Cholesterol gallstones are primarily made up of cholesterol. They are the most common type of gallstone, comprising 80% of gallstones in individuals in Europe and the Americas. Cholesterol is one of the substances that liver cells secrete into bile. Secretion of cholesterol into bile is an important mechanism by which the liver eliminates excess cholesterol from the body.
In order for bile to carry cholesterol, the cholesterol must be dissolved in the bile. Cholesterol is a fat, however, and bile is an aqueous or watery solution; fats do not dissolve in watery solutions. In order to make the cholesterol dissolve in bile, the liver also secretes two detergents bile acids and lecithin-into the bile. These detergents, just like dish-washing detergents, dissolve the fatty cholesterol so that it can be carried by bile through the ducts. If the liver secretes too much cholesterol for the amount of bile acids and lecithin it secretes, some of the cholesterol does not dissolve. Similarly, if the liver does not secrete enough bile acids and lecithin, some of the cholesterol does not dissolve. In either case, the undissolved cholesterol sticks together and forms particles of cholesterol that grow in size and eventually become gallstones.
There are two other processes that promote the formation of cholesterol gallstones though neither process is able to form cholesterol gallstones by itself. The first is an abnormally rapid formation and growth of cholesterol particles into gallstones. Thus, with the same concentrations of cholesterol, bile acids and lecithin in the bile, patients with gallstones form particles of cholesterol more rapidly than individuals without gallstones. The second process that promotes the formation and growth of gallstones is reduced contraction and emptying of the gallbladder that allows bile to stay in the gallbladder longer than normally so that there is more time for cholesterol particles to form and grow into gallstones.