Cholesterol is a fat-like substance that is an important constituent of body cells and also is involved in the formation of hormones and bile salts. Cholesterol is made by the liver from various foods, especially those containing saturated fats (see fats and oils), although a small amount is absorbed directly from cholesterol-rich foods such as eggs and shellfish. Both cholesterol and fats (triglycerides) are transported in the blood as lipoproteins. These are particles with a core, which is made of varying proportions of cholesterol and triglycerides, and an outer layer made of proteins.
High blood cholesterol levels increase the risk of atherosclerosis (accumulation of fatty deposits on the lining of the arteries) and with it the risk of coronary artery disease or of stroke (damage to part of the brain due to interruption of its blood supply).
In general, cholesterol transported in the bloodstream as low-density lipoproteins (LDLs) or as very low-density lipoproteins (VLDLs) is a risk factor for these conditions, while cholesterol in the form of highdensity lipoproteins (HDLs) seems to protect against arterial disease. Levels of cholesterol in the blood are influenced by diet, genetic factors, and metabolic diseases such as diabetes mellitus.
Cholesterol levels can be measured by blood tests. They are measured in the UK in millimoles per litre (mmol/L). The optimum level is less than 5.0. Higher levels may require dietary modification. Sometimes, however, medication (e.g. a statin such as simvastatin) may be required to reduce the risk of arterial diseases.