Cholesterol is a fatty lipid found in the body tissues and blood plasma of vertebrates. It can be found in large concentrations in the liver, spinal cord, and brain. Mostly insoluble in water, it often forms plaque deposits on the inside of arteries, a condition known as atherosclerosis, which is a major contributor to coronary heart disease when the buildup is such that it inhibits blood flow to the heart.
Cholesterol is a steroid that is primarily synthesized from acetyl-CoA in the liver, although other sites include the intestines, adrenal glands and reproductive organs. Cholesterol is the major precursor for the synthesis of vitamin D3, of the various steroid hormones, including cortisol, cortisone, and aldosterone in the adrenal glands, and of the sex hormones progesterone, estrogen, and testosterone. Cholesterol is also utilized in membrane synthesis. Cholesterol is excreted from the liver in the form of a secretion known as bile; it sometimes crystallizes in the gall bladder to form gallstones.
Research has shown that the relative abundance of certain protein complexes, called lipoproteins, to which cholesterol becomes attached may be the real cause of cholesterol buildup in the blood vessels. High-density lipoprotein (HDL) carries cholesterol out of the bloodstream for excretion, while low-density lipoprotein (LDL) carries it back into the system for use by various body cells. The ratio of Hdl to Ldl is a commonly performed test, and the higher the ratio of these two, the lower the risk of heart disease.