Lipid metabolism

Lipids are body fats that are either synthesized within cells (endogenous lipids) or derived from dietary fat (exogenous lipids). They are typically characterized by their insolubility in water, and have a diverse range of biological functions in cell membranes as phospholipids, and as a major source of stored energy in adipose tissue as triacylglycerols (TAGs). Serum lipids, including TAGs, phospholipids, cholesterol and their component fatty acids, are transported between sites of synthesis in the liver and intestine to peripheral tissues, for utilization and storage in macromolecular complexes of lipid and protein called lipoproteins. The physiology of circulating serum lipoproteins is described in terms of the transport of exogenous and endogenous chylomicrons and very low-density lipoproteins, respectively. These TAG-rich lipoproteins are remodelled under the action of lipoprotein lipase and lipid transfer proteins into smaller chylomicron remnants and low-density lipoproteins (LDLs). These cholesterol-rich lipoproteins deliver cholesterol back to the liver and peripheral tissues via specific membrane receptors, but can promote atherosclerosis by depositing themselves and their cholesterol in artery walls. In contrast to this forward transport of cholesterol into tissues, reverse cholesterol transport describes the pathway which leads cholesterol away from peripheral tissues and back to the liver. This function is performed by high-density lipoproteins (HDLs), and confers protection against atherosclerosis. The metabolism of TAG-rich lipoproteins, and the inter-relationship between these processes and HDL, particularly in the post-prandial period, determines the extent to which LDL and HDL can promote and prevent atherosclerosis, respectively.