Excretion of Human Milk Proteins by Term and Premature Infants

Human milk provides a source of high quality available protein for the suckling infant. Among the various proteins in human milk, however, several serve not solely in a nutritional role but also have other physiological functions. ¹ These are the proteins which contribute to defense against infection, a role which necessitates maintenance of their structural integrity in the gut. Physiological functions have been described for secretory IgA, lactoferrin, and lysozyme. Secretory IgA is the major immunoglobulin in human milk ^{2 , 3} (Chapter 4) and acts locally in the infant’s gut to prevent attachment of microbes to intestinal cells and therefore initiation of infection. ^{4 , 5} Lactoferrin is the major iron-binding protein in human milk and is believed to prevent microbial proliferation through its strong affinity for iron which prevents use of this mineral by iron-requiring bacteria ^{6 , 7} (Chapter 3). Lysozyme affects the cell wall of several bacteria, causing lysis ⁸ (Chapter 6). Previous reports of the presence of intact human milk proteins, including lactoferrin, secretory IgA, and lysozyme, in the feces of breast-fed infants ^9–13 provide support for the fact that these proteins play a physiologically active role in the infant gut.