesterol is also absorbed by a protein-facilitated mechanism but, in contrast to dietary TAGs, only about 40% of dietary cholesterol will be absorbed directly.
Figure 6.6 Interruption of the entero-hepatic circulation. LDL, low-density lipoprotein;HMG-CoA,3-hydroxy-3-methyl-glutaryl-coenzyme A.
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