Stearic acid is saturated and has 18 carbons in a straight chain, octadecanoic. Dissolve some in water and the solution is clear until the tiniest trace of Ca, Mg, Sr, etc is added and then soap scum precipitates. Mag stearate is a component of the gunk in your shower. I would think some is naturally present in the gut by action of bile salts acting on oils. In the stomach I'd guess it would remain insoluble because of pH and the carbon side chain might be digested if something is present in gastric juices that munches hydrocarbons.