Cardiovascular disease is the leading cause of illness and death in the United States. One of the major contributing factors to cardiovascular disease is cholesterol. Yet cholesterol is an essential chemical in the body; without it, we couldn't survive. It is used in the synthesis of hormones like estrogen and testosterone, and it is necessary for the formation of cell membranes.
Cholesterol circulates in the bloodstream in complexes called "lipoproteins" containing triglycerides (another type of fat), phospholipids (mostly lecithin), and proteins. There are four classes of these "lipoprotein" complexes:
- Lipoproteins
- chylomicrons,
- very-low-density lipoproteins (VLDL),
- low-density lipoproteins (LDL), and
- high-density lipoproteins (HDL)
Chylomicrons appear in the bloodstream after a meal and transport dietary triglycerides from the gut to sites where the triglycerides are used and stored.
VLDL transport triglycerides and cholesterol that are synthesized by the liver to similar sites for utilization or storage. Many people with high triglycerides and cholesterol make too much VLDL in the liver because of an inherited tendency.
When chylomicrons and VLDL reach capillary beds in various tissues such as muscle or fat, an enzyme breaks down triglycerides into fatty acids and glycerol. The remaining chylomicron remnants continue to circulate until they are taken up or absorbed by the liver.
LDL
The VLDL remnants are converted primarily to LDL, which is removed from the circulation mostly by being absorbed into liver cells. For liver cell absorption of LDL to occur, the LDL must bind to the LDL receptor on the cell surface. People with familial hypercholesterolemia lack these receptors, and the result is they have LDL-cholesterol levels that are often two or three times normal.
The LDL-cholesterol complex is small and dense compared to chylomicrons and VLDL, and when it is present in high concentrations it tends to deposit inside the blood vessel wall. This contributes to atherosclerosis (the build-up of fatty plaque in the arteries; "hardening of the arteries").
HDL
HDL has a different function in the body. It removes excess cholesterol from cells and helps transport it back to the liver. High HDL levels are associated with a reduced risk of heart disease and low levels with an increased risk of early heart disease. For this reason HDL-cholesterol is known as the "good" cholesterol. However, we don't yet have any direct evidence that increasing HDL can prevent or treat heart disease.
Cholesterol and Coronary Heart Disease
Cholesterol and lipoproteins are implicated in the development of the blood vessel plaque that causes heart attacks and most strokes. In parts of the world where blood cholesterol and LDL levels are low, atherosclerosis and heart attacks are almost unknown. Therefore, it was suggested decades ago that lowering cholesterol levels might be one major way of decreasing risk of coronary heart disease. As a result of many studies, especially several that have been completed and announced over the last few years, it is now well established that lowering cholesterol does decrease the risk of having a second heart attack in people who already have known heart disease. Thanks to the large amount of information, it is now widely agreed that most people with known heart disease should be taking some type of cholesterol-lowering medication. Therefore, treatment to lower cholesterol has become a standard part of the management of people who have heart disease or are at high risk for developing heart disease.
Articles about Cholesterol and Cholesterol Lowering Drugs
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