What is Cholesterol?

Cholesterol is a fatty substance (a lipid) that is an important part of the outer lining (membrane) of cells in the body of animals. Cholesterol is also found in the blood circulation of humans. The cholesterol in a person's blood originates from two major sources; dietary intake and liver production. Dietary cholesterol comes mainly from meat, poultry, fish, and dairy products. Organ meats, such as liver, are especially high in cholesterol content, while foods of plant origin contain no cholesterol. After a meal, cholesterol is absorbed by the intestines into the blood circulation and is then packaged inside a protein coat. This cholesterol-protein coat complex is called a chylomicron.

Click for Larger ImageThe liver is capable of removing cholesterol from the blood circulation as well as manufacturing cholesterol and secreting cholesterol into the blood circulation. After a meal, the liver removes chylomicrons from the blood circulation. In between meals, the liver manufactures and secretes cholesterol back into the blood circulation.

 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. LDL lipoprotein deposits cholesterol on the artery walls, causing 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.

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 level of HDL cholesterol (low LDL/HDL ratios) are desirable.

Total cholesterol is the sum of LDL (low density) cholesterol, HDL (high density) cholesterol, VLDL (very low density) cholesterol, and IDL (intermediate density) cholesterol.

 What determines the level of LDL cholesterol in the blood?

Click for Larger Image The liver not only manufactures and secretes LDL cholesterol into the blood; it also removes LDL cholesterol from the blood. A high number of active LDL receptors on the liver surfaces is associated with the rapid removal of LDL cholesterol from the blood and low blood LDL cholesterol levels. A deficiency of LDL receptors is associated with high LDL cholesterol blood levels.

Both heredity and diet have a significant influence on a person's LDL, HDL and total cholesterol levels. For example, familial hypercholesterolemia (FH) is a common inherited disorder whose victims have a diminished number or nonexistent LDL receptors on the surface of liver cells. People with this disorder also tend to develop atherosclerosis and heart attacks during early adulthood.

Diets that are high in saturated fats and cholesterol raise the levels of LDL cholesterol in the blood. Fats are classified as saturated or unsaturated (according to their chemical structure). Saturated fats are derived primarily from meat and dairy products and can raise blood cholesterol levels. Some vegetable oils made from coconut, palm, and cocoa are also high in saturated fats.

 Does lowering LDL cholesterol prevent heart attacks and strokes?

Lowering LDL cholesterol is currently the primary focus in preventing atherosclerosis and heart attacks. Most doctors now believe that the benefits of lowering LDL cholesterol include:

  • Reducing or stopping the formation of new cholesterol plaques on the artery walls;
  • Reducing existing cholesterol plaques on the artery walls;
  • Widening narrowed arteries;
  • Preventing the rupture of cholesterol plaques, which initiates blood clot formation;
  • Decreasing the risk of heart attacks; and
  • Decreasing the risk of strokes. The same measures that retard atherosclerosis in coronary arteries also benefit the carotid and cerebral arteries (arteries that deliver
    blood to the brain).

 How can LDL cholesterol levels be lowered?

Lowering LDL cholesterol involves losing excess weight, exercising regularly, and following a diet that is low in saturated fat and cholesterol.

 Medications to lower cholesterol

Medications are prescribed when lifestyle changes cannot reduce the LDL cholesterol to desired levels. The most effective and widely used medications to lower LDL cholesterol are called statins. Most of the large controlled trials that demonstrated the heart attack and stroke prevention benefits of lowering LDL cholesterol used one of the statins. Other medications used in lowering LDL cholesterol and in altering cholesterol profiles include nicotinic acid (niacin), fibrates such as gemfibrozil (Lopid), resins such as cholestyramine (Questran), and ezetimibe, Zetia. The cholesterol medications such as colestipol, cholesteramine, and colesevelam can lower LDL cholesterol. These sequestrants combine bile acids in the intestine and cause most of the bile acids to be eliminated in bowel movements.

 What are “normal” cholesterol blood levels?

There are no established “normal” blood levels for total and LDL cholesterol. In most other blood tests in medicine, normal ranges can be set by taking measurements from large number of healthy subjects. For example, normal fasting blood sugar levels can be established by performing blood tests among healthy subjects without diabetes mellitus. If a patient's fasting blood glucose falls within this normal range, he/she most likely does not have diabetes mellitus, whereas if the patient's fasting blood sugar tests higher than the normal range, he/she probably has diabetes mellitus and further tests can be performed to confirm the diagnosis. Medications, such as insulin or oral diabetes medications can be prescribed to lower abnormally high blood sugar levels.

Unfortunately, the normal range of LDL cholesterol among “healthy” adults (adults with no known coronary heart disease) in the United States may be too high. The atherosclerosis process may be quietly progressing in many healthy adults with average LDL cholesterol blood levels, putting them at risk of developing coronary heart diseases in the future.

 Key Points

Cholesterol is a fat-like substance that is made in your body. Cholesterol is also in some foods that you eat. Your body needs some cholesterol to work the right way. Your body makes all the cholesterol it needs.

  • Too much cholesterol in the blood is called high blood cholesterol
    or hypercholesterolemia.
  • High blood cholesterol increases the chance of having a heart attack or some other symptom of heart disease, like chest pain (angina).
  • Lowering cholesterol is important for everyone-young, middle-aged, and older
    adults, and both men and women.
  • Eating too much saturated fat and cholesterol raises the level of cholesterol in your blood.
  • Too much cholesterol in your blood can build up in the walls of arteries.
    This is called plaque.
  • There are no signs or symptoms of high blood cholesterol. Many people don't
    know that their cholesterol level is too high.
  • High blood cholesterol is diagnosed by checking cholesterol levels in your blood.
  • A blood test called a lipoprotein profile measures the cholesterol levels in your
    blood and is the recommended test.
  • It is important that everyone age 20 and older get their cholesterol checked at
    least once every 5 years.
  • Many people are able to lower their cholesterol levels by eating a low saturated
    fat and low cholesterol diet, exercising, and losing weight if needed.
  • Some people will need to take medicines prescribed by their doctor to lower their cholesterol in addition to eating a low saturated fat and low cholesterol diet,
    exercising, and losing weight if needed.

Additional information on Cholesterol is available for download by clicking on the following links:

High Blood Cholesterol - What You Need to Know (PDF format)

The National Heart, Lung, and Blood Institute as a part of the National Institutes of Health and the U.S. Department of Health and Human Services is the source for the information provided on this page and at the above links.
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