Breastfeeding: Unraveling the Mysteries of Mother's Milk

, Georgetown University Medical Center

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Human milk contains a great number of enzymes, many of which have specific transport functions (Table VIII). For instance, xanthine oxidase acts as a carrier of iron[65] and glutathione peroxidase carries selenium.[66] Although proteases are present in human milk, it is not known how much of that activity is expressed because of the antiprotease activity of human milk itself.[66] One can postulate that antiproteases might protect the mammary gland from local proteolysis (caused by leukocytic or lysosomal proteases) and might prevent the proteolytic breakdown of milk proteins, many of which have to reach the infant intact (eg, immunoglobulins, digestive enzymes). The antitryptic and antichymotryptic activity of human milk might prevent the absorption of endogenous and bacterial proteases in infants and thereby contribute to the passive protection of extraintestinal organs such as the liver.[67] The high activity of antiproteases in colostrum coincides with the period of greatest transfer of nonimmunoglobulin protein from the intestine to the systemic circulation of the newborn.

The digestive enzymes in milk (amylase and digestive lipase) act in the newborn to compensate for immature pancreatic function. These enzymes are remarkably stable for years in milk stored at low temperature (-20deg.C or -70deg.C). Moreover, activity is unchanged after storage for 24 hours at 38deg.C. The stability of enzymes and of other proteins in milk might be due to the antiprotease activity of milk. Furthermore, many enzymes are stable in the gastrointestinal tract of the newborn.

Amylase,[68] an enzyme identified in milk more than a century ago,[69] may be more important to the infant after initiation of starch supplements[70] or when formula that contains oligosaccharides hydrolyzed by amylase is fed to partially breast-fed infants. Amylase activity in the duodenum of the newborn is only 0.2% to 0.5% of the adult level. At the time of supplementation (after 4 to 6 months of exclusive breast-feeding), the infant is still deficient in endogenously produced amylase.[71] The latter secreted from salivary glands and pancreas does not reach adequate levels until 2 years after birth. Other infants and toddlers who might benefit from milk amylase are those with pancreatic insufficiency caused by diseases such as cystic fibrosis[72] or malnutrition.[73,74,75] Because of the potential of bile salt-dependent lipase in milk[76] to compensate for the low pancreatic lipase in the newborn,[77,78] this enzyme has received great attention in the past decade.[44,66] The characteristics of the digestive enzymes of human milk are summarized in Table IX.


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