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Bioavailability of Calcium in Soy Beverages

Health professionals are increasingly aware of the importance of mineral bioavailability in the foods we eat. Calcium is of particular importance, as it represents one of the most under-consumed nutrients in North America. Because of the current popularity of calcium, many calcium-fortified products have appeared on the market. Among these are fortified soy beverages.

The two most common types of calcium used to fortify these beverages are tricalcium phosphate and calcium carbonate. Heaney et al. conducted a study to evaluate tricalcium phosphate bioavailability in fortified soy beverages in comparison to the calcium found in cow's milk. Their results showed that the bioavailability of tricalcium phosphate accounted for 75% of the calcium found in cow's milk.1

In addition to its lower bioavailability, the calcium added to soy beverages can remain at the bottom of the container, regardless of the type of calcium used. A study analyzed the physical properties of fortified beverages, such as soy beverages.2,3 The authors evaluated eight national brands of calcium-fortified soy beverages2 and in most cases they observed the presence of residue at the bottom of the empty cartons, even when they had been vigorously shaken.3 This residue had the consistency of toothpaste.4 Soy beverage cartons that were not shaken provided on average 30% of the quantity of calcium indicated on the nutrition label, whereas the cartons that were shaken provided 59%.2 The authors thus concluded that the amount of calcium found in calcium-fortified soy beverages might not correspond to the amount indicated on the label.3

References Heaney RP et al. Bioavailability of the calcium in fortified soy imitation milk, with some observations on method. Am J Clin Nutr 2000; 71:1166-69. Heaney RP and Rafferty K. The settling problem in calcium-based soybean drinks. (Letter) J Am Diet Assoc 2006;11:1753-1754. Heaney RP et al. Not all calcium-fortified beverages are equal. Nutr Today 2005;40:39-44. Rafferty K. Bioavailability: what the “nutrition facts” panel doesn’t reveal. Presentation at the Symposium “What’s for Dinner? Fresh Approaches for Improving Canadians’ Eating Habits,” Dairy Farmers of Canada, November 2006.

Keywords: calcium