Hyperlipidemia, hyperlipoproteinemia, or hyperlipidaemia (British English) involves abnormally elevated levels of any or all lipids and/or lipoproteins in the blood. It is the most common form of dyslipidemia (which includes any abnormal lipid levels).
Lipids (fat-soluble molecules) are transported in a protein capsule. The size of that capsule, or lipoprotein, determines its density. The lipoprotein density and type of apolipoproteins it contains determines the fate of the particle and its influence on metabolism.
Hypercholesterolemia (also spelled hypercholesterolaemia) is the presence of high levels of cholesterol in the blood. It is a form of "hyperlipidemia" (elevated levels of lipids in the blood) and "hyperlipoproteinemia" (elevated levels of lipoproteins in the blood).
Cholesterol is a sterol; see the diagrammatic structure at the right. It is one of three major classes of lipids which all animal cells utilize to construct their membranes and is thus manufactured by all animal cells. Plant cells do not manufacture cholesterol. It is also the precursor of the steroid hormones, bile acids and vitamin D.
Low-density lipoprotein (LDL) is one of the five major groups of lipoproteins, which, in order of molecular size, largest to smallest, are chylomicrons, very low-density lipoprotein (VLDL), intermediate-density lipoprotein (IDL), LDL, and high-density lipoprotein (HDL). Lipoprotein molecules enable the transportation of lipids (fats), such as cholesterol, phospholipids, and triglycerides, within the water around cells (extracellular fluid), including the bloodstream. Studies have shown that higher levels of type-B LDL particles (as opposed to type-A LDL particles) are associated with health problems, including cardiovascular disease.
Although the nickname is overly simplistic and thus misleading, LDL molecules are often informally called bad cholesterol because they can transport their content of many fat molecules into artery walls, attract macrophages, and thus drive atherosclerosis. In contrast, HDL molecules are frequently referred to as good cholesterol or healthy cholesterol, because they can remove fat molecules from macrophages in the wall of arteries.