Cholesterol is a waxy, fat-like substance (lipid). Although it's often discussed as if it were a poison, you can't live without it. It is essential to your body's cell membranes, to the insulation of your nerves, for the production of energy and hormones, and it is used by the liver to make bile, which helps to digest the fat in your food.
Approximately 80% of cholesterol is manufactured in the liver, with the remaining 20% coming from the diet. And although the amount of fat and cholesterol you eat may influence your cholesterol levels, other factors can also play a role. Cholesterol can only be carried in the blood after it is coated with special proteins. Once coated they form a package called lipoproteins. Some of the lipoproteins are called low-density lipoproteins, or LDLs, and contain mostly cholesterol. Others are called high-density lipoproteins, or HDLs, which contain mostly protein.
LDL is often referred to as "bad cholesterol" and HDL as "good cholesterol". Although this is not strictly true, it does give an indication of the importance of the balance between the two. LDL particles are the main source of cholesterol required by the body. The LDL particles carrying the cholesterol attach to receptors on cell surfaces and the cholesterol is then transferred into the cell. The problem arises if the cells become saturated with cholesterol because there are too many LDL particles in the blood due to excessive cholesterol intake in the diet, increased production of cholesterol by the liver, or reduced excretion of cholesterol by the liver. When this occurs, cholesterol can be deposited into the artery walls as plaques and this can eventually lead to blocked arteries. The HDL particles, on the other hand, pick up cholesterol deposited in the artery walls and transport it to the liver for disposal.
This is why a high HDL level relative to the LDL level is good. It helps to protect you from developing blocked arteries.