Cholesterol is a white, waxy substance that is produced by the liver and carried into the bloodstream by special proteins that are called lipoproteins. Some foods, such as meat, milk and egg yolks, also contain cholesterol, and this is sometimes referred to as dietary cholesterol.
Cholesterol is an important part of a healthy body because it is needed to form cell membranes, bile and some hormones such as oestrogen and testosterone. However, a high level of cholesterol in the blood (hypercholesterolaemia) is recognised as a major risk factor for coronary heart disease, which can lead to a heart attack.
Major types of cholesterol
Cholesterol cannot be dissolved in our blood and first has to be coated with a layer of protein, to form a lipoprotein, before it can be transported around the body.
There are several types of lipoproteins: very low-density lipoprotein (VLDL); low-density lipoprotein (LDL) and high-density lipoprotein (HDL).
VLDLs are made in the liver and their job is to carry fats to different parts of the body. Once they drop off some of their fat load, they become low-density lipoprotein (LDL).
LDL is the major carrier of cholesterol in the blood and is also known as ‘bad’ cholesterol. When too much cholesterol circulates through the blood, it builds up on the walls of the arteries that supply the heart and brain. When this happens it can cause atherosclerosis, or hardening of the arteries.
High-density lipoprotein (HDL) is often called the ‘good’ cholesterol as it can actually help carry cholesterol away from the arteries to the liver, where it is processed and excreted from the body. So, it's not just your total cholesterol measurement that's important, but how much LDL-cholesterol (‘bad’) and how much HDL-cholesterol (‘good’) you have.
What causes high blood cholesterol?
Factors that contribute to high cholesterol levels include a diet high in saturated fats and/or cholesterol, diabetes and an underactive thyroid gland. Being overweight also tends to increase cholesterol levels. Regular physical activity can improve your cholesterol profile, so being inactive won't help improve your results.
Age and gender also influence your cholesterol levels, causing them to increase from about age 20 onwards. Generally women have lower cholesterol levels than men until they reach menopause. This is thought to be due to the protective effect of oestrogen.
There may also be hereditary causes. If close members of your family have high blood cholesterol, there is an increased chance that you will also. In some cases, there may be an inherited inability to process LDL cholesterol and these people may have an elevated blood cholesterol even with a diet that is low in saturated fats.
The National Heart Foundation recommends that adults over 45 years of age have their blood cholesterol level tested regularly, in order to detect abnormal levels early so that they can be treated. They also recommend that all adults at high risk should have their cholesterol tested every year, regardless of their age.
Last Reviewed: 04 March 2003