Cholesterol is a lipid (fat). It is manufactured by the liver from the fatty foods that we eat, and plays a vital part in allowing the body to function normally. 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 of the adrenal cortex. It 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 promote arterial disease. 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 are another type of fatty substance present in the blood. They are found in dairy products, meat and cooking oils. Triglycerides are also produced by the liver. Those who are overweight, have a diet that is high in fatty or sugary foods, or drink a large amount of alcohol, have an increased risk of having a high triglyceride level.
The amount of cholesterol present in the blood can range from 3.6 to 7.8 mmol/litre. A level above 6 mmol/litre is considered as high, and a risk factor for arterial disease. Government advice recommends a target cholesterol level of less than 5. However, in the UK, two in three adults have a total cholesterol level of 5 or above. In England, men, 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 blood cholesterol levels increase. If other risk factors, such as high blood pressure and smoking, are present, the risk increases even more.
