Digestion absorption and transport of dietary fat

3.6.1 The plasma lipoproteins

The plasma lipoproteins are a family of globular proteins, each of which consists of a core of neutral lipid (predominantly triglyceride or cholesteryl ester) surrounded by a coat of phospholipid and protein. These particles can be divided into four broad categories: (1) chylomicrons, which primarily transport dietary triglyceride and cholesterol; (2) very-low-density lipoproteins (VLDL), which primarily transport triglycerides that have been synthesized in the liver;

and two lipoproteins that function primarily in the transport of endogenous cholesterol, namely (3) low-density lipoprotein (LDL) and (4) high-density lipoprotein (HDL). There is also an intermediate-density particle (IDL), having a density between that of VLDL and LDL. The IDL particles are very short-lived in the bloodstream, however, and seem to have little nutritional or physiological importance.

The protein components of the plasma lipoproteins are known as apolipoproteins. At least six different kinds of apolipoprotein have been identified in the intestinal lymph of humans: apoA, apoB (three different molecular sizes), apoC and apoE.

3.6.2 Digestion, absorption and transport

Absorption of the fat-soluble vitamins takes place mainly in the proximal jejunum and depends on the proper functioning of the digestion and absorption of dietary fat. The fat content of a typical Western diet is composed mainly of triglycerides accompanied by smaller amounts of phospholipids and sterols. The efficiency of absorption of fat-soluble vitamins parallels that of fat absorption and is affected by the nature of the lipid component of the diet.

The stomach is the major site for emulsification of dietary fat. The coarse lipid emulsion, on entering the duodenum, is emulsified into smaller globules by the detergent action of bile salts aided by the churning action of the intestine. The adsorption of bile salts on to the surface of the fat globules increases the lipolytic activity of pancreatic lipase, which hydrolyses triglycerides at the 1 and 3 positions and yields 2-monoglyceride and free fatty acids (Fig. 3.26). Secretion of bicarbonate from the pancreas and the biliary tract is needed to neutralize the pH to the optimal range for lipolysis. A cofactor called coli-pase is present in pancreatic juice and is required for lipase activity when bile salt is present. During their detergent action, bile salts exist in a monomolecular solution. Above a critical concentration of bile salts, the bile constituents (bile salts, phospholipid and cho-

CH2OCOR

CHOCOR2 I

CH2OCOR;

Triglyceride

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