The National Heart Lung and Blood Institute created the National Cholesterol Education Program in 1985 to educate the American public about the dangers of high blood cholesterol levels. High blood cholesterol increases your risk for heart disease, which remains the leading cause of death in the United States. Your diet contributes to your cholesterol level, but your liver produces approximately 1,000 mg of cholesterol per day to maintain a constant cholesterol level, since your body continually uses and excretes cholesterol.
Function
Although too much cholesterol causes health problems, your body needs some cholesterol to survive. Cholesterol is necessary for the production of steroid hormones including sex hormones that regulate fertility, glucocorticoid hormones that regulate blood sugar and mineralcorticoid hormones that regulate blood pressure. The cells also use cholesterol to add structure and support to the cell membrane. Since your body continually produces hormones and new cells, it needs a continue supply of cholesterol. When cells uptake and use cholesterol, it gets removed from your bloodstream.
Production
Your liver cells produce approximately 75 percent of your total blood cholesterol. Enzymes in your liver, including HMG CoA reductase, stimulate the conversion of fatty acids into cholesterol. In order to travel through the blood to reach the cells in the body, cholesterol must bind to specialized proteins known as lipoproteins. Your liver produces several types of lipoproteins that differ in the amount of protein they contain and the amount of cholesterol they can carry. Low-density lipoprotein, LDL, contains a small amount of protein and can carry a large amount of cholesterol. High-density lipoprotein contains a higher amount of protein and less cholesterol.
Transport
LDL cholesterol carries the cholesterol produced in your liver through the bloodstream to the cells throughout the body. Cells uptake the cholesterol and either use it as a structural component of the cell membrane or convert it into a steroid hormone, therefore removing the cholesterol molecule. HDL plays a vital role in the process known as reverse cholesterol transport. During this process, HDL picks up excess cholesterol from the tissues and blood vessels and carries it back to the liver. Specialized receptors in the liver bind to the cholesterol and use it to produce bile acids.
Bile Acids
Liver cells use the returned cholesterol to produce cholic and chenodeoxycholic acids, substances known as the bile acids. The liver then releases the bile acids to the gall bladder, which stores them until needed in the process of digestion. As food moves through your digestive tract and enters the small intestine, the gall bladder sends the bile acids to the small intestine. These acids break down fat molecules into tiny droplets to allow enzymes to break down the fat into molecules the intestines can absorb. Although bile acids recirculate through the system, they eventually get excreted with your body waste. Your body converts approximately 500 mg of cholesterol into bile acids each day, serving as the main route of cholesterol elimination, according to Colorado State University.
References
- National Heart Lung and Blood Institute: National Cholesterol Education Program
- Colorado State University; Secretion of Bile and the Role of Bile Acids in Digestion; November 2001
- National Heart Lung and Blood Institute; High Blood Cholesterol; September 2008
- "Journal of Lipid Research"; Morphology and Structure of Lipoproteins; Zhang et. al.; January 2011
- American Heart Association; About Cholesterol; May 2011
