Cholesterol plays an essential role in fat metabolism and additionally functions to maintain biological membrane structure and is the starting molecule for steroid hormone synthesis. Dietary cholesterol and that produced in the liver is transported in the circulatory system for use by other tissues. The metabolism and utilization of cholesterol is a tightly regulated process so that excess cholesterol does not build up in blood vessels, causing atherosclerosis.
Fat Digestion
The major use of cholesterol is for bile acid synthesis. Liver cells synthesize bile acids from cholesterol. The bile acids are transported to the gall bladder and stored until dietary fat enters the small intestine. This triggers the release of bile from the gall bladder. Bile acids have a hydrophobic component and a hydrophilic component, so that they can surround fats in the small intestine. This process, called emulsification, allows for the breakdown of fat aggregates into small particles and their transportation in the aqueous fluid of the small intestine.
Fat Transport
Chylomicrons are large molecules made from fats, proteins and cholesterol by cells of the small intestine. These molecules transport fats out of the intestines and into the circulatory system. Some of the fat is stored in fat tissue, and some is transported to the liver.
LDLs
The liver produces low density lipoproteins, also referred to as LDLs. The LDLs, similar to chylomicrons, are made up of fats, proteins and cholesterol. The LDLs transport cholesterol and fats from the liver to other tissues in the body. LDLs are sometimes called the bad cholesterol because high circulating levels of LDLs are a risk factor for cardiovascular disease.
HDLs
When cells have too much cholesterol, transport molecules called high-density lipoproteins, or HDLs, pick up the excess cholesterol and transport it back to the liver. The cholesterol is either reused to make predominately bile acids or excreted. HDL is sometimes called the good cholesterol because it functions to remove excess cholesterol from the body, so higher levels of HDLs are indicative of a decreased risk for cardiovascular disease.
Cholesterol Excretion
The primary route of cholesterol elimination in the body is through the bile. The bile, containing free cholesterol, bile acids and bile salts, that is released from the gallbladder during digestion is excreted in the feces. However, a review from Colorado State University reports that more than 90 percent of bile acids are recycled back to the liver and reused for dietary fat digestion.
