High-density lipoprotein (HDL) is one of the five major groups of lipoproteins. Lipoproteins are complex particles composed of multiple proteins which transport all fat molecules (lipids) around the body within the water outside cells. They are typically composed of 80–100 proteins per particle (organized by one, two or three ApoA ; more as the particles enlarge picking up and carrying more fat molecules) and transporting up to hundreds of fat molecules per particle. Overview Lipoproteins have long been divided into 5 subgroups, by density/size (an inverse relationship), which also correlates with function and incidence of cardiovascular events. Unlike the larger lipoprotein particles  which deliver fat molecules to cells, HDL particles remove fat molecules from cells which need to export fat molecules . The lipids carried include cholesterol, phospholipids, and triglycerides ; amounts of each are quite variable. Increasing concentrations of HDL particles are strongly associated with decreasing accumulation of atherosclerosis within the walls of arteries . This is important because atherosclerosis eventually results in sudden plaque ruptures, cardiovascular disease, stroke and other vascular diseases. HDL particles are popularly referred to as "good cholesterol", because they can transport fat molecules out of artery walls, reduce macrophage accumulation, and thus help prevent or even regress atherosclerosis. However, studies have shown that HDL-lacking mice still have the ability to transport cholesterol to bile, suggesting that there are alternative mechanisms for cholesterol removal. Testing Because of the high cost of directly measuring HDL and LDL (low-density lipoprotein) protein particles, blood tests are commonly performed for the surrogate value, HDL-C, i.e. the cholesterol associated with ApoA-1/HDL particles. In healthy individuals, about 30\% of blood cholesterol, along with other fats, is carried by HDL. This is often contrasted with the amount of cholesterol estimated to be carried within low-density lipoprotein particles, LDL, and called LDL-C. HDL particles remove fats and cholesterol from cells, including within artery wall atheroma, and transport it back to the liver for excretion or re-utilization; thus the cholesterol carried within HDL particles (HDL-C) is sometimes called "good cholesterol" (despite being the same as cholesterol in LDL particles). Those with higher levels of HDL-C tend to have fewer problems with cardiovascular diseases, while those with low HDL-C cholesterol levels (especially less than 40 mg/dL or about 1 mmol/L) have increased rates for heart disease. Higher native HDL levels are correlated with better cardiovascular health, but it does not appear that further increasing one's HDL improves cardiovascular outcomes . Structure and function With a size ranging from 5 to 17 nm, HDL is the smallest of the lipoprotein particles. It is the densest because it contains the highest proportion of protein to lipids. Its most abundant apolipoproteins are apo A-I and apo A-II. A rare genetic variant, ApoA-1 Milano, has been documented to be far more effective in both protecting against and regressing arterial disease; atherosclerosis. The liver synthesizes these lipoproteins as complexes of apolipoproteins and phospholipid, which resemble cholesterol-free flattened spherical lipoprotein particles, whose NMR structure was recently published; the complexes are capable of picking up cholesterol, carried internally, from cells by interaction with the ATP-binding cassette transporter A1 (ABCA1). A plasma enzyme called lecithin-cholesterol acyltransferase (LCAT) converts the free cholesterol into cholesteryl ester (a more hydrophobic form of cholesterol), which is then sequestered into the core of the lipoprotein particle, eventually causing the newly synthesized HDL to assume a spherical shape. HDL particles increase in size as they circulate through the bloodstream and incorporate more cholesterol and phospholipid molecules from cells and other lipoproteins, for example by the interaction with the ABCG1 transporter and the phospholipid transport protein (PLTP). While high HDL levels might correlate with better cardiovascular health, specifically increasing one's HDL might not increase cardiovascular health. While higher HDL levels are correlated with cardiovascular health , no medication used to increase HDL has been proven to improve health. The remaining possibilities are that either good cardiovascular health causes high HDL levels, there is some third factor which causes both, or this is a coincidence with no causal link.
High-density lipoprotein (HDL) is one of the five major groups of lipoproteins. Lipoproteins are complex particles composed of multiple proteins which transport all fat molecules (lipids) around the body within the water outside cells. They are typically composed of 80–100 proteins per particle (organized by one, two or three ApoA . HDL particles enlarge while circulating in the blood, aggregating more fat molecules) and transporting up to hundreds of fat molecules per particle. Overview Lipoproteins are divided into 5 subgroups, by density/size (an inverse relationship), which also correlates with function and incidence of cardiovascular events. Unlike the larger lipoprotein particles , which deliver fat molecules to cells, HDL particles remove fat molecules from cells . The lipids carried include cholesterol, phospholipids, and triglycerides , amounts of each are variable. Increasing concentrations of HDL particles are associated with decreasing accumulation of atherosclerosis within the walls of arteries , reducing the risk of sudden plaque ruptures, cardiovascular disease, stroke and other vascular diseases. HDL particles are commonly referred to as "good cholesterol", because they transport fat molecules out of artery walls, reduce macrophage accumulation, and thus help prevent or even regress atherosclerosis. Testing Because of the high cost of directly measuring HDL and LDL (low-density lipoprotein) protein particles, blood tests are commonly performed for the surrogate value, HDL-C, i.e. the cholesterol associated with ApoA-1/HDL particles. In healthy individuals, about 30\% of blood cholesterol, along with other fats, is carried by HDL. This is often contrasted with the amount of cholesterol estimated to be carried within low-density lipoprotein particles, LDL, and called LDL-C. HDL particles remove fats and cholesterol from cells, including within artery wall atheroma, and transport it back to the liver for excretion or re-utilization; thus the cholesterol carried within HDL particles (HDL-C) is sometimes called "good cholesterol" (despite being the same as cholesterol in LDL particles). Those with higher levels of HDL-C tend to have fewer problems with cardiovascular diseases, while those with low HDL-C cholesterol levels (especially less than 40 mg/dL or about 1 mmol/L) have increased rates for heart disease. Higher native HDL levels are correlated with lowered risk of cardiovascular disease in healthy people . Structure and function With a size ranging from 5 to 17 nm, HDL is the smallest of the lipoprotein particles. It is the densest because it contains the highest proportion of protein to lipids. Its most abundant apolipoproteins are apo A-I and apo A-II. A rare genetic variant, ApoA-1 Milano, has been documented to be far more effective in both protecting against and regressing arterial disease; atherosclerosis. The liver synthesizes these lipoproteins as complexes of apolipoproteins and phospholipid, which resemble cholesterol-free flattened spherical lipoprotein particles, whose NMR structure was recently published; the complexes are capable of picking up cholesterol, carried internally, from cells by interaction with the ATP-binding cassette transporter A1 (ABCA1). A plasma enzyme called lecithin-cholesterol acyltransferase (LCAT) converts the free cholesterol into cholesteryl ester (a more hydrophobic form of cholesterol), which is then sequestered into the core of the lipoprotein particle, eventually causing the newly synthesized HDL to assume a spherical shape. HDL particles increase in size as they circulate through the blood and incorporate more cholesterol and phospholipid molecules from cells and other lipoproteins, such as by interaction with the ABCG1 transporter and the phospholipid transport protein (PLTP). While higher HDL levels are correlated with lower risk of cardiovascular diseases , no medication used to increase HDL has been proven to improve health. Numerous lifestyle changes and drugs are under study to increase HDL levels, as of 2017.
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