Bile salts and fat absorption.

There has been a recent resurgence of interest in bile salt metabolism. Normally the body contains about 3 g. of bile salts. These comprise conjugates of cholic acid and chenodesoxycholic acid. The free acids are derived from the oxidation of cholesterol in the liver and once formed are conjugated with either taurine or glycine, after which they are secreted into the bile and so enter the small bowel.' They pass down the gut and are mainly reabsorbed by the ileum. This early observation2 has recently been confirmed3 and an active transport system confined to the ileum has been demonstrated .4 This is teleologically satisfactory for it allows a high concentration of bile salts to be maintained in the lumen of the jejunum and yet prevents too great a loss of the total bile salt pool from the body; normally this is 0O8 g. a day'. Once passed the ileocaecal valve the conjugated salts are both deconjugated and degraded. Many metabolites are formed, one of which is desoxycholic acid. Some of these are partially absorbed from the caecum, pass up the portal vein, are then conjugated and excreted in the bile. Thus, in man conjugates of desoxycholic acid represent previous bacterial degradation of cholic acid.' Various functions have been attributed to bile salts. They are extremely active surface agents and they probably facilitate emulsification of the long-chain fatty acid triglycerides and so aid the action of pancreatic lipase, which to be effective has to work on an emulsion at an oil/water interface,5 but their precise effect on pancreatic lipolysis is still open to question. Even so, it is doubtful if impaired digestion of triglyceride as the rate-limiting step in fat absorption in the absence of bile, for under these circumstances triglycerides are absorbed as fast as free fatty acids.6' 7 Another property is to disperse fat into a water-soluble form.8 Verzair9 showed that oleic acid dissolved in a bile salt solution could pass through a membrane and suggested that bile might pass through the mucosa in this form. This property has been studied and its relation to fat absorption explored by Hofman and Borgstrom using techniques developed by detergent chemists.'0 11, 12 Bile salts after reaching a critical concentration form aggregates rather than precipitate out of solution and so they behave like long-chain paraffin soaps. The micro aggregates of detergents are called micelles and may entrap lipid soluble compounds. The concentration …