What are Lipoproteins?
When one hears about blood cholesterol levels, there is a bit of a misunderstanding about what exactly is being discussed. The body’s lipids – cholesterol, triglycerides and phosphoplipids – are transported in the blood attached to lipoproteins. Without these lipoproteins, cholesterol or any other type of fat for the matter, cannot stay dissolved in the blood. Although these lipids are separate entities, its interaction is closely related to the extent that one can increase or decrease the other.
HDL, LDL, VLDL and IDL
The good cholesterol is known as HDL-cholesterol or high-density lipoprotein cholesterol. HDL carries very small quantities of cholesterol and phospholipids and transport it away from the tissues to the liver where it can be broken down and excreted. The bad cholesterol known as LDL-cholesterol is when cholesterol is attached to low-density lipoproteins. These protein carriers can transport a high concentration of cholesterol and moderate quantities of phospholipids to the body’s tissues. There are two other types of lipoproteins – VLDL (very-low density lipoprotein) and IDL (intermediate density lipoprotein) which carry high and moderates amounts of triglycerides respectively as well as cholesterol and phospholipids.
Triglycerides and Cholesterol in the Blood
The body has to use triglycerides for energy at some point or the other. To do this, triglycerides are called up from the fat tissue or liver and has to be transported to the cells where it will be used for energy production. Since fats are not soluble in blood, the lipoproteins bind to it and carry it to various destinations. VLDL (very-low density lipoprotein) and IDL (intermediate-density lipoprotein) are primarily responsible for carrying triglycerides. As the triglycerides are delivered to the target cells, the VLDL becomes IDL.
Function of HDL and LDL Cholesterol
Cholesterol and phospholipids are also present in VLDL and IDL. Eventually once all triglycerides are removed, the remaining lipoprotein laden with cholesterol and phospholipids is known as LDL. Much of this LDL-cholesterol is removed by the liver but circulating LDL will deposit cholesterol in the tissue cells. Fortunately HDL-cholesterol, the good cholesterol, is a protective mechanism which carries some of this cholesterol back to the liver to be excreted in bile so that it does not accumulate in the liver tissues.
How triglycerides affect HDL and LDL Cholesterol
However, high levels of triglycerides disrupts this mechanism. In the presence of high triglyceride levels, a compound known as CETP (cholesterol ester transfer protein) in the blood reduces HDL-cholesterol. It also reshapes LDL-cholesterol to make it into small densely-packed particles that are more athrogenic, meaning that it contributes to and promotes plaques in the artery wall. LDL-cholesterol is a key factor in the development of atherosclerosis, the narrowing of arteries by the build up of atheromatous plaques.