Since the intestine is primarily an absorptive organ, it must
have the means of exporting newly absorbed lipids. The
enterocyte re-esterifies fatty acids and monoglycerides to
form triglycerides and phospholipids. Absorbed cholesterol
is esterified to form cholesterol esters. Under ordinary
circumstances, the vast majority of the core lipids
in the chylomicron are triglycerides; however, after a
cholesterol-rich meal, the intestine manufactures cholesterol ester-rich chylomicron particles. Triglycerides and
cholesterol esters are then packaged into the core of chylomicrons,
which are secreted into the lymphatics. By secreting
chylomicrons into the lymphatics, they gain entrance
into the general circulation via the thoracic duct.
This guarantees that extrahepatic tissues, principally adipose
tissue and muscle, are the first to be exposed to the
newly secreted chylomicrons—if chylomicrons were secreted
directly into the bloodstream, they would first be
delivered to the liver via the portal vein.
TABLE I Buoyant Density and Size of Plasma
Chylomicrons are very large (up to 1 μm in diameter).
Thus a plasma sample containing chylomicrons is milky
in appearance. A sample of fasting plasma is typically
clear in appearance, even if there is an elevation in LDL
particles—LDL particles are not large enough to scatter
In terms of net transport, the bulk of lipid flux is triglyceride →
adipose tissue and muscle. How is the triglyceride transferred from the chylomicron particle to these tissues?
As was the case with the transfer of lipids from the intestinal
lumen to the intestinal epithelial cells, this transfer
begins with a lipolytic reaction to convert an oily lipid,
triglyceride, into an amphipathic lipid, free fatty acid. In
this case, the reaction is catalyzed by