Solubility and relative absorption of copper, iron, and zinc in two milk-based liquid infant formulae

The relative solubility of copper, iron, and zinc in two experimental liquid infant formulae are examined. The results of these trials suggest that substituting organic forms of copper and iron in the mix results in an almost three-fold increase in their solubility (iron lactate, 73.4% vs ferrous sulfate, 27.6% and copper gluconate, 11.3% vs cupric sulfate, 3.0%). Organic zinc substitutes did not show this pattern of increased solubility Electron microscopy was employed to document the changes in protein-protein and protein-lipid interactions and to examine the pattern of electron-dense precipitates in the two experimental formulae. Electron micrographs of the liquid infant formula that had been formulated using inorganic salts (sulfates) showed extensive attachment of denatured whey proteins and casein micelles to the oil droplet surfaces and the surface of the oil droplets were also punctuated by electron-dense granule. The oil droplets of the formula produced using organic versions of the mineral salts were smooth and clear of electron-dense deposits. Experiments were designed to determine whether the observed changes in solubility and microstructure were correlated with increases in relative absorption of the minerals. We applied a technique of in vitro acidification followed by a peptic digestion of the two experimental infant formulae with human milk samples as controls. Coupling this in vitro digestion with an absorption model consisting of live isolated intestinal loops from guinea pig we were able to assess the relative absorption of copper, iron and zinc in the test digests. The relative absorption of the three minerals from digests of human milk was significantly higher than for either of the experimental formulae. Relative mineral absorption from digests of the two experimental infant formulae tested was only significantly different (P < 0.05) for Fe. Based on the results from this study we can conclude that substituting organic forms of iron in bovine milk-base infant formulae would have beneficial effects on both the solubility and bioavailability of this important micronutrient.