With the launching of the Framingham Heart Study in 1948, scientists began gathering information about behavioral and biological risk factors for heart disease. In 1961, Framingham's researchers determined that an elevated cholesterol level was associated with atherosclerosis, or hardening of the arteries, and a higher risk for cardiovascular disease. Subsequently, they discovered that different forms of cholesterol in your bloodstream conferred varying levels of risk.
Cholesterol Synthesis
Cholesterol is produced by nearly every cell in your body because this lipid, or fat, is essential for normal cellular function. Several enzymes are needed for cholesterol synthesis, but the most important of these is called HMG-CoA reductase, which is the target of many cholesterol-lowering drugs. Cholesterol is also transported through your bloodstream, but it must be packaged in a water-soluble form first. This packaging process, which involves the combining of cholesterol with proteins to form lipoproteins, takes place primarily in your liver.
Lipoproteins
The movement of cholesterol from place to place in your body is facilitated by several distinct lipoproteins. The ones that are familiar to most people are high-density lipoprotein, or HDL, and low-density lipoprotein, or LDL. LDL transports cholesterol from your liver and deposits it in other tissues, including the inner walls of your arteries. Conversely, HDL "vacuums" cholesterol from your peripheral tissues and returns it to your liver for processing. LDL tends to worsen atherosclerosis, while HDL helps to reduce it. Thus it is desirable to keep your LDL low and your HDL high.
Deciphering the Numbers
The numbers you see on your lipid panel, which is a test that measures the different kinds of lipids in your body, are generated by measuring the levels of those lipids from a small volume of your blood. Your total cholesterol is determined by adding the amounts of cholesterol that are contained in your HDL, LDL and several other cholesterol-bearing lipoproteins. Some of these "minor" lipoproteins don't appear on your lipid panel, which is why simply adding the HDL and LDL will not equal your total cholesterol.
American Heart Association Recommendations
After consolidating the massive amount of scientific data that has accumulated during the past six decades, the American Heart Association has disseminated recommendations for optimal levels of total cholesterol, HDL, LDL and triglycerides. Your total cholesterol should be less than 200, your HDL should be above 40 and your triglycerides should be less than 150. If you are healthy -- that is, you currently have no heart disease or cardiac risk factors such as smoking or high blood pressure -- an optimal LDL level is below 100. If you already have heart disease or if you exhibit risk factors such as smoking, obesity, high blood pressure or diabetes, your doctor may want you to get your LDL even lower.
