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Chapter: Introduction to Human Nutrition: Nutrition and Metabolism of Lipids

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Absorption of solubilized fat

Absorption of solubilized fat
The ilial or absorptive phase involves the transit of dietary fats from mixed micelles into the enterocyte.

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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