Nutritional proteins in milk and the protein requirement of normal infants.

All whey proteins in human milk are not absorbed and thus not nutritionally available. There is evidence that secretory immunoglobulin A (SIgA), lactoferrin, and lysozyme are resistant to proteolytic action, and the major part of these protective proteins is excreted in the infant's stool. An exclusively breast-fed infant would thus have a mean intake of 1.3 g of protein per kilogram of body weight per day during the first month and about 0.9 g/kg/d during the third month. Term infants fed with formulas containing 1.5 g of protein per deciliter show metabolic changes that indicate that the infants receive more protein than they require for growth; these changes are not seen in breast-fed infants or in infants receiving a whey-predominant formula containing only 1.1 g/dL of protein. A reevaluation of protein requirement and intake during infancy is suggested.