Nutrition and Metabolism of Lipids 93secreted by the palate, although its contribution to
lipolysis in adults is questionable and thought to be
more important in young suckling infants, in which
its release is stimulated by suckling and the presence
of milk. It is possible that this lingual lipase is carried
into the stomach, where it acts as a human gastric
lipase (HGL) that has been shown to degrade up to
10% of ingested fat. Although these early products of
fat digestion, fatty acids and monoacylglycerols, rep-
resent a relatively minor component of fat digested,
their entry into the duodenum is believed to supply a
major stimulus for the production of the hormone
cholecystokinin (CCK), which inhibits gut motility.
The stomach serves mainly as an organ of mechan-
ical digestion and, by churning its contents, produces
a coarse creamy emulsion known as chyme. The cir-
cular pyloric sphincter muscle that separates the
stomach from the duodenum and, with other factors,
controls the rate of gastric emptying opens twice a
minute to release approximately 3 ml of chyme. Since
emulsifi ed fat in chyme is less dense than the aqueous
material, the two fractions separate with the fat col-
lecting above the aqueous layer. As a result, the entry
of emulsifi ed fat into the duodenum is delayed, allow-
ing suffi cient time for the minor breakdown products
to act on CCK.
The duodenal phase involves the breakdown of the
emulsifi ed fat by a process known as lipolysis and the
solubilization of the products of lipolysis. The entry
of chyme containing minor lipolytic products into
the duodenum stimulates the:
● release of CCK, which inhibits gut motility
● secretion of bile acids from the gall bladder
● release of pancreatic juice containing a battery of
lipases.
Lipolysis is an enzyme-catalyzed hydrolysis that
releases fatty acids from lipids (TAGs, phospholipids,
and CEs). It involves the hydrolytic cleavage of bonds
between a fatty acid and the glycerol backbone of
TAGs and phospholipids, and cholesterol in CEs, and
occurs not only in the digestive tract but also in cir-
culating and intracellular lipids (Figure 6.4). The
hydrolysis of emulsifi ed dietary fat entering the duo-
denum is catalyzed by a battery of pancreatic enzymes
including a pancreatic lipase that acts chiefl y on TAG
and phospholipase A 2 and a cholesterol ester hydro-
lase acting on phospholipids and CEs. The hydrolysis
of TAG by pancreatic lipase occurs in a sequential
fashion with the initial removal of a fatty acid from
position 1 and then position 3 from the glycerol back-
bone, generating a 2,3-diacylglycerol, followed by a
2-monoacylglycerol (2-MAG).Solubilization of emulsifi ed fat
With the notable exceptions mentioned previously
(Section 6.2), fats are insoluble in water and must be
rendered soluble before they can be absorbed in the
gut and transported within cells and in the circula-
tion. In each of these situations, this is achieved by
the hydrophobic fat or lipid associating with mole-
cules that are capable of interfacing with both hydro-
phobic and hydrophilic environments. Molecules
with these characteristics are called amphipathic mol-
ecules, examples of which are phospholipids, bile
salts, and specialized proteins known as apoproteins
(Figure 6.5). In the small intestine emulsifi ed fats are
solubilized by associating with bile salts produced in
the liver and stored and released from the gallbladder,
and phospholipids to form complex aggregates known
as mixed micelles. Lipids within cells and in the cir-
culation are solubilized by combining with specifi c
proteins known as fatty acid-binding proteins (FABPs)
and apolipoproteins (ApoA, B, C, E), respectively.
Further details of the structure and function of these
specialized proteins are given in Section 6.5.
The action of pancreatic lipase on TAG yields free
fatty acids and 2-MAG. Fatty acids of short- and
medium-chain length (≤14 carbons) are absorbed
directly into the portal circulation with free glycerol
and transported bound to albumin to the liver, where
they are rapidly oxidized. In contrast, long-chain fatty
acids (LCFAs; >14 carbons) associate with bile salts in
bile juice from the gallbladder and are absorbed into
the enterocyte for further processing and packaging
into transport lipoproteins. The primary bile salts,
cholic and chenodeoxycholic acids, are produced
from cholesterol in the liver under the action of the
rate-limiting enzyme 7-α-hydroxylase. These bile
salts act effectively as detergents, solubilizing lipids by
the formation of mixed micelles. These are spherical
associations of amphipathic molecules (with hydro-
phobic and hydrophilic regions) with a hydrophilic
surface of bile salts and phospholipids that encapsu-
lates a hydrophobic core of more insoluble LCFAs
and 2-MAG (see Figure 6.4). The micelle core will
also contain some lipid-soluble vitamins including
tocopherols and carotenoids. The formation of mixed