In Vitro Solubility Characteristics of Six Calcium Salts.

Six calcium salts (lactate, carbonate, citrate, gluconate, phosphate, and citrate malate) were evaluated for ionic and total soluble calcium content utilizing a sequential pH treatment to simulate gastrointestinal pH conditions. At pH 2.0 (gastric pH), 80-90% of all soluble calcium present was in the ionic form. However, when the pH was brought to 7.0 (intestinal pH), a unique solubility pattern for each calcium source was evident. Both calcium citrate and calcium citrate malate formed significantly higher levels of a soluble complex. By contrast, calcium phosphate precipitated extensively under the same conditions. These results indicate the importance of pH adjustment during in vitro testing of calcium bioavailability.