Cholesterol and other fats can not dissolve in the blood. They have to be transported to and from the cells by special carriers called lipoproteins. These are of two kinds.  Low-density lipoprotein, or LDL, is known as the "bad" cholesterol. Too much LDL cholesterol can clog your arteries, increasing your risk of heart attack and stroke. High-density lipoprotein, or HDL, is known as the "good" cholesterol. Your body makes HDL cholesterol for your protection. It carries cholesterol away from your arteries. Studies suggest that high levels of HDL cholesterol reduce your risk of heart attack.

A high level of LDL in the blood may mean that cell membranes in the liver have reduced the number of LDL receptors due to increased amounts of cholesterol inside the cell. After a cell has used the cholesterol for its chemical needs and doesn't need it any more, it reduces its number of LDL receptors. This enables LDL levels to accumulate in the blood. When this happens, the LDLs begin to deposit cholesterol on artery walls, forming thick plaques. In contrast, the HDLs-the "good" guys-act to remove this excess cholesterol and transport it to the liver for disposal. (To know more, refer to the article on HDL).

A third group of carrier molecules, the very low-density lipoproteins (VLDL) are converted to LDL after delivering triglycerides to the muscles and adipose (fat) tissue.

The levels of HDL, LDL and total cholesterol are all indicators for atherosclerosis and heart attack risk. People who have a cholesterol level of 275 or higher (200 or less is desirable) are at significant risk for a heart attack, despite a favourable HDL level. In addition, people who have normal cholesterol levels but low HDL levels are also at increased risk for a heart attack.