Introduction to Human Nutrition

94 Introduction to Human Nutrition

micelles increases the solubility of fat by 100- to 1000-
fold. They create an acidic microenvironment for the
lipid core which, through protonation, facilitates the
dissociation of LCFAs and 2-MAG from the micelle
and diffusion 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 fl ow into the cell.
FABPs have numerous roles within cells and specifi c-

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-esterifi cation 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 effi cient, with up to 90% being absorbed.

Dietary cholesterol also associates within mixed

micelles and is absorbed in a similar manner by spe-

cifi c 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.







Liver

Apoproteins

and

phospholipids

Cholesterol

esters

OH

OH

OH

OH

OH

Short- and medium-chain

fatty acids

Gall bladder

Bile acids

Thoracic duct Chylomicrons

lymph

Plasma

Mixed micelles

TAGs MAGs Fatty acids

Unabsorbed material

Dietary

fats

PL

Feces

Bile acid conjugates

Duodenum Jejunum Ileum

Hydrophobic lipid core

TAG

ACAT

Pancreatic

lipase

P P

Figure 6.4 Reception, emulsifi cation, lipolysis, solubilization, and absorption of fats. ACAT, acyl-CoA-cholesterol acyltransferase; MAG, mono-
acylglycerol; TAG, triacylglycerol; PL phospholipid; P, phosphate.