Cholesterol is a chemical compound produced naturally by the body which made up of a combination of lipid (fat) and steroid. Cholesterol is a building block for cell membranes and for hormones like estrogen and testosterone. While only a small amount comes from diet, around 80% of the body's cholesterol is produced by the liver. After a meal, dietary cholesterol is absorbed from the intestine and stored in the liver. The liver regulates cholesterol levels in the blood stream and can secrete cholesterol if it is needed by the body.
LDL cholesterol is known as "bad" cholesterol because elevated levels of LDL cholesterol are linked with an increased risk of heart disease, stroke, and artery disease. This bad cholesterol deposits cholesterol along the inside of artery walls, resulting in the formation of a hard, thick substance called cholesterol plaque. Over time, cholesterol plaque causes thickening of the artery walls and narrowing of the arteries (a process called atherosclerosis) which reduces blood flow through the constricted area.
HDL cholesterol is known as the "good cholesterol" because HDL cholesterol particles reverse the process of atherosclerosis by extracting cholesterol from the artery walls and disposing of them through the liver.
High levels of LDL cholesterol and low levels of HDL cholesterol (high LDL/HDL ratios) are risk factors for atherosclerosis, whilst low levels of LDL cholesterol and high levels of HDL cholesterol (low LDL/HDL ratios) are desirable and protect against heart disease and stroke.
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