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), is used in the production of bile for digestion as well as fulfilling some other actions. 

Cholesterol in our bodies is derived from two main sources:

• It is manufactured in all cells but the liver is the chief organ involved in cholesterol balance in all humans (and animals).
• We ingest cholesterol 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. Cholesterol is carried in different particles (lipoproteins).  Excess LDL (“bad”) cholesterol may be deposited in arteries, for example in 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 in structures containing proteins and some other lipids (fats). 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 indicate a lower risk of heart disease. 
• Low-density lipoprotein (LDL) cholesterol – lipoproteins with more fat than protein. This is the "bad" cholesterol. This constitutes the bulk of the cholesterol in the blood. 

HDL consists of mostly protein. Its chief action seems to be to clear cholesterol from the body by picking up 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.

Increasing 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, apo B, unlike other lipoproteins that have many (apo)proteins. 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 onto artery walls and other body tissues such as tendons.

Blockage of the arteries

Cholesterol deposited onto the walls of arteries form oily collections that later gain crystals of cholesterol and inflammation with infiltration of cells, and clotting can occur on the affected parts of the arteries, called plaques.

The plaques seldom become thick enough to protrude and obstruct 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 process results in partial blockage of the arteries of the heart, angina follows: blood supply at rest is adequate but not on exercise. Exercise thus induces chest pain while resting relieves the pain.

A total blockage can cause inadequate flow of blood with death of the muscle tissue, resulting in continuous pain or a heart attack, which can be fatal. 

Other risk factors

Though there is much muscle in reserve at the heart, a very large region of damage may harm the pump function. The heart rhythm may also be disturbed by a heart attack.  Problems in the arteries of the brain can cause a stroke.

This is not painful but the area that no longer receives blood will no longer provide its function – power, speech, sensation, balance, etc. Problems in the blood supply to 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.

^{Reviewed by Prof David Marais, FCP(SA), Head of Lipidology at Groote Schuur Hospital and the University of Cape Town, January 2018}

^{Image credit: iStock}