American Journal of Clinical Nutrition, Vol 22, 273-278, Copyright © 1969 by The American Society for Clinical Nutrition, Inc.

Current Concepts of Bile Acid Absorption

¹ From the Gastroenterology-Liver Section, Department of Internal Medicine, the University of Texas Southwestern Medical School at Dallas, Dallas, Texas

It is thus apparent that there are a variety of different transport phenomena that act in concert to maintain effectively an enterohepatic circulation in which there is a high bile acid concentration in the proximal small bowel and a progessively reduced concentration in the distal small bowel. This difference in intraluminal bile acid concentrations can be readily explained in terms of the different transport mechanisms operative at different levels of the small intestine. In the jejunum there is no active transport and since normally only conjugated bile acids are present in the jejunum, the relatively rapid mechanism of passive nonionic diffusion does not occur. Thus, proximal bile acid absorption is dependent on the relatively slow passive ionic diffusion and on passive micellar diffusion. In contrast, in the ileum active transport occurs; it is of teleologic interest that those bile acids (trihydroxy) that are passively transported the slowest, are actively transported the fastest. In addition, a large proportion of unconjugated bile acids, normally found in the distal small bowel, is probably absorbed via nonionic passive diffusion. Finally, passive ionic diffusion and passive micellar diffusion also probably take place, in the ileum, as in the jejunum. These various mechanisms working together, then, bring about rapid and nearly complete absorption of bile acid from the ileum.

It is apparent that in the past 10 years a great deal of information has been obtained concerning some of the properties of bile acid absorption. It should be equally apparent, however, that a number of features of these transport systems remain to be elucidated.

First, no detailed information is available concerning the relative quantitative importance of passive versus active transport mechanisms in overall bile acid absorption in the intact animal and, in particular, in man.

Second, additional information must be obtained on the quantitative importance of bile acid absorption from the colon in the intact animal. Such data may have particular relevance in such pathophysiologic states as seen in man after ileectomy.

Third, the precise mechanisms of passive absorption of bile acids from solutions above the critical micellar concentration must be elucidated.

Fourth, there currently exists a controversy concerning the stereospecificity of the ileal active transport site. It must be clarified whether it is primarily the configuration on the side chain as proposed by Lack and Weiner (22) or the number of hydroxyl groups on the steroid nucleus (23) that primarily determines the transport kinetics.

Fifth, since the micellar characteristics of the intestinal contents are highly complex, it must yet be determined how the presence of mixed micelles in the intestinal contents alters the transport of bile acid monomers, both ionized and unionized.