However, having an excessively high level of lipids in your blood - a condition known as hyperlipidemia - can have a serious effect on your health, as it increases your risk of having a heart attack, or stroke.

Cholesterol is present in the membrane (outer layer) of every cell in the body. It insulates nerve fibres, and is an essential building block for hormones, such as the sex hormones and the hormones made and released by the adrenal glands.

Cholesterol also enables the body to produce bile salts.

Cholesterol is carried in the blood by molecules called lipoproteins. There are several different lipoproteins, but the three main types are:

Low density lipoprotein (LDL).

This is often known as bad cholesterol and is thought to increase arterial disease (i.e. in the arteries). It carries cholesterol from the liver to the cells and can cause a harmful build-up if there is too much for the cells to use. Normally, the blood contains about 70% of LDL, but the level will vary from person to person.

High density lipoprotein (HDL).

This is often referred to as 'good cholesterol', and is thought to prevent arterial disease. It takes cholesterol away from the cells and back to the liver, where it is either broken down or is passed from the body as a waste product.

Triglycerides

Triglycerides are another type of fatty substance present in the blood. They're found in dairy products, meat and cooking oils. Triglycerides are also produced by the liver. People who are overweight, eat a diet high in fatty or sugary foods or drink a large amount of alcohol have an increased risk of a high triglyceride level.

The amount of cholesterol present in the blood can range from 3.6 to 7.8 mmol/litre. More than 6mmol/litre is considered high, and is a risk factor for arterial disease. Government advice recommends a target cholesterol level of less than 5. However, in the UK, two out of three adults have a total cholesterol level of 5 or above. Men in England, on average, have a level of 5.5, and women have a level of 5.6.

Evidence strongly indicates that high cholesterol levels can cause narrowing of the arteries (atherosclerosis), heart attacks and strokes. The risk of coronary heart disease also rises as the blood's cholesterol level increases. If other risk factors, such as high blood pressure and smoking, are present, the risk increases even more.