Nutrition Series: The Basics - Lipoproteins

Here is the next installment of the Nutrition Series! It may help to revisit the last post in this series, about cholesterol. This one explains a bit about how lipids are carried around in our bodies, and with topics like HDL and LDL abounding in the media, it can be beneficial to know a bit about these hard working molecules that have been indiscriminately labeled good and bad, respectively.

THE BASICS - LIPOPROTEINS

A chylomicron; ApoA, B, C, and E are apolipoproteins, T (Triacylglycerol); C (Cholesterol); green (Phospholipids)

Lipoproteins are complexes of phospholipids and apolipoproteins, as shown in the picture above. Lipoproteins play a hugely important role in the body, by facilitating the transport of non-water soluble triglycerides, cholesterol, and cholesterol esters in the aqueous(water based) environment of our bloodstream. These lipoprotein particles are classified by their density and size, with larger and less dense lipoproteins consisting of more fat than protein. Some of the largest particles may be up to 200 times the size of the smallest ones, but even the the largest, chylomicrons, can only be about 1/20th the width of the thinnest human hair.

Here are the various classifications of lipoproteins, in order from largest/least dense to smallest/most dense:

Chylomicrons are created by the enterocytes in the small intestine. Enterocytes function to absorb nutrients from the gastrointestinal tract. Chylomicrons carry exogenous(created outside the body) sources of lipids, absorbed by enterocytes, to the liver, heart, adipocytes(fat storage cells), and skeletal muscles. After they’ve unloaded their contents, the remnants of these particles are recycled by the liver into VLDL.

(VLDL) Very Low Density Lipoprotein particles are created by the liver and carry endogenous(created within the body) products to peripheral tissues. As triglycerides from VLDL are deposited into the cells of tissues, it becomes an IDL particle.

(IDL) Intermediate Density Lipoprotein are either recycled by the liver, or deposit more lipids and become LDL particles.

(LDL) Low Density Lipoprotein, or the so called “bad cholesterol”, has the role of transferring the relatively large amount of cholesterol and cholesterol esters to peripheral tissues. There are varying sizes of LDL particles, and only the smaller, denser LDL particles have been associated with heart disease.

(HDL) High Density Lipoprotein, often called “good cholesterol”, carries excess cholesterol from cells back to the liver, to be metabolized into bile acids and salts.

Together, HDL and LDL maintain the cholesterol balance of the body, with LDL transferring cholesterol to cells, and HDL carrying cholesterol from cells.

The image below demonstrates the differences between small and large LDL particles, and shows the metabolism of lipoprotein particles in general as well. (LPL stands for lipoprotein lipase, an enzyme which hydrolyzes lipids in lipoproteins)