Cholesterol is a fatty substance known as a lipid. It is mostly made by the liver from the fatty foods we eat and is vital for the normal functioning of the body (see box, left).
Having an excessively high level of lipids in your blood (hyperlipidemia) can have a serious effect on your health as it increases your risk of having a heart attack or stroke.
Cholesterol is needed in the body to:
- make up the structure of the membrane (outer layer) of every cell in the body,
- insulate nerve fibres,
- make hormones, such as sex hormones and steroid hormones, and
- make bile acids, which are needed for the digestion and absorption of fats.
'Good' and 'bad' cholesterol
Cholesterol cannot travel around the body on its own because it does not dissolve in water. Instead, it is carried in your blood by molecules called lipoproteins.
The two main lipoproteins are LDL and HDL.
- Low-density lipoprotein (LDL). LDL is the main cholesterol transporter and carries cholesterol from your liver to the cells that need it. If there is too much cholesterol for the cells to use, this can cause a harmful build-up in your blood. Too much LDL cholesterol in the blood can cause cholesterol to build up in the artery walls, leading to disease of the arteries. For this reason, LDL cholesterol is known as 'bad cholesterol', and lower levels are better.
- High-density lipoprotein (HDL). HDL carries cholesterol away from the cells and back to the liver, where it is either broken down or passed from the body as a waste product. For this reason, it is referred to as 'good cholesterol', and higher levels are better.
The amount of cholesterol in the blood (including both LDL and HDL) can be measured with a blood test.
Your doctor or nurse may also measure your level of triglycerides. Triglycerides are the fats you use for energy and come from the fatty foods you eat. You store what you do not use in the fatty tissues of your body. Excess triglycerides in the blood also increase heart problems.
Normal cholesterol level
Blood cholesterol is measured in units called millimoles per litre of blood, often shortened to mmol/L.
The government recommends that cholesterol levels should be less than 5mmol/L.
In the UK, two out of three adults have a total cholesterol level of 5mmol/L or above. On average, men in England have a cholesterol level of 5.5mmol/L and women have a level of 5.6mmol/L.
The UK population has one of the highest average cholesterol concentrations in the world.
Risks of high cholesterol
Evidence strongly indicates that high cholesterol levels can cause narrowing of the arteries (atherosclerosis), heart attack and stroke.
This is because cholesterol can build up in the artery wall (see Symptoms), restricting the flow of blood to your heart, brain and the rest of your body. It also increases the chance of a blood clot developing.
Your risk of coronary heart disease (when your heart's blood supply is blocked or disrupted) rises as your blood's cholesterol level increases. Other factors, such as high blood pressure and smoking, increase this risk even more.
Who is at risk?
There are many factors that can increase your chance of having heart problems or stroke if you have high cholesterol. These are called risk factors.
- Some risk factors, such as an unhealthy diet and smoking, can be changed by altering your lifestyle.
- Some risk factors, such as having diabetes or high blood pressure, can be treated with medication.
- Some risk factors, such as having a family history of stroke or heart disease, cannot be changed.
For more information about risk factors, see Causes.