1. The digestion of zinc present in human milk, cow's milk and a commercial babyfood was compared, using the piglet as a model for the human infant.
2. In piglets given human milk the pH of stomach contents was approximately 1 and 0.4 units lower than that of animals given respectively cow's milk and babyfood. The pH values of intestinal contents were approximately neutral and did not vary with the type of feed.
3. Hard casein curds were present throughout the stomachs and small intestines of animals fed on cow's milk or babyfood and between 55 and 70% Zn in these digesta samples were recovered in an insoluble form by centrifugation. In contrast, little solid material was observed in the digesta of animals fed on human milk, and 57 and 93% respectively of the Zn in digesta were recovered in a soluble form in the stomach and small intestine.
4. Soluble fractions prepared by centrifugation of digesta were analysed by filtration on Sephadex G-150. After any of the three feeds, soluble Zn in stomach contents was mainly in a low-molecular-weight form. In intestinal samples, however, Zn was present in low- and high-molecular-weight forms. Whilst there were similar amounts of Zn in the low-molecular-weight form in all samples, approximately three times as much of the total intestinal Zn was in a soluble high-molecular-weight form complexed to proteins in the animals fed on human milk compared with those fed on cow's milk or babyfood.
5. Analysis of protein-bound soluble Zn in intestinal samples on SDS-polyacrylamide gels resulted in a similar pattern of proteins for all feeds. Results indicated that at least some of these proteins were derived from intestinal secretions of the piglet.
6. Some implications of these results in respect of the mode of digestion of Zn and its biological availability to the human infant are discussed.