Mechanisms of bile acid and fatty acid absorption across the unstirred water layer and brush border of the intestine

The role of the bile acid micelle in enhancing fat absorption must be re defined. The fact that the presence of a micellar phase increases fatty acid and steroid uptake in the intestine derives from the fact that the rate limiting step to uptake of these lipids is the unstirred water layer. The bile acid micelle overcomes this resistance by delivering a large mass of lipid across this rate limiting diffusion barrier so that a maximum concentration of fatty acid monomer can be maintained against the lipid cell membrane. The actual act of absorption then takes place by passive diffusion of the fatty acid across the epithelial cell brush border at a rate determined by the concentration of fatty acid monomer in equilibrium with the micelle and the appropriate permeability coefficient.