What is cholesterol?
Cholesterol is a chemical compound that the body requires as a building block for cell membranes and for hormones like estrogen and testosterone. The liver produces about 80% of the body's cholesterol and the rest comes from dietary sources like meat, poultry, eggs, fish, and dairy products. Foods derived from plants contain no cholesterol.
Cholesterol content in the bloodstream is regulated by the liver. After a meal, cholesterol in the diet is absorbed from the small intestine and metabolized and stored in the liver. As the body requires cholesterol, it may be secreted by the liver.
When too much cholesterol is present in the body, it can build up in deposits called plaque along the inside walls of arteries, causing them to narrow.
What are the different types of cholesterol?
Cholesterol does not travel freely through the bloodstream. Instead, it is attached or carried by lipoproteins (lipo = fat) in the blood. There are three types of lipoproteins that are categorized based upon how much protein there is in relation to the amount of cholesterol.
Low-density lipoproteins (LDL) contain a higher ratio of cholesterol to protein and are thought of as the “bad” cholesterol. Elevated levels of LDL lipoprotein increase the risk of heart disease, stroke, and peripheral artery disease, by helping form cholesterol plaque along the inside of artery walls. Over time, as plaque buildup (plaque deposits) increases, the artery narrows (atherosclerosis) and blood flow decreases. If the plaque ruptures, it can cause a blood clot to form that prevents any blood flow. This clot is the cause of a heart attack or myocardial infarction if the clot occurs in one of the coronary arteries in the heart.
High-density lipoproteins (HDL) are made up of a higher level of protein and a lower level of cholesterol. These tend to be thought of as “good” cholesterol. The higher the HDL to LDL ratio, the better it is for the individual because such ratios can potentially be protective against heart disease, stroke, and peripheral artery disease.
Very low-density lipoproteins (VLDL) contain even less protein than LDL. VLDL like LDL has been associated with plaque deposits.
Triglycerides (a type of fat) may increase cholesterol-containing plaques if levels of LDL are high and HDL are low.
Total cholesterol score is the sum of HDL cholesterol, LDL cholesterol and 20% of triglycerides as determined by a blood test. A high score indicates an increased risk for developing cardiovascular disease and/or strokes.
Cholesterol Levels
What are LDL and HDL Cholesterol?
LDL cholesterol is called "bad" cholesterol, because elevated levels of LDL cholesterol are associated with an increased risk of coronary heart disease, stroke, and peripheral artery disease. LDL lipoprotein deposits cholesterol along the inside of artery walls, causing the formation of a hard, thick substance called cholesterol plaque.
HDL cholesterol is called the "good cholesterol" because HDL cholesterol particles prevent atherosclerosis by extracting cholesterol from the artery walls and disposing of them through the liver. Thus, high levels of LDL cholesterol and low levels of HDL cholesterol (high LDL/HDL ratios) are risk factors for atherosclerosis, while low levels of LDL cholesterol and high levels of HDL cholesterol (low LDL/HDL ratios) are desirable and protect against heart disease and stroke.
Why is high cholesterol dangerous?
Elevated cholesterol levels are one of the risk factors for heart disease, stroke, and peripheral artery disease. The mechanism involving cholesterol in all three diseases is the same; plaque buildup within arteries decreases blood flow affecting the function of the cells and organs that these blood vessels supply.
Atherosclerotic heart disease or narrowed coronary arteries in the heart can cause the symptoms of angina, when the heart muscle is not provided with enough oxygen to function.
Decreased blood supply to the brain may be due to narrowed small arteries in the brain or because the larger carotid arteries in the neck may become blocked. This can result in a transient ischemic attack (TIA) or stroke.
Peripheral artery disease describes gradual narrowing of the arteries that supply the legs. During exercise, if the legs do not get enough blood supply, they can develop pain, called claudication.
Other arteries in the body may also be affected by plaque buildup causing them to narrow, including the mesenteric arteries to the intestine and the renal arteries to the kidney.
Where does cholesterol come from?
The liver is responsible for managing the levels of LDL in the body. It manufactures and secretes LDL into the bloodstream. There are receptors on liver cells that can "monitor" and try to adjust the LDL levels. However, if there are fewer liver cells or if they do not function effectively, the LDL level may rise.
Diet and genetics both play a factor in a person's cholesterol levels. There may be a genetic predisposition for familial hypercholesterolemia (hyper=more = cholesterol + emia=blood) where the number of liver receptor cells is low and LDL levels rise causing the potential for atherosclerotic heart disease at a younger age.
In the diet, cholesterol comes from saturated fats that are found in meats, eggs, and dairy products. Excess intake can cause LDL levels in the blood to rise. Some vegetable oils made from coconut, palm, and cocoa are also high in saturated fats.
What are normal cholesterol levels?
Blood tests are required to measure total cholesterol and lipoproteins. For a complete lipoprotein analysis, the patient should be fasting for at least 12 hours.
The National Cholesterol Education Program endorsed by the American Heart Association suggests the following risk guidelines for levels of total cholesterol, HDL, and LDL:
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