Zinc, copper, calcium, and magnesium in human milk.

Human milk was fractionated and analyzed for the distribution of copper, zinc, calcium, and magnesium among various fractions. In whole milk the concentration of copper was 0.27 +/- 0.13 micrograms/ml, zinc 1.10 +/- 1.19 microgram/ml, calcium 241.2 +/- 61.9 microgram/ml, and magnesium 41.4 +/- 15.2 micrograms/. Most of the Cu, Zn, Ca, and Mg was found in the skim milk, but significant amounts of Cu, Zn, and Ca were found in the fat, most likely associated with the fat globule membrane. Less than 4% of each of the elements was found in the casein. A low molecular weight fraction contained 26% of total Cu. 25% of Zn, 34% of Ca, and 54% of MG. Gel filtration demonstrated Zn and Cu in skim milk binding to two different proteins with molecular weight (MW) greater than 400,000 and 60 to 80,000, as well as to LMW compounds. The unique distribution and binding of these elements in the milk may, in part, explain the known high bioavailability of trace elements and minerals in human milk.