High density lipoprotein.

High density lipoprotein (HDL) is a discoidal particle comprising phospholipid, cholesteryl esters and several apolipoproteins. It serves in transporting cholesterol from the periphery to the liver by the process of "reverse cholesterol transport". Compatible with this is the finding that the mass of the tissue cholesterol pools is inversely related to plasma HDL concentration. The plasma levels of components of HDL are determined by various physiological and pathological factors. The serum HDL levels are lower with advancing age, male sex, and in genetically predisposed, obese, sedentary persons. The effect of diet on serum HDL levels is not established; mild to moderate alcohol intake is associated with high serum HDL level. The main diseases affecting serum HDL levels are uncontrolled diabetes mellitus, uraemia and hyperthyroidism. Anabolic steroids, sex hormones, oral contraceptives, hypocholesterolaemics and beta blockers have been shown to affect serum HDL level variably. There is increasing epidemiological evidence to show that high levels of HDL are protective against coronary heart disease (CHD). A low serum HDL cholesterol concentration (less than 35 mg/dl) is associated with a significant increase in coronary risk in both men and women. Guidelines published by the National Cholesterol Education Programme do not recommend routine measurement of HDL cholesterol and adaptation of therapeutic modalities aiming to raise the low HDL levels. They recommend hygienic means (i.e. smoking cessation, aerobic exercises and weight loss) to raise the HDL cholesterol levels.