Cholesterol is in a class of molecules known as lipids, which includes fats, fatty acids and the fat-soluble vitamins A, D, E and K along with cholesterol. While cholesterol is sometimes vilified as only being a part of an unhealthy diet, the body needs to synthesize cholesterol for certain functions. Consult a physician or registered dietitian regarding dietary choices.
Types of Cholesterol
Chemically, cholesterol is a four-ringed structure. One section can dissolve in water, and one can dissolve in fat or oil-like substances. High levels of cholesterol can contribute to heart disease. As measured in the blood, substances called "lipoproteins" are commonly called good and bad cholesterol. Lipoproteins are actually a combination of cholesterol, fat, fatty acids and protein. They are classified by density, with low density lipoprotein -- LDL -- considered to be the bad cholesterol and high density lipoprotein -- HDL -- the good cholesterol.
Metabolism
Cholesterol is absorbed in the diet or synthesized in the liver. It is transported through the blood as various lipoproteins, such as LDL and HDL, plus others, such as very low density lipoprotein -- VLDL -- and intermediate density lipoprotein -- IDL. LDL primarily takes the cholesterol to the tissues where it can be deposited. In the blood vessels, this can form plaque that can clog the arteries, a condition known as atherosclerosis. HDL takes cholesterol from the tissues and returns it to the liver to be processed.
Functions
Cholesterol is necessary for good health. However, the body is generally able to synthesize its own cholesterol, so dietary cholesterol can easily become excessive and contribute to bad health. Normal biochemical functions of cholesterol include the synthesis of steroid hormones, such as the stress hormone cortisol, the blood pressure, fluid and electrolyte hormone aldosterone, and the sex steroids testosterone and estrogen. It is also used to synthesize vitamin D. In addition, it is an important part of the cell membrane, analogous to the skin of the cell, where it helps the membrane to be flexible and withstand temperature changes.
Consequences
The normal biochemical pathways for cholesterol are necessary for the body. However, high LDL can decrease the blood supply to the organs, leading the body to increase blood pressure, which can further narrow the vessels. Over time, this narrowing can lead to heart attacks, strokes, kidney disease and other problems. The diagnosis and management of cholesterol should be tailored to the individual with the help of a physician, but lifestyle modifications, including proper diet and exercise, are first-line treatments, followed by medical therapy. Some medications inactivate enzymes that process and make LDL cholesterol. For example, statins inhibit a liver enzyme called HMG-CoA-reductase.
References
- "Lehninger Principles of Biochemistry"; David L. Nelson, et al.; 4th Ed 2004
- "Harrison's Principles of Internal Medicine"; Anthony S Fauci, et al.; 17th Ed 2008
- "Molecular Biology of the Cell"; Bruce Alberts et al.; 4th Ed 2002
