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Bile Salt-Stimulated Lipase in Human Milk: Evidence of Activity in Vivo and of a Role in the Digestion of Milk Retinol Esters

Summary: 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.When infants were fed test meals of pasteurized human milk both esterase and lipase activities in their duodenal contents fell, presumably because the milk diluted their endogenous enzymes. In contrast, when fresh milk was fed, the esterase activity did not decrease. The difference in esterase activity 1 hr after feeding fresh and pasteurized milk corresponded to the activity in the milk.The bile salt-stimulated lipase of milk but not the pancreatic lipase is inhibited by eserine. After feeding fresh milk a fraction of the lipase activity of duodenal contents was inhibited by eserine; the amount so inhibited corresponded to the activity of the milk. Duodenal contents collected 1 hr after feeding fresh milk were found to hydrolyze retinol palmitate severalfold faster than duodenal contents obtained after feeding pasteurized milk; again the difference corresponded to the activity of the milk fed. It is concluded that the bile salt-stimulated milk lipase is active in the infant's intestine and may contribute to the digestion of the milk lipids, particularly the retinol esters.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.