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Low-density lipoprotein (LDL) and high-density lipoprotein (HDL) are the two main types of lipoproteins; they differ markedly from one another in their ratio of cholesterol and triglycerides to protein, and thus, in their effect on blood levels of cholesterol.

LDL: the "bad" cholesterol. LDLs carry a great deal of cholesterol, because they help transport it from the liver to the rest of the body. Too much LDL cholesterol circulating in the blood can cause buildup on the inner walls of the arteries that feed the heart and brain. Over time, LDL cholesterol can contribute to the formation of plaque - a thick, hard deposit that clogs arteries, resulting in atherosclerosis. As arteries narrow, the oxygen supply to the heart is gradually compromised, and the risk for a heart attack grows.

HDL: the "good" cholesterol. HDLs carry relatively little cholesterol - they actually circulate in the bloodstream, helping remove excess cholesterol from blood and tissues. Specifically, evidence suggests that HDLs tend to carry cholesterol away from the arteries and back to the liver, where it is then eliminated from the body; and that HDLs also remove excess cholesterol from plaque in the arteries, resulting in a slower potential buildup of plaque.

TRIGLYCERIDES transport and store fat in the body. It's clear that high triglyceride levels aren't good for the body and can be particularly problematic when combined with excess levels of LDL cholesterol. People with high triglycerides often have high total cholesterol, high LDL cholesterol, and low HDL cholesterol. And some lipoproteins are rich in triglycerides that also contain cholesterol.

The National Cholesterol Education Program, developed by the National Heart, Lung and Blood Institute of the National Institutes of Health, has set the "safe" total cholesterol level at 200 mg/dL. A score of 200 or higher signifies increased risk for developing heart disease; over 240 puts you in the high-risk category.

An LDL cholesterol level of less than 130 mg/dL is acceptable for most people; less than 100 mg/dL is ideal. The desirable level of LDL cholesterol depends on whether an individual has risk factors for coronary artery disease, or a pre-existing health condition caused by atherosclerosis or diabetes.

The AHA and the American Diabetes Association currently recommend HDL levels of at least 40 mg/dL for men and at least 50 mg/dL for women. A blood level of 60 mg/dL or above is ideal.

The HDL:LDL ratio is crucial - perhaps more so than the total cholesterol score. For example, if you have a total cholesterol score of less than 200 mg/dL, you could still be at risk for developing cholesterol-related atherosclerosis - if your LDL level constitutes a large percentage of that score. By comparison, if your total cholesterol is higher than 200 mg/dL, you may actually be at less risk, if your HDL score is high.