The major dietary lipid for human, animan and plant are triacylglycerols, sterols, and membrane phospholipids. Fatty acids may be oxidized to acetyl-CoA (β-oxidation) or esterified with glycerol, forming triacylglycerol (fat) as the body’s main fuel reserve. You can see the overview of fatty acid in Figure 1.
Figure 1-Overview of fatty acid metabolism showing the major pathways and end products. Ketone bodies comprise the substances acetoacetate, 3-hydroxybutyrate, and acetone.
Lipids in the diet (Figure 2) are mainly triacylglycerol and are hydrolyzed to monoacylglycerols (MG) and fatty acids in the gut, then reesterified in the intestinal mucosa. The triacylglycerols are packaged with protein and secreted into the lymphatic system and thence into the blood stream as chylomicrons, the largest of the plasma lipoproteins.
Chylomicrons also contain other lipidsoluble nutrients, eg, vitamins. Unlike glucose and amino acids, chylomicron triacylglycerol is not taken up directly by the liver. It is first metabolized by tissues that have lipoprotein lipase, which hydrolyzes the triacylglycerol, releasing fatty acids that are incorporated into tissue lipids or oxidized as fuel. The other major source of long-chain fatty acid is synthesis (lipogenesis) from carbohydrate, mainly in adipose tissue and the liver.
Adipose tissue triacylglycerol is the main fuel reserve of the body. On hydrolysis (lipolysis) free fatty acids are released into the circulation. These are taken up by most tissues (but not brain or erythrocytes) and esterified to acylglycerols or oxidized as a fuel. In the liver, triacylglycerol arising from lipogenesis, free fatty acids, and chylomicron remnantsis secreted into the circulation as very low density lipoprotein (VLDL). This triacylglycerol undergoes a fate similar to that of chylomicrons.
Partial oxidation of fatty acids in the liver leads to ketone body production (keto- genesis). Ketone bodies are transported to extrahepatic tissues, where they act as a fuel source in starvation.
Figure 2-Transport and fate of major lipid substrates and metabolites.
(FFA, free fatty acids; LPL, lipoprotein lipase; MG, monoacylglycerol; TG, triacylglycerol; VLDL, very low density lipoprotein.) [Source: Harper, 2003]Content of the new page
Figure 3-Chylomicron
