Caffeine, an ingredient in colas and some other soft drinks, has been identified as a risk factor for osteoporosis. (In our study, however, the loss of bone mineral density was similar for diet soda, and just slightly less for decaffeinated options.) Also, cola is the only kind of soft drink that contains phosphoric acid, which interferes with calcium absorption and contributes to bone loss. As for carbonated water, it's generally considered neutral with respect to bone health. But a better choice is mineral water. It contains calcium and magnesium and, when consumed every day, has been shown to protect bone mineral density in adults.
-Katherine Tucker, Ph.D., director of the Nutritional Epidemiology Program in the Friedman School of Nutrition Science and Policy at Tufts University in Boston, Mass.