THE BODY'S CHOLESTEROL RECYCLING SYSTEM

The body's cholesterol recycling system

Think of the liver as not only a bile and lipoprotein manufacturing plant but also a cholesterol removal and recycling plant. How does the liver recycle cholesterol? The liver cells build bile acids from cholesterol. The bile acids are then secreted in bile (along with free cholesterol) into the upper section of the small intestine, where they aid in fat digestion. Once in the acidic environment of the small intestine, the bile acids convert into bile salts. Bile salts are continuously recycled from the intestine and returned to the liver. Each bile salt is actually reused about twenty times before it is excreted out of the body in the feces.

The liver, therefore, doesn't need to churn out too much new "homemade" cholesterol because it has this neat recycling program (called enterohepatic circulation) in which the bile acids (formerly cholesterol) are continuously recycled between the intestines and the liver. Almost 95 percent of the bile acids entering the intestine are recirculated and absorbed back into the bloodstream, heading straight to the liver, where they are once again secreted into the intestine in bile. This recycling system decreases the liver's requirement for new cholesterol and actually inhibits production of new bile acids from cholesterol. How much cholesterol the liver uses up each day to make bile acids depends on the return flow of bile salts from the intestine. Sequestering bile salts in the intestine so that they are excreted in waste and not recycled will force the liver to increase LDL clearance from the blood to replenish the cells' internal cholesterol stores (used for making new bile). Another class of cholesterol - lowering drugs works in this fashion. An important concept to keep in mind is that there is only one primary route out of the body for cholesterol (in the form of either free cholesterol or bile salts), and that is through the feces by way of the intestinal tract. If you block the absorption of free cholesterol or interrupt the recirculation of bile salts by trapping them in the intestine and excreting them, the end result is a drop in circulating LDL cholesterol. Several steps in the Cholesterol Down Plan work to lower LDL in this manner.