A substance which is insoluble in water, but soluble in fat solvents such as alcohol and ether. The main lipid groups are the triglycerides, phospholipids, and glycolipids. They play an important role in nutrition, health (particularly in the functioning of the cell membranes, and the immune response), and disease (notably cardiovascular disease). There is a strong correlation between the concentration of CHOLESTEROL in the blood (transported as LIPOPROTEINS) and the risk of developing ATHEROMA and coronary heart disease (see HEART, DISEASES OF). Lipoproteins are classified by their density and mobility, the chief groups being low-density (LDL), very low-density (VLDL), and high-density (HDL). High SERUM concentrations of LDL increase the risk of cardiovascular disease, while HDL is thought to protect the vessel wall by removing cholesterol, and has an inverse relationship to risk. The various serum lipid abnormalities have been classified into five groups, according to the cause and the particular raised lipoprotein. Most important are type II (increased LDL, genetically determined) and type IV (increased VLDL, associated with obesity, diabetes, and excess alcohol). Various lipid-lowering drugs are available (see LIPID-REGULATING DRUGS) but any drug treatment must be combined with attention to diet, reduction of blood pressure, and cessation of smoking.