The Four Types of Lipoproteins

Cholesterol Down: Ten Simple Steps to Lower Your Cholesterol in Four Weeks - Without Prescription Drugs
by Janet Bond Brill, Ph.D., R.D., LDN

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Lipoproteins are divided into four main classes according to density: chylomicrons, VLDL, LDL, and HDL. Lower-density lipoproteins are characterized by a higher fat-to-protein ratio (fat is lighter) and therefore float more easily in the blood. Here are the basics about the four classes of lipoproteins in order of increasing density:

Chylomicrons are the least dense of all the lipoproteins and are basically just big balls of fat (triglycerides), with a makeup of about 90 percent fat, a touch of phospholipids, some cholesterol, and a smidgin of protein.

VLDL (very low-density lipoprotein) carries a great amount of fat, some phospholipids, and cholesterol. The high fat content of VLDL makes a large quantity of this lipoprotein in the blood undesirable.

LDL (low-density lipoprotein, or "bad" cholesterol) has only a fraction of the fat and double the protein of VLDL and is very high in cholesterol. This lipoprotein carries the majority of cholesterol in the blood and is considered the unhealthy one.

HDL (high-density lipoprotein, or "good" cholesterol) is a spherical blob of mostly protein (albeit a type different from that found in LDL), some cholesterol, phospholipids, and very little fat. The densest of all the lipoproteins, HDL is the healthy one.

Measuring Your Cholesterol

Your doctor will ask you to fast overnight before having your blood drawn to measure your lipoprotein levels. Fasting ensures that the chylomicrons are gone and have no effect on the sum total of cholesterol or triglycerides swimming around your bloodstream. The lab report will analyze your blood specimen for HDL, LDL, and VLDL. It will show the amount of "good" cholesterol (HDL), "bad" cholesterol (LDL), and triglycerides (blood fat) in the bloodstream. Your doctor may also test you for some risky particle characteristics- such as small and dense LDLs, small HDLs, or big VLDL particles-if you are at high risk for heart disease.

The key to a healthy blood test lies in the cholesterol transport: where the cholesterol goes, how it gets there, and how much of it accumulates. The blood test measures the quantity of cholesterol transport vehicles (aka lipoproteins). A healthy blood test shows a high number of HDL cholesterol transporters (the lipoprotein that carries cholesterol out of the arteries back to the liver for degradation). Too much LDL is unhealthy because it can build up in the inner arterial wall that feeds the heart and brain. In combination with other substances, the cholesterol in LDL forms plaque, which clogs the arteries (atherosclerosis). If a blood clot forms, cells downstream die and a heart attack or stroke occurs. Atherosclerosis is therefore a disease related to problems with cholesterol transport.

HDL, The Lifesaving Lipoprotein

HDL is manufactured in the small intestine and the liver, its primary source. As it moves through the bloodstream, it takes in excess cholesterol that leaches out of tissue cells and, most important, excess cholesterol building up in the inner arterial wall. HDLs unload this cholesterol in the liver, where it is then excreted via bile, a process referred to as "reverse cholesterol transport." Other lipoproteins bring cholesterol into the cells, so it is this reverse transport from the cells to the liver that distinguishes HDL as the "good" cholesterol. An elevated level of HDL has been associated with a reduced risk for heart disease. HDL confers heart-protective benefits in four ways:

1. HDL circulates around the body, picking up excess cholesterol and bringing it back to the liver for disposal (reverse cholesterol transport).

2. HDL is an antioxidant, capable of dismantling rogue free radical molecules that oxidize the protein in the LDL particles-a contributing factor to atherosclerosis. (Oxidation is the same process that produces rust on metals.)

3. HDL is an anti-inflammatory (like aspirin) and can decrease the inflammation linked with the atherosclerotic process.

4. HDL lessens the ability of the blood to form clots, thus reducing the risk of heart attack or stroke.

LDL, The Deadly Lipoprotein

LDL is the chief cholesterol carrier in the blood, ferrying approximately 70 percent of all the blood cholesterol around the network of arteries. Basically, this lipoprotein is loaded with cholesterol. Unlike HDL, LDL is not directly manufactured in the liver. Instead, a different type of lipoprotein is first produced by the liver, the parent molecule called VLDL. After circulating around the bloodstream, VLDL loses much of its fat (triglyceride) cache to various bodily cells to become LDL. LDL is designed to take cholesterol to cells that have run short, as cells require some cholesterol to maintain proper functioning. Typically, the LDL is taken into the cell and broken down, and then the cholesterol is used to make membranes or hormones. However, when the amount of LDL in the blood gets too high, the situation can become injurious.

Copyright © 2006 by Dr. Janet Brill.

About the Author

Dr. Janet Brill is a registered and licensed dietitian/nutritionist, exercise physiologist, and certified wellness coach. She has been published in the International Journal of Obesity and the International Journal of Sport Nutrition, as well as in the popular press.