Cholesterol excretion

Cholesterol is lost by the shedding of cells e.g. gut and skin, and by excretion within bile.

Biliary excretion is dependent on bile salts. Bile salts have a steroid backbone which is hydrophobic and hydroxyl, glycine or taurine side chains which are hydrophilic. Cholesterol is normally immiscible with water, but it associates with the hydrophobic portion of bile salts within the interior of micelles.

Mixed micelles, containing cholesterol, bile salts, lecithin and phospholipids, form when the concentration of bile salts reaches a critical threshold within bile. They consist of a hydrophilic exterior surrounding the hydrophobic interior.

The relative concentration of bile salts, cholesterol and phospholipid within bile can be shown to have a very close association with the tendency to precipitation of cholesterol. Cholesterol gallstones may result.