High cholesterol: introduction
Cholesterol is a fat-like substance present normally in the blood, and in every cell of the body. The level at which it is present in the blood varies, and the higher the cholesterol level in an individual, the greater the risk of coronary heart disease (CHD) and stroke. A raised cholesterol level is therefore a risk factor for these diseases, and is especially important when other risk factors are also present. High cholesterol is also linked to diabetes and high blood pressure.
There is some evidence that there is virtually no lower limit at which further lowering of cholesterol does not decrease risk of CHD and stroke. Introduction of new guidelines from the Scottish Intercollegiate Guidelines Network (SIGN) in 1999 reduced the threshold for 'raised cholesterol' in the UK from 6.5 mmol/l to 5.0 mmol/l. The consensus in the UK is that 5.0 mmol/l is the point at which the total cholesterol level becomes 'raised'. Of note, however, a report from the World Health Organization has defined non-optimal cholesterol as having a mean cholesterol of >3.8 mmol/l (Lawes et al, 2004).
High cholesterol in the blood is related to the intake of saturated fat in the diet, rather than to cholesterol intake.
Cholesterol is transported in the blood bound to lipoproteins, of which there are two broad types:
- High density lipoprotein (HDL) is thought to transport cholesterol away from tissues and arteries and into the liver, and a high level in the blood is therefore protective against diseases of the heart and arteries. Cholesterol in this form is known as 'good' cholesterol.
- Low density lipoprotein (LDL) is thought to have the opposite function, and cholesterol in this form is thought of as 'bad'.
Whilst this distinction is important, LDL makes up 60-70% of total blood cholesterol, and it is total cholesterol which is considered in this section of the website.