Fat Absorption and Transport

Dietary fat is hydrolyzed in the lumen of the small intestine (mostly by pancreatic lipase) to yield glycerol, free fatty acids, monoacylglycerols, and diacylglycerols. The hydrolysis products of this digestion are combined back into triacylglycerols (fats) in the endoplasmic reticula and Golgi complexes of the intestinal mucosa cells. Fats are combined with apoproteins to form chylomicrons, which transport the fats through blood and lymph (Figure 18.3). Chylomicrons are thus the transport vehicle for dietary cholesterol. Note that fats in chylomicrons are digested in capillaries (to produce chylomicron remnants), like the VLDLs described below. Free fatty acids are rarely found in the bloodstream. Rather, they are complexed to serum albumin.

The liver also plays an important role in fat metabolism. Fats synthesized in the liver are combined with another set of apoproteins to form very low density lipoproteins (VLDLs), which are hydrolyzed en route to peripheral tissues at the inner surface of capillaries.

Hydrolysis of fats in capillaries by lipoprotein lipase yields intermediate-density lipoproteins (IDLs) from VLDLs and chylomicron remnants from chylomicrons (Figure 18.7). IDLs are taken up by the liver and further processed to low-density lipoproteins (LDLs). LDLs are the primary form by which cholesterol is transported to tissues and high-density lipoproteins (HDLs) serve to transport cholesterol from tissues back to the liver.

See also: Bile Salts, Pancreatic Lipase, Lipoprotein Lipase, Lipoproteins

INTERNET LINK: Lipid Transport