The steroid cholesterol is the most prevalent steroid in all animals and is essential for animal life. Cholesterol has multiple physiological roles that include its structural presence in all mem branes; here it facilitates permeability and fluidity in the various cell membranes as well as the function of clathrin coated pits and invaginated caveolae. Cholesterol in the caveolae has been shown to be involved in cell signaling processes, by the formation of lipid rafts in the plasma membrane which allows liganded receptor proteins, like the estrogen, progesterone, and 1α,25(OH)2D3 nuclear receptors to be in close proximity with high concentrations of a variety of second messengers which can initiate rapid biological responses.
Cholesterol also is the starting point in the biosynthesis of all steroid hormones, vitamin D3 and its steroid hormone daughter metabolite, 1α,25(OH)2D3 as well the bile acids.
The level of the total body cholesterol is determined by a complex interplay of dietarily available cholesterol, de novo synthesis of cholesterol, and excretion of cholesterol and bile salts. In an average 70-kg man, 0.8–1.4 g of cholesterol is turned over per day. The daily ingestion of cholesterol in Western countries is 0.5–2.0 g; the efficiency of intestinal absorption ranges from 30 to 50%. However, it is known that dietary cholesterol is not the exclusive source of the bodily pool of cholesterol. The liver and intestine together account for more than 60% of the body’s daily biosynthesis of this sterol. When considering the turnover of body cholesterol, it has been suggested that there are two principal components. Pool A, which comprises ~30% of the total body cholesterol, consists of the cholesterol present in liver, bile, plasma, erythrocytes, and intestine. Pool B constitutes the remaining ~70% of the exchangeable body cholesterol; it principally comprises the cholesterol in the skin, adipose tissue, and skeleton.
