Cholesterol is a waxy fat-like substance the body needs to build strong cell membranes, produce hormones and make bile acids used to digest dietary fat. Although necessary for normal body functions, too much cholesterol in the blood vessels leads to a buildup that can restrict the flow of blood and contribute to heart disease, the leading cause of death in the United States. Your body produces approximately 75 percent of your total amount of cholesterol and uses several biological processes to control it.
Cholesterol Production
Your body converts fatty acids into cholesterol through a complex series of chemical reactions that occur in the liver. One of the first steps in cholesterol production requires an enzyme known as 3-hydroxy-3-methylglutaryl-coenzyme A, or HMG CoA reductase. The most commonly prescribed type of cholesterol-lowering medication, known as a statin drug, inhibits the activity of HMG CoA reductase, therefore slowing the production of cholesterol. In subsequent steps, the body converts HMG CoA reductase into another enzyme known as 3-hydroxy-3-methylglutaryl-CoA reductase, or HMGR. The body controls the rate of cholesterol production by inhibiting or stimulating HMGR. In the presence of excess cholesterol, the production of HMGR decreases, which reduces the amount of cholesterol produced.
Reverse Cholesterol Transport
Once the liver produces cholesterol, it must travel through the blood to reach the cells throughout the body. Since cholesterol, as a fat, is waxy and the blood consists mainly of water, the liver produces specialized proteins known as lipoproteins to bind to and carry cholesterol. Low-density lipoprotein, or LDL, binds to the majority of the cholesterol in the liver and takes it to the cells. LDL is commonly known as the "bad" cholesterol. High-density lipoprotein, or HDL, extracts excess cholesterol from the tissues and transfers it back to LDL molecules, which carry it back to the liver in a process known as reverse cholesterol transport, according to the Medical Biochemistry Page. HDL is known as the "good" cholesterol. By increasing your HDL level, through a healthy diet, increasing your physical activity level and maintaining a healthy weight, you stimulate this biological control mechanism for cholesterol.
Bile Acids
The human body produces approximately 1 g of cholesterol per day to support necessary functions. Colorado State University reports that the majority of that cholesterol gets converted into bile acid -- a substance critical for the digestion of dietary fat. To produce bile acid, specialized LDL receptors in the liver bind to and uptake cholesterol carried by LDL in the reverse cholesterol transport. A series of enzymatic reactions convert the cholesterol into bile acid, which eventually gets excreted from the body with the waste. The production of bile acids is the main biological control for the elimination of excess cholesterol from the body.
Role of the Thyroid
The thyroid gland, located in the neck, produces and releases two hormones known as T3 and T4 that regulate nearly all the enzyme reactions in the body. Thyroid hormones stimulate the expression of the sterol regulatory element-binding protein necessary for the production of the LDL receptors in the liver. A decrease in thyroid hormone inhibits the protein, which decreases the number of LDL receptors. A decrease in the number of LDL receptors decreases the amount of cholesterol converted into bile acids. This means that patients with hypothyroidism, a condition of low thyroid hormone levels, may experience an increase in blood cholesterol levels.
References
- The Medical Biochemistry Page: Introduction to Cholesterol Metabolism; April 2011
- Colorado State University: Secretion of Bile and the Role of Bile Acids in Digestion; November 2001
- "Journal of Biological Chemistry"; Thyroid Hormone Regulation and Cholesterol Metabolism; D-J. Shin and T.F. Osborne; 2008
- American Heart Association: About Cholesterol; May 2011
