504
chapter
22
Metabolic Homeostasis
TG
I Lipoprotein
^
lipase
FA
ADIPOSE
TISSUE
F I G U R E 2 2 - 1 8
Triacylglycerol storage in high-fat and high-carbohydrate diets. FA, Fatty acid; TG, triacylglycerol.
supported by amino acids from 75 g of protein and 16 g
of glycerol.
22.6 Lipid Homeostasis
The two principal types of lipid in fuel homeostasis are
triacylglycerols and fatty acids. Other lipids serve roles
in cellular membrane structure and other functions. Suffi-
cient triacylglycerol normally is stored in humans to sup-
port many weeks of fasting (Table 22-2). Triacylglycerols
constitute a very compact form of energy and their com-
plete oxidation yields a high amount of energy. They are
transported between organs as components of lipoproteins.
Free fatty acids are amphiphilic and act as a detergent. For
interorgan transport, fatty acids are bound to albumin.
Lipid Digestion and Absorption
Dietary lipid consists mostly of triacylglycerol from plant
and animal sources. It supplies about 45% of the energy
in a typical Western diet. Except for the essential polyene
fatty acids (Chapter 18), the dietary requirement for lipid
can be met by carbohydrate or protein. However, the
Eskimo has a satisfactory diet consisting of 80-90% lipid.
Digestion of triacylglycerol is discussed in Chapter 12. A
small proportion of the free fatty acids, notably the short-
chain acids in milk and milk products, is absorbed directly
into the portal blood and transported to the liver. The bulk
of the fatty acids are esterified in intestinal cells into tria-
cylglycerol and packaged within the Golgi apparatus into
chylomicrons that contain triacylglycerol, phospholipids,
cholesterol and cholesterol esters, and apoproteins. Chy-
lomicrons are secreted into lymphatic capillaries that sur-
round the intestinal cells and fuse into larger vessels to
form the thoracic duct, which opens into the systemic cir-
culation by a one-way valve at the junction of the left
jugular and subclavian veins. Chylomicrons appear in the
plasma shortly after a fatty meal and reach a maximum
concentration within 4-5 hours. Their large size results
in light scattering and gives plasma a turbid appearance
termed
postalimentary lipemia.
Disposition of Absorbed Triacylglycerol
In various tissues of the body, chylomicrons are acted upon
by lipoprotein lipase that hydrolyzes the associated tria-
cylglycerol. This lipase is secreted by adipocytes, h├ępa-
tocytes, and cardiac and mammary tissue, and it becomes
associated with the external surface of endothelial cells of
the vasculature in response to the need of these tissues for
fatty acids. Its half-life is a matter of hours. Its synthesis
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