Updated 01 October 2014

What is cholesterol?

We cannot live without cholesterol, but when levels of low density lipoprotein (LDL) are too high, it can be dangerous to our health.

What is cholesterol?

Cholesterol is a fatty substance which is essential to the human body. It forms part of the lining membrane of every cell, is the basic substance used for the production of hormones (e.g. oestrogen and testosterone) and is used in the production of bile for digestion.

Cholesterol in our bodies is derived from two main sources:
•    It is manufactured in the liver of all humans (and animals).
•    We ingest it through foods derived from animals.

Cholesterol from the bile used in digestion is reabsorbed and recycled to the liver for re-use.

What is high cholesterol?

High cholesterol (hypercholesterolaemia) is a condition in which the amount of cholesterol in the blood exceeds normal values. Excess LDL ("bad") cholesterol may be deposited in arteries, for example in the the coronary arteries of the heart, the carotid arteries to the brain, and the arteries that supply blood to the legs.

‘Good’ and ‘bad’ cholesterol

Cholesterol travels in the blood attached to proteins. These cholesterol-protein packages, called lipoproteins, can be divided into two major types, depending on their composition. The more protein they contain, the higher the density.

•    High-density lipoprotein (HDL) cholesterol – lipoproteins with more protein than fat. This is the "good" cholesterol and higher levels of HDL cholesterol actually have a protective function in the body.
•    Low-density lipoprotein (LDL) cholesterol – lipoproteins with more fat than protein. This is the "bad" cholesterol.

HDL consists of mostly protein with only a small amount of fat. HDL cholesterol helps clear cholesterol from the body by picking up leftover cholesterol from cells and carrying it back to the liver for disposal.

Low levels of HDL cholesterol increase the risk of coronary artery disease (CAD) and other forms of atherosclerotic disease. High levels of HDL cholesterol appear to help protect against heart disease.

Abnormally high levels of LDL cholesterol in the blood are associated with an increased risk of atherosclerosis (hardening of the arteries), coronary heart disease (leading to angina and heart attacks) and stroke.

LDL cholesterol, consists of mostly fat and about a quarter is protein. LDL carries cholesterol from the liver to other parts of the body where it is needed for cell repair and other activities. This lipoprotein tends to deposit its cholesterol part into artery walls and other body tissues such as tendons.

Cholesterol deposited into the walls of arteries form oily collections called plaques. The plaques seldom become thick enough to protrude towards the inside of the artery, but they can tear, forming clots. This could impair or stop the blood supply, causing tissue or an organ to malfunction temporarily or permanently.

If this happens in the arteries of the heart, a partial blockage can cause angina, and a total blockage can cause a heart attack, which can be fatal. Problems in the arteries of the brain can cause a stroke, and problems in the leg arteries can cause gangrene.

Although high cholesterol is an important risk factor for these conditions, it is only one of many contributory factors. Other risk factors such as smoking, high blood pressure, obesity, diabetes and a family history of vascular disease may be as, or even more, important than your cholesterol level.

Read more:

Causes of high cholesterol

Diagnosing high cholesterol
Knowing cholesterol better

Image: Cholesterol blocked artery from Shutterstock

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