Isotopic exchange of ingested calcium between labeled sources. Evidence that ingested calcium does not form a common absorptive pool

Summary We studied the extent of salt dissociation during absorption of calcium from sources of differing absorbability by measuring fractional absorption from loads in the range of 200–300 mg in healthy adult women. Sources were labeled both intrinsically and extrinsically with⁴⁵Ca and⁴⁷Ca, respectively, and were fed alone and in combination with one another. We first confirmed our previous observation of superior absorbability of calcium oxalate over spinach calcium in a randomized cross-over design in 20 women. Spinach calcium exhibited only half the absorbability of the same load of calcium presented as the oxalate. Then, in 14 women fed spinach with both an intrinsic and an extrinsic label, apparent absorption of the extrinsic label averaged 0.130±0.041 and of the intrinsic label, 0.029±0.023. Thus, the extrinsic tag was partially, but not completely, bound by the spinach. In the same 14 women, milk absorption averaged 0.331±0.092 when ingested alone. However, when coingested with spinach, apparent milk calcium absorption fell to 0.267±0.079 and apparent spinach calcium absorption rose to 0.111±0.039. Thus, there was significant but incomplete label exchange between the two sources, indicating that at least some of the calcium from both sources enters a common preabsorptive, ionic pool. By contrast, we had previously shown no tracer exchange when labeled oxalate was co-fed with labeled milk. We conclude that (1) the presence of calcium as the oxalate in spinach is not a sufficient explanation for the poor absorbability of spinach calcium; and (2) oxalate calcium and spinach calcium are absorbed by different mechanisms, one involving a common preabsorptive pool and the other not. We suggest that oxalate calcium absorption is by passive diffusion of the intact complex and spinach calcium absorption by active transport of the free cation.