Lipoproteins: What Are They and What Do They Do?

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If you've had your cholesterol checked before, you have probably noticed a lot of different types of cholesterol listed on your lab result. LDL, VLDL, HDL - what do they all mean? All of these types of cholesterol may be made up of similar parts, but their functions in the body are different. Having elevated or lowered levels of some of these forms of cholesterol may increase your risk of developing heart disease.

Cholesterol and triglycerides are fatty molecules. Because of their fat-like properties, they are not able to easily circulate in the bloodstream. In order for cholesterol and triglycerides to travel in the blood, they are often carried by proteins that make the cholesterol and triglycerides more soluble in blood. This lipid and protein complex is referred to as a lipoprotein. When triglycerides and cholesterol are removed from this lipoprotein complex, and you have the protein alone, the protein component is referred to as an apolipoprotein. Different types of apolipoproteins are associated with different lipoproteins.

There are five different types of lipoproteins in the blood, and they are commonly classified according to their density.  The main types of lipoproteins that are analyzed in a lipid panel include:

  • Very Low-Density Lipoproteins (VLDL). These lipoproteins consist of mainly triglycerides, some cholesterol molecules, and less protein. The more fat a lipoprotein contains, the less density it has. In this case, VLDL is less dense than most lipoproteins because of its high lipid composition. VLDL is made in the liver and is responsible for delivering triglycerides to cells in the body, which is needed for cellular processes. As triglycerides get delivered to cells, VLDL is made up less of fat and more of protein, leaving cholesterol on the molecule. As this process occurs, VLDL will eventually become an LDL molecule.
  • Low-Density Lipoproteins (LDL). LDL consists of more cholesterol than triglycerides and protein. Because it contains less lipid and more protein in comparison to VLDL, its density is greater. LDL is responsible for carrying cholesterol to cells that need it. Elevated LDL levels are associated with an increased risk of cardiovascular disease. Certain forms of LDL - specifically small, dense LDL (sdLDL) and oxidized LDL (oxLDL) - have been associated with promoting the formation of atherosclerosis by depositing fats on the walls of arteries in the body. Because increased levels of LDL are associated with the development of cardiovascular disease, LDL is also known as the “bad” cholesterol.
  • High-Density Lipoprotein (HDL). Compared to LDL, HDL consists of less cholesterol and more protein, making these lipoproteins the most dense. HDL is made in the liver and in the intestines. It is responsible for carrying cholesterol from cells back to the liver. Because of this, HDL is also considered the “good” cholesterol.

There are also other lipoproteins that also function in transporting fats to cells, but are not commonly measured in a routine lipid panel. These include:

  • Chylomicrons. These lipoproteins are the least dense out of all of the lipoproteins. These molecules are primarily made up of triglycerides and a small amount of protein. Chylomicrons are responsible for transporting lipids from the intestinal tract to cells in the body.
  • Intermediate Density Lipoprotein (IDL). These lipoproteins are less dense than LDL molecules but denser than VLDL particles. As the triglycerides on VLDL are broken down by the cells that need it, the particle becomes more dense due to the change in the lipid to protein ratio. This results in VLDL being converted into IDL. As triglycerides and cholesterol are delivered to more cells in the body, IDL will gradually be converted into LDL.

    Sources:
    Baron RB. Lipid Disorders. In: Papadakis MA, McPhee SJ, Rabow MW. eds.Current Medical Diagnosis & Treatment 2015New York, NY: McGraw-Hill; 2014. 

    Rader DJ, Hobbs HH. Disorders of Lipoprotein Metabolism. In: Kasper D, Fauci A, Hauser S, Longo D, Jameson J, Loscalzo J. eds. Harrison's Principles of Internal Medicine, 19eNew York, NY: McGraw-Hill; 2015.

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