Cholesterol is a fat-like substance (lipid). About 80 percent of cholesterol is manufactured in the liver. The remaining cholesterol is consumed in cholesterol-rich foods such as meat, eggs or dairy products. Cholesterol is vital to good health. The body uses cholesterol to:

Cholesterol is carried through the bloodstream by lipoproteins. Lipoproteins are proteins that wrap around cholesterol and other fatty materials and transport them through the bloodstream. There are five major kinds of lipoproteins, each of which has a different function in the body. They are:

• High-density lipoproteins (HDL). “Good” cholesterol, HDLs move easily through the blood and are actually beneficial. They are stable and do not stick to artery walls. They help prevent heart disease by carrying cholesterol away from the arteries and back to the liver, where the process of its removal from the body begins. Liver damage, from alcohol abuse or other conditions, can undo the beneficial effects of HDLs.
  
  
• Low-density lipoproteins (LDL). “Bad” cholesterol, LDLs contain more fat and less protein than HDLs. LDLs are unstable and tend to fall apart. They  are more likely to adhere to the walls of the artery and penetrate the protective inner lining of cells (e.g., the endothelium). Once cholesterol has migrated into the inner wall of the artery, it oxidizes and attracts other fatty substances (e.g., triglycerides), sticky blood-clotting materials (e.g., fibrin and platelets) and white blood cells. Together, these substances form the building materials for plaque deposits, which are the hallmark of atherosclerosis, or "hardened arteries." Atherosclerosis is the leading cause of coronary artery disease, which is the leading cause of heart attack and death in the United States. LDL reduction is a primary therapeutic goal among people with abnormal cholesterol levels (e.g., dyslipidemia). Many people with high levels of "bad" cholesterol also have high triglyceride levels because both types of fats have similar risk factors (e.g., obesity and diabetes).
  
  
• Very low-density lipoproteins (VLDL). These are extremely harmful lipoproteins that carry triglycerides and cholesterol.
  
  
• Intermediate-density lipoproteins. Like VLDLs, these also carry triglycerides and cholesterol.
  
  
• Chylomicrons. These are very large particles that are rich in triglycerides.

Together, LDL, VLDL and the intermediate-density lipoproteins are sometimes referred to as non-HDLs. A high non-HDL is an indicator of increased risk for heart attack and angina.

Finally, advances in research techniques have made it possible to measure the levels of individual lipoproteins in the blood. Lipoprotein “a” – abbreviated Lp(a) – is associated with increased risk of heart disease when values are greater than 30 milligrams per deciliter. Lp(a) can sometimes be detected in patients with premature coronary heart disease, which is defined as heart disease that appears in men who are younger than 55 years old and women who are younger than 65 years old.

A second blood lipid known as apolipoprotein B (apo-B) may be helpful in determining cardiovascular risk. According to some studies, apo-B may even be a better predictor that LDL cholesterol because apo-B has been implicated in the early formation of arterial plaque. Some researchers believe that one of the first steps in the atherosclerotic process is retention of apo-B in the deep arterial wall. 

Researchers are also finding that the size and number of cholesterol particles may be associated with human longevity. Studies have shown that people with higher LDL particle number are more likely to suffer from a heart attack than people with lower LDL particle number, independent of overall LDL levels. Also, some research has shown that smaller, more dense LDL particles are more dangerous than larger, looser LDL particles.

Newer cholesterol tests are emerging that can measure for these characteristics. For instance, some researchers believe that the ratio of apolipoprotein A to apolipoprotein B may be a better indicator of heart attack risk than total cholesterol. Still other tests can measure for Lp(a) and LDL particle size and number. Currently, these tests are not part of the standard lipid profile. However, patients wishing to know more information about them should consult their cardiologist.