This discussion of lipid metabolism involves lipoprotein components, the known effects of contraceptive steroids on lipoprotein metabolism and the relationships of lipid and lipoprotein parameters to cardiac risk.
PIP: Concern about a possible association between oral contraceptives (OCs) and cardiovascular problems has led to investigation of the effects of OC use on plasma lipids and lipoproteins. The lipoprotein metabolic system involves chylomicrons derived from dietary fat, very-low-density lipoproteins (VLDL) derived primarily from the liver, and high-density lipoproteins (HDL). VLDLs form remnantlike particles termed intermediatedensity lipoproteins (IDLs), which ultimately are converted to low-density lipoproteins (LDLs). Understanding of how these particles relate to the atherogenic process is incomplete, although the spectrum of VLDL-LDL particles is involved in atherosclerosis. Whereas the LDLs and IDLs can deliver cholesterol to the artery wall, the HDLs seem to be involved in bidirectional cholesterol transport and are capable of accepting cholesterol from tissues and other lipoprotein classes. This property has caused HDL to become known as an antiatherogenic lipoprotein. Estrogen appears to increase VLDL production rates as well as the clearance of remants and LDL, although genetic factors may be involved. Estrogen's effect on HDL seems to be a more uniform increase in production. In contrast, synthetic progestational steroids appear to reduce HDL, in part due to the increased metabolism of HDL lipids mediated by hepatic lipase activity. They appear to interact with estrogen to raise LDL and VLDL levels. Epidemiologic studies suggest that low- and high-density cholesterol provide the best clinical indices of cardiovascular risk. When the HDL level is low, coronary risk amplifies; when the HDL level is high, the LDL effect on coronary risk compresses. There is as yet no precise information on the extent to which changes in lipoproteins, especially HDL, affect the development or progression of coronary artery disease.