An approach to assessing trace element bioavailability from milk in vitro. Extrinsic labeling and proteolytic degradation.

Both in vitro and in vivo, the use of a radioisotope can significantly enhance the sensitivity of methods for trace element studies. An essential prerequisite for this approach, however, is that the added (extrinsic) radiolabel equilibrates with the native (cold) element within all compartments of the diet. By using ultracentrifugation, ultrafiltration, and gel filtration chromatography, we have shown that the method is valid for zinc, copper, and manganese when using milks and formulas. For iron, however, extrinsic labeling does not necessarily yield results similar to the native distribution. We have used extrinsic labeling to follow the distribution of Zn, Cu, and Mn between high molecular weight compounds (proteins) and low molecular weight complexes in human and bovine milk after in vitro proteolysis. Peptic digestion at various pHs and pancreatic digestion for varying times were used to mimic digestion in the infant. After limited proteolysis, a large proportion of trace minerals in human milk was found in the low molecular weight fraction, whereas in cow's milk a large proportion was bound to incompletely digested casein. These findings may, at least in part, explain the higher bioavailability of trace elements from human milk compared to cow's milk.