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, hépa-

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