What is cholesterol?

[Naveed Yaseen]

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.

The 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?

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.

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).

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.

Why is HDL the good cholesterol?

HDL is the good cholesterol because it protects the arteries from the atherosclerosis process. HDL cholesterol extracts cholesterol particles from the artery walls and transports them to the liver to be disposed through the bile. It also interferes with the accumulation of LDL cholesterol particles in the artery walls.

The risk of atherosclerosis and heart attacks in both men and is strongly related to HDL cholesterol levels. Low levels of HDL cholesterol are linked to a higher risk, whereas high HDL cholesterol levels are associated with a lower risk.

Very low and very high HDL cholesterol levels can run in families. Families with low HDL cholesterol levels have a higher incidence of heart attacks than the general population, while families with high HDL cholesterol levels tend to live longer with a lower frequency of heart attacks.

Like LDL cholesterol, life style factors and other conditions influence HDL cholesterol levels. HDL cholesterol levels are lower in persons who smoke cigarettes, eat a lot of sweets, are overweight and inactive, and in patients with type II diabetes mellitus.

HDL cholesterol is higher in people who are lean, exercise regularly, and do not smoke cigarettes. Estrogen increases a person’s HDL cholesterol, which explains why women generally have higher HDL levels than men do.

For individuals with low HDL cholesterol levels, a high total or LDL cholesterol blood level further increases the incidence of atherosclerosis and heart attacks.

Therefore, the combination of high levels of total and LDL cholesterol with low levels of HDL cholesterol is undesirable whereas the combination of low levels of total and LDL cholesterol and high levels of HDL cholesterol is favorable.

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