FIGURE 3.2 Triacylglycerols consist of a glycerol backbone with three fatty acid side chains (R). Removal of the fatty acid from the middle carbon would yield 1,3-diacylglycerol. Removal of the fatty acids from the two end carbons would yield 2-monoacylglycerol.

In addition to TAGs, the other dietary fat components consist largely of glycero-phospholipids and cholesterol esters. The glycerophospholipids (Figure 3.3) are primarily in the form of phosphatidylcholine (~70%) and phosphatidylethanolamine.22 Pancreatic phospholipase A2 hydrolyzes the ester-bond-linking fatty acids to the sn-2 position of phospholipids.23 The resulting lysophospholipids are absorbed and may be reesterified and packaged into intestinal lipoproteins in combination with cholesterol and TAGs. The cholesterol esters are hydrolyzed by cholesterol ester hydrolase to yield free fatty acids and cholesterol.24 The free fatty acids are readily absorbed, but the cholesterol needs to be incorporated into micelles with fatty acids, monoacylglycerols, lysophospholipids, and conjugated bile acids for it to be absorbed.

After absorption and repackaging, fat is distributed to the body by chylomicrons, which first enter the lymphatic circulation before making their way to the blood. In the capillary beds of muscle and adipose tissue, the chylomicrons come in contact with the enzyme lipoprotein lipase, which hydrolyzes fatty acids from the TAGs. The fatty acids may be oxidized by the tissues for energy, taken up by adipose tissue and reesterified into TAGs, or circulated back to the liver bound to albumin. The chylomicron remnant consists primarily of cholesterol esters and contains apolipo-protein E (apo E) on its surface. The remnant is cleared by the liver, which has receptors for apo E.

There is also an endogenous pathway in the body for the synthesis and distribution of lipids needed in energy metabolism. Saturated fatty acids may be synthesized in the liver from acetyl-CoA derived from glucose metabolism. These fatty acids may be stored as TAGs or packaged in very low-density lipoproteins (VLDLs) in combination with cholesterol, phospholipids, and lipoproteins. The TAG-enriched VLDL particles are released into the blood circulation, and the TAGs are utilized in the same manner as those from chylomicrons. As TAGs are removed, the VLDL particles transition first to an intermediate-density lipoprotein, and then to low-density lipoprotein (LDL) particles. The LDL particles contain apolipoprotein B-100 (apo B-100) on the surface, and this is also cleared via the liver, which has receptors for this ligand.

Fatty acids of all classes can ultimately be oxidized to carbon dioxide and water. However, each of the different dietary lipid classes has preferential routes of utilization and localization in different tissue compartments. The long-chain saturated fatty acids (LC-SFA > 16 carbon) may by oxidized in muscle mitochondria to supply

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