Bile acid metabolism. Studies on the mechanism of intestinal absorption

Bile acids, the products of oxidative catabolism of cholesterol by the liver, play an important role in the intestinal absorption of fat. Recent work suggests that these amphipathic sterols act not only as simple emulsifying agents but, more impor-tantly, also as the basic micellar structure into which the poorly soluble amphiphylic monoglycer-ides and long chain fatty acids become solubilized (1). It is largely as a consequence of the forma-tion of these "mixed micelles " that absorption of fat into the mucosal cells of the gastrointestinal tract is facilitated. It has been appreciated for many years that bile acids undergo an enterohepatic circulation with ab-sorption from the gastrointestinal tract followed by rapid clearance from the blood as the liver se-cretes these acid sterols back into the bile. The intestinal absorptive phase of this cycle is of con-siderable theoretical as well as clinical interest, since an understanding of the transport mecha-nisms whereby bile acids normally are able to penetrate the mucosal cell barrier no doubt will be necessary before the elucidation of the pathophysi-ology of certain malabsorption syndromes associ-ated with deranged bile acid metabolism is pos-sible. Yet, despite the potential physiologic im-portance of bile acid absorption, little detailed data are currently available concerning the transport of these sterols. For example, several studies have shown that the ileum is capable of moving conjugated and unconjugated bile acids against a * Submitted for publication August 19, 1965; accepte