Human milk contains an enzyme which, in the presence of certain bile salts, has high lipase and esterase activity. These activities are lost on pasteurization of the milk. Bile salt-stimulated lipase was demonstrated in gastric contents during the first 2 hr after feeding fresh human milk to infants, indicating that it is stable in the stomach and passes with the milk into the duodenum where it is activated by bile salts.Speculation
The bile salt-stimulated lipase has been found in milk only from primates; it is not present in lower animals. There are few examples of a new enzyme function which appears so late in evolution and an important physiologic role for this lipase is implicated. It may ensure an adequate lipolytic capacity in the intestine under conditions when the infant's endogenous enzyme activities are critically low. Since the bile salt-stimulated lipase was found to enhance severalfold the rate at which retinol esters are hydrolyzed by intestinal contents of the newborn, it seems possible that it may considerably enhance the utilization of the retinol esters in milk. These are the main source of vitamin A for the newborn, and must be hydrolyzed before they are absorbed. This vitamin is necessary for normal growth and development but the amounts present in human milk are often barely sufficient to cover the needs of the newborn, particularly when the mother is malnourished. Vitamin A deficiency is common in many nonprivileged societies and a role for the bile salt-stimulated lipase in enhancing its absorption could be of great nutritional significance.