Date of Award

1-1-1986

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Biochemistry and Molecular Biology

Abstract

Humans absorb zinc, iron and copper more readily from human milk than from bovine milk or infant formulae. Given that solubility is requisite to absorption, the solubilities of select components, including zinc, were examined in preparations of bovine and human milk and in model solutions based on the low-molecular-weight components of the two milks. Solubility was examined over a pH range similar to that found in the human digestive system. Factors which could alter zinc solubility were examined by changing the composition of the model solutions and by fortifying milks with various compounds.Each aliquot of the models or milks was adjusted to a pH between 2 and 8.5. After incubation, each aliquot was centrifuged and/or filtered. By experimental definition, the supernatant fractions and the filtrates from the models and milks, respectively, contained the soluble components.Overall, zinc in bovine milk was soluble only up to pH 6, while zinc in human milk was soluble up to at least pH 7. Iron and copper were also more soluble in human milk than in bovine milk over the pH range studied. Insoluble milk components, including over 90% of the zinc, were recovered in precipitates from bovine samples.In both models and human milk, the pH at which precipitable calcium phosphate formed was altered by the concentrations of calcium and inorganic orthophosphate. Whenever a precipitate of calcium phosphate formed, zinc was also removed from solution. Of four amino acids tested, histidine was the only one found to increase zinc solubility, albeit slightly. Addition of ethylene diamine tetraacetic acid (EDTA) increased zinc, iron and copper solubility in bovine milk.As bovine and human milks were subjected to increasing alkalinity, the formation of precipitable calcium phosphate led to the simultaneous precipitation of a large percentage of the zinc. Precipitation of calcium phosphate may also contribute to diminished iron and copper solubility. Complexation of a trace metal by EDTA may inhibit simultaneous precipitation with calcium phosphate. The interactions between calcium phosphate and trace metals may be the basis for differences in trace metal absorption from foods such as human and bovine milks.

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