Although the therapeutic benefits of calcium supplementation have yet to be fully determined, popular literature has promoted this practice to combat osteoporosis. This study examined the potential bioavailability of calcium from various supplement and antacid forms. Four dosage levels were studied for each of five sources, including leading supplements Caltrate and Os-Cal, two "calcium rich" antacids, Tums and Rolaids, and a calcium carbonate control. These samples were subjected to four gastric treatments in an in vitro digestion procedure designed to observe the influence of acid secretion on calcium solubility. Depending on the degree of acidity of these solutions, solubility behavior of CaCO3 was highly variable. Normal and reduced acid treatments adequately solubilized between 85 and 88% of the total calcium, whereas negligible quantities were dissolved in deionized water or the achlorhydric solution. Relative availability of calcium from the various sources differed according to gastric treatment. Tums, Caltrate and Os-Cal had dissolution rates that were similar to reagent grade CaCO3. Rolaids provided a significantly lower amount of soluble calcium under all of the experimental conditions (P < 0.001). Amounts of available calcium increased with increasing dosages. The effectiveness of fortification techniques used to increase the calcium content of various food systems was also examined. Fortified orange juice and yogurt samples were found to contain a quantity of available calcium comparable to that found in milk. It was concluded that both of these approaches are reasonable for increasing calcium intakes in individuals with normal gastric secretion. / Master of Science