Calcium Partitioning in Human and Bovine Milk

The partitioning of calcium in human milk was studied experimentally and compared with the distribution of calcium among the compartments of bovine milk. Care was taken to handle the milk samples to produce minimal disturbance of the milk fat globule membranes and the CO2 content. About 15% of the milk calcium was associated with casein in human milk; the remainder was in the aqueous phase. The equilibrium between ionized calcium and calcium complexes with smaller anions was examined. Ionized calcium varied between 2.3 and 4.0 mM across individuals (mean 3.0 +/- .1 mM) at 90 d of lactation. The remainder of the calcium in the aqueous phase was associated with citrate and phosphate. Changes in total calcium in human milk during lactogenesis and late lactation reflected changes in citrate and casein rather than alterations in ionized calcium.