LDL and HDL are best thought of as cholesterol taxis. LDL is the “bad” taxi, because it transports cholesterol and fat from the liver into the bloodstream, where it can clog arteries. HDL is the “good” taxi, because it transports cholesterol from the blood back to the liver. As such, a low LDL-to-HDL ratio is a good thing, because it can reduce your risk of cardiovascular diseases.
LDL is an abbreviation for low-density lipoprotein, which is a type of cholesterol and triglyceride carrier within your bloodstream. LDL is often called the “bad” cholesterol, but it’s actually part of the delivery system. LDL transports cholesterol from your liver to the rest of your body via the blood, but it tends to deposit cholesterol and fat onto arterial walls, which clogs blood vessels over time and increases the risk of atherosclerosis, heart attack and stroke. Healthy LDL blood levels range from about 100 to 129 milligrams per deciliter, although you should aim for levels less than 100 if you are at higher risk for heart disease.
HDL stands for high-density lipoprotein. HDL is also a cholesterol and triglyceride carrier, but it’s smaller than LDL and mainly responsible for transferring cholesterol from the blood back to the liver for elimination, storage or recycling. In fact, HDL is responsible for transferring up to 30 percent of your blood cholesterol back to your liver, which reduces the risk of cardiovascular diseases. Ideal HDL blood levels are 60 milligrams per deciliter or higher, and levels below 40 are considered too low.
As you’ve probably guessed by now, it's best to have low levels of LDL combined with high levels of HDL. You can calculate cholesterol ratios a couple of different ways. One way is to compare your total blood cholesterol number to your HDL number. For example, if your total cholesterol is 200 milligrams per deciliter of blood and your HDL is 50 milligrams per deciliter then your ratio is 4:1, which is pretty good, but the optimum ratio is a little lower at 3.5:1. In this example, a lower ratio indicates a lower risk of cardiovascular disease. Another type of ratio is obtained by comparing HDL to LDL. For example, if your HDL is 50 milligrams per deciliter of blood and your LDL is 150 milligrams per deciliter, then your ratio is 0.33. In this comparison, ratios above 0.4 are ideal. In other words, your HDL levels should be 40 percent or more of your LDL levels.
Calculating cholesterol ratios provides useful information about your risks for cardiovascular disease, but not for determining the best way to reduce the risk if it’s high. For treatment purposes, it's more important to know the actual numbers for all your types of cholesterol and triglycerides. People most at risk of cardiovascular disease are those with very low HDL levels or very high LDL levels.
- Pocket Guide to Diagnostic Tests; Diana Nicoll et al.
- Human Biochemistry; Charles Dreiling
- MayoClinic.com: How important is cholesterol ratio?