What is Cholesterol?

In order to understand the role of the liver in cholesterol production, it is important to first note what cholesterol actually is and how it originates.  Cholesterol is a sterol, a lipid, a fatty substance that is a major component of the outer membrane of all cells.  It has a complex chemical structure.

Cholesterol is also vastly located within the bloodstream.  Cholesterol originates in the body in two main ways.  First, dietary intake is responsible for cholesterol presence.  Secondly, the liver is responsible for not only the production and secretion of certain amounts of cholesterol, but is the main organ responsible for the regulation of cholesterol levels circulating throughout the bloodstream.

Cholesterol from Dietary Intake

Cholesterol that is received through dietary intake must be processed and absorbed.  Cholesterol is found in a variety of foods including, fish, poultry, dairy products and meat.  Foods that are derived from plants typically contain no cholesterol.  Once the cholesterol has been ingested, it is absorbed in the intestinal tract, where it circulates the blood encapsulated by a protein coat known as chylomicron.

The Liver

The liver is capable of removing chylomicron encapsulated cholesterol from the blood.  It can also produce and secrete cholesterol.  Ingestion of cholesterol activates the liver to remove these chylomicrons from circulating blood.  Before and after meals, when cholesterol is not readily available, the liver will manufacture and secrete certain amounts of cholesterol into the circulation of the blood.

Cholesterol: Total, LDL, and HDL

Total cholesterol is available in two main types, LDL and HDL.  LDL is known as “bad” cholesterol because it is associated with the formation of a hard substance within the arteries known as plaque.  The build up of plaque within the arteries is cause for reduced blood flow or even stopping blood flow, known as a blood clot.  This causes a narrowing of the arteries, known as atherosclerosis, associated with increased risk for heart disease, stroke, and heart attacks.

HDL cholesterol is known as “good” cholesterol.  This is because it is responsible for the removal of LDL cholesterol from arterial walls.  High levels of HDL present can more readily control LDL levels.

Liver: Cholesterol Production

The liver is not only capable of producing and secreting cholesterol, but it is responsible for the overall maintenance and regulation of cholesterol levels.  Cholesterol is necessary in the body, but it must be controlled to prevent specific conditions.  The liver is responsible for the isolation of cholesterol from the blood stream absorbed through ingested food.  Cholesterol that is absorbed and circulates the bloodstream can be taken in by the liver for recycling or elimination.

Typically, the primary path for synthesis of cholesterol is accomplished through use of a substance known as acetyl-CoA through a specific reductase pathway known as HMG-CoA.  The liver is responsible for the production of approximately 25% of cholesterol used by the body.  The rest of the cholesterol utilized must be ingested and absorbed.  However, the liver is responsible for the regulation of all cholesterol within the bloodstream.

Resources:

http://www.webmd.com/cholesterol-management/guide/cholesterol-basics

http://www.medicinenet.com/cholesterol/article.htm