Four types of beverages were evaluated. One caffeinated and 1 non-caffeinated beverage contained citric acid as an acidulant, and 2 cola-type beverages were either caffeinated or non-caffeinated and contained phosphoric acid as an acidulant. Milk and water were also tested as control beverages. Small but significant increases in urinary calcium excretion resulted from consumption of the 2 caffeinated beverages, whereas the 2 non-caffeinated beverages had no effect. No calcium loss was due to either phosphoric or citric acid acidulation. Phosphoric or citric acid acidulation had no effect on calcium loss.
Though the caffeine in the drinks was primarily responsible for excess calcium excretion, previous studies of the effect of caffeine have shown a compensatory drop in calcium excretion over the 24-hour period following ingestion. The fact that the small calcium loss from carbonated beverages was offset by reduced excretion later in the day, and the habituation of the subjects to frequent consumption, lead the authors to conclude that the main cause of calcium loss from carbonated beverages was their lack of the nutrients needed for bone health. Carbonated beverages displace milk in the diet, eliminating a major source of bone-building calcium.
Heaney, Robert P. and Karen Rafferty. Carbonated beverages and urinary calcium excretion. Am J. Clin Nutr 2001;74:343-7.
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American Journal of Clinical Nutrition