THE ROLE OF MICELLAR SOLUBILIZATION IN LIPID ABSORPTION

Summary Results with everted sacs, in vitro , suggest that lipid uptake is limited by diffusion across an unstirred aqueous layer adjacent to the cell membrane. This step is facilitated by micellar solubilization. There is a linear relation between uptake and concentration of solubilized fatty acid. Bile acids play a non‐specific role as solubilizers since, for the same concentration of solubilized fatty acid, uptake is the same, irrespective of bile acid concentration, and for a different detergent the difference in uptake is consistent with the different micellar diffusion rates. Micellar aggregates carry absorbable lipid up to the cell membrane but lipid ultimately penetrates as single molecules, each type at its own rate. Metabolic events within the absorptive cell, such as resynthesis of triglyceride from absorbed fatty acid and monoglyceride, are affected by the rate of uptake but not, it seems, specifically by the presence or absence of bile acids. Absorption from the whole intesti ne in vivo does not always seem to fit the model suggested by isolated segments, in vitro . Comparison of results for the same mixture but with varying absorptive situations helps to reconcile some discrepancies. For example, adaptation in the proportion of absorptive area utilized can compensate for variations in absorption per unit area. The physicochemical properties of bile acids are admirably suited to a role as micellar solubilizers under the conditions prevailing in the lumen of tlie small intestine. There is no compelling evidence for an additional, specific role in absorption of fatty acid and monoglyceride. However, a specific requirement for bile acids in cholesterol absorption is suggested, particularly by experiments in vivo .