Bioavailability of the calcium in fortified soy imitation milk, with some observations on method

Background: Calcium-fortified soy milk is growing in popularity, particular ly among vegetarians, but the bioavailability of its calcium was not previo usly known. Additionally, the validity of isotopic labeling methods for for tified liquid products had not been established. Objectives: The objectives of this study were to compare the bioavailabilit y of the calcium in fortified soy milk with that of calcium in cow milk and to evaluate the method of labeling soy milk for bioavailability testing. Design: A within-subject comparison of extrinsically labeled cow milk with intrinsically and extrinsically labeled soy milks was undertaken in 16 heal thy men. In all tests, 300-mg Ca loads were given as a part of a light brea kfast after an overnight fast. The milks were physically partitioned into l iquid and solid phases to enable evaluation of tracer distribution. Results: Calcium from intrinsically labeled soy milk was absorbed at only 7 5% the efficiency of calcium from cow milk. Extrinsic labeling of soy milk did not produce uniform tracer distribution throughout the liquid and solid phases and resulted in a 50% overestimate of true absorbability. Conclusion: Calcium-fortified soy milk does not constitute a calcium source comparable to cow milk, and extrinsic labeling of such calcium particulate suspensions does not produce the uniform tracer distribution needed for bi oavailability testing. Hence, intrinsic labeling of the fortificant is requ ired for such liquid suspensions.