Chylomicrons

Chylomicrons are triglyceride-rich lipoprotein particles produced by enterocytes in the small intestine. They are the largest circulating lipoproteins, with diameters ranging from roughly 75 to 1200 nanometers, and are composed mainly of triglycerides, along with cholesterol esters, phospholipids, and apolipoproteins, notably apoB-48. In circulation they acquire apolipoproteins C and E from high-density lipoproteins.

Biogenesis and secretion occur after dietary fat digestion. Fatty acids and monoglycerides are re-esterified in enterocytes to form triglycerides, which are assembled with apoB-48 by microsomal triglyceride transfer protein (MTP) to form pre-chylomicrons. These particles are secreted into the lymphatic system via intestinal lacteals and then enter the bloodstream through the thoracic duct, bypassing the liver on first pass.

Metabolism involves delivery of dietary triglycerides to peripheral tissues. In capillaries of adipose and muscle, lipoprotein lipase (LPL) hydrolyzes the triglycerides in chylomicrons, releasing free fatty acids for uptake by tissues; apoC-II acts as a cofactor for LPL. The remaining particles—chylomicron remnants enriched in cholesterol esters and apoE—are cleared from circulation by the liver through receptor-mediated endocytosis, primarily via hepatic apoE receptors.

Clinical relevance includes disorders of chylomicron metabolism. Hyperchylomicronemia can result from LPL deficiency, apoC-II deficiency, or related genetic conditions (type I hyperlipoproteinemia), and may present with pancreatitis, eruptive xanthomas, or lipemia retinalis. In healthy individuals, chylomicrons are typically cleared within hours after a fat-containing meal; remnants can contribute to atherogenic risk when elevated.