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Reviewed by the Faculty of Harvard Medical School

What Is Cholesterol?

Bacon & Eggs

Cholesterol is essential for human life. It builds and repairs cells, it is used to produce sex hormones like estrogen and testosterone, it is converted to bile acids to help you digest food and it is found in large amounts in brain and nerve tissue. The liver produces enough cholesterol to satisfy these functions.

Cholesterol produced by the liver can't just float loose in the water-based bloodstream. Instead, it is transported in special protein packages called lipoproteins. A typical lipoprotein contains triglycerides (another type of blood fat) and cholesterol in the center, surrounded by phospholipids and water-soluble proteins on the outer surface to help the lipids move through the watery fluids of the blood. The four types of lipoproteins differ from one another in their content of protein, triglycerides and cholesterol. Two types of cholesterol — high-density lipoprotein (HDL) cholesterol and low-density lipoprotein (LDL) cholesterol — have become quite familiar to most people concerned with the health of their heart and blood vessels.

HDL cholesterol is the "good" cholesterol because high concentrations in the blood are associated with a low risk of heart attack. HDL contains more protein than triglycerides or cholesterol and helps remove cholesterol from artery walls. HDL carries cholesterol from body cells to the liver, either to be reused, converted to bile acids or disposed of in the bile.

LDL cholesterol is the "bad" cholesterol that's associated with a higher risk of heart disease. LDL becomes oxidized and deposits in the walls of arteries to initiate the condition known as "atherosclerosis," or hardening of the arteries. This condition causes 500,000 heart attacks each year. Others risk factors that may contribute to atherosclerosis are a family history of the disease, age, male sex, cigarette smoking, hypertension and diabetes mellitus.

When you get a blood test for cholesterol levels, your doctor will be most interested in your levels of HDL and LDL cholesterol and triglycerides. The total cholesterol is actually less important. This table provides a guide to interpreting cholesterol levels, based on guidelines recently issued by the National Cholesterol Education Program Expert Panel.

LDL Cholesterol
Less than 100 Optimal
100 to 129 Normal
130 to 159 Borderline high
160 to 189 High
190 or greater Very high
HDL Cholesterol
Less than 40 Low
60 or greater High
Less than 150 Optimal

Tips To Help Improve Cholesterol Levels
  • Strive for a BMI of 25 or less
  • Exercise daily

Last updated January 2, 2009