Digestion of Lipids

Emulsification of Fats
Fatty acids can be liberated by
simple hydrolysis of the ester bonds
in triglycerides, but the
insolubility of the trigly
cerides presents a problem
; digestion
occurs following dispersion of dietary fat into
small particles with
sufficiently exposed
surface area for rapid attack by digestive enzy
mes. This is achieved by detergent action
and mechanical mixing, with the
detergent effect
being supplied by several components,
both in the diet and in the dige
stive juices, but esp
ecially by partially digested fats (fatty
acid soaps and monacylglycerols) and by
bile salts
The bile salts such as cholic acid contain a hydrophobic side and a
hydrophilic side
, thus allowing bile salts to dissolve
at an oil-water interface, with the
hydrophobic surface in contact with the nonpolar
phase and the hydrophilic surface in the
aqueous medium. This detergent action
emulsifies fats and yields mixed micelles,
which allow attack by wat
er-soluble digestive enzymes
and facilitate the absorption of
lipids through the intestinal mucosa.
Mixed Micelles also serve as transport vehicles
for those lipids that are less water-soluble than
fatty acids, such as
cholesterol or the
fat-soluble vitamins A, D, E, and K.
Thus, efficient absorption of lipids depends on the
presence of sufficient bile acids to
solubilize the ingested lipids.
2.
Digestion of Fats
.
The emulsification of fats rende
r them susceptible to hydrolysis by
enzymes secreted by the pancreas. Th
e most important enzyme involved is
pancreatic
lipase
.
Pancreatic lipase is virtually specific for
the hydrolysis of pr
imary ester linkages,
the 1 or the 3 ester bonds, but not the bond in
the central 2 position (see below). As a
result of this conversion,
2-monoglycerides (2-monoacylglycerols) are major end-
products of triglyceride digestion.
Less than 10% of trigly
cerides remain unhydrolyzed
in the intestine.