Development of an Assay to Determine the Amount of Ca-Fatty Acid Soaps in Feces

BACKGROUND: The quantification of fecal Ca-fatty acid soaps is important to understand how fatty acids behave in the gastrointestinal tract. OBJECTIVE: As current methods to extract Ca-fatty acid soaps from feces give low recoveries an accurate assay to determine the amount of fatty acid soaps in feces was developed. METHOD: Ca-fatty acid soaps are determined indirectly after non-soap fatty acid compounds have been extracted from the feces. Synthetic Ca-fatty acid soaps of different chain lengths (C12-C18) and degree of saturation (C18:0-C18:2) were incubated with several solvents to find the solvents that least-solubilize the Ca-fatty acid soaps. A three-step extraction was devised using extractions with hexane, hexane-isopropanol and water either at room temperature or at 60°C, 37°C, or 80°C, respectively. Feces were spiked with free fatty acids, Ca-fatty acid soaps, Na-fatty acid salts, and phospholipids. RESULTS: All of the free fatty acids and phospholipids and almost all of the Na-fatty acid salts were removed and 98% of Ca-lauric acid soap, 99% of Ca-stearic acid soap, and 93% of oleic acid soap were recovered. CONCLUSIONS: The method is suitable for determining fatty acids in the form of Ca-fatty acid soaps in feces. HIGHLIGHTS: New method to determine fecal Ca-fatty acid soaps. Consistent and high recovery of fatty acid-soaps.