Cholesterol is a soft, waxy substance found among the lipids (fats) in the bloodstream and in all body cells. Cholesterol forms part of the outer membrane that surrounds every cell. It’s used to insulate nerve fibers (and so make nerve signals travel properly) and make hormones, which carry chemical signals around the body. Without cholesterol, your body wouldn’t work – it’s vital to ensuring the body’s normal functioning.

According to the lipid hypothesis, abnormally high cholesterol levels (hypercholesterolemia) and abnormal proportions of LDL and HDL are associated with cardiovascular disease, by promoting atheroma development in arteries (atherosclerosis). This disease process leads to myocardial infarction (heart attack), stroke, and peripheral vascular disease. As high LDL contributes to this process, it is termed “bad cholesterol,” while high levels of HDL (“good cholesterol”) offer a degree of protection.

The balance can be redressed with exercise, a healthy diet, and sometimes medication.

The cholesterol in a person’s blood originates from two major sources; dietary intake and liver production. Dietary cholesterol comes mainly from meat, poultry, fish, and dairy products. Organ meats, such as liver, are especially high in cholesterol content, while foods of plant origin contain no cholesterol.

After a meal, cholesterol is absorbed by the intestines into the blood circulation, and is then packaged inside a protein coat. This cholesterol-protein coat complex is called a chylomicron. Cholesterol and other fats can’t dissolve in the blood. They have to be transported to and from the cells by special carriers called lipoproteins. There are several kinds, but the ones to focus on are low-density lipoprotein (LDL) and high-density lipoprotein (HDL).

The human body contains about 100g of cholesterol. Most of this is incorporated in the membranes from which cells are constructed, and is an indispensable component of them. The insulating layers of myelin wound around neurons are especially rich in cholesterol. In far smaller quantities, but no less important, cholesterol is the starting ingredient for the synthesis of the steroid hormones Progesteron, Estrogens, Androgens (e.g., testosterone), Glucocorticoids (e.g., cortisol), and Mineralocorticoids (e.g., aldosterone).

Total cholesterol is the sum of HDL cholesterol, LDL cholesterol and 20% of the triglyceride value.