Bile acids are derived from cholesterol and have detergent properties which aid in fat digestion and absorption (Figure 18.4). Bile acid synthesis is the major metabolic end product of cholesterol, accounting for about half of the 800 mg/day that is made in the body. By contrast, synthesis of steroid hormones from cholesterol requires only about 50 mg/day of cholesterol. Although the body synthesizes about 400 mg of bile acids per day, closer to 20-30g of bile acids per day are secreted into the upper small intestine. To meet this need, the body recycles bile acids from the lower small intestine to the liver via enterohepatic circulation.

Bile acids are eliminated in the feces but, due to recycling, only about 0.5g/day or less are eliminated in this way.

Cholic acid and chenodeoxycholic acid are the predominant bile acids in humans. The bile acid deoxycholate is abundant in the bile of some other mammals. Conjugates of cholic acid with glycine and taurine form salts called glycocholate and taurocholate, respectively.

Synthesis of bile acids and salts from cholesterol is shown in Figure 19.23. These reactions involve enzymes called microsomal P450 mixed-function oxidases. The first reaction, in which cholesterol is converted to 7--hydroxycholesterol, is the rate limiting step.