Solubility of calcium soaps of long-chain fatty acids in simulated intestinal environment

Calcium soaps of long-chain fatty acids are often referred to as being insoluble. These soaps make up most of the fecal lipid in infants fed high calcium diets and are responsible for increased fat malabsorption in patients with exocrine pancreatic insufficiency receiving enzymes and calcium carbonate antacids. We investigated the solubility of the calcium soaps of long-chain fatty acids in simulated human intestinal contents. Calcium soaps of the ¹⁴C]fatty acids of lauric (120), palmitic (160), stearic (180), and oleic acids (181) were synthesized. Individual soaps were incubated (1 hr at 37°C) in 140 mM NaCl solution containing 10 mM bile salts; 75% glycine conjugates (35% cholate, 25% chenodeoxycholate, 15% deoxycholate), and 25% taurine conjugates (12% cholate, 8% chenodeoxycholate, 5% deoxycholate) with 1.45 mM lecithin at various pHs ranging from 5.0 to 6.8. Soap solubility was assessed by analyzing the supernatant for radioactivity after centrifugation at 12,500g for 15 min. Fatty acids were uniformly more soluble than the respective calcium soaps. The solubility of the free fatty acid can be compared to its calcium soap by two methods: either as the total amount of fatty acid in solution or as the molar solubility of the fatty acid and the soap (a calcium soap molecule contains two fatty acid molecules). By molar solubility the free fatty acid was 13 times more soluble than the calcium soap, whereas comparison of the quantity of fatty acid in solution revealed an average of 6.5 times more free fatty acid than that present in the respective calcium soap. The solubility of the calcium soaps was negatively correlated with acid concentration H⁺], carbon chain length (n), and fatty acid saturation. The solubility could be described by a multilinear regression (r=0.84) incorporating chain length and acid concentration as the independent variables; log soap solubility =–0.0379(n)–19298.2H⁺]–0.074.