High milk lipase activity associated with breast milk jaundice

Human milk samples that inhibit bilirubin-UDP-glucuronyl transferase (UDPGT) activity in vitro have been associated with prolonged unconjugated hyperbilirubinemia in newborn infants. We measured the concentration of nonesterified fatty acids (total and individual fatty acids), total fat and protein, and lipase activities (with and without bile salt stimulation) in milk samples from two groups of women. Women whose infants had prolonged unconjugated hyperbilirubinemia and whose milk inhibited the activity of UDPGT were in the first group (N = 9). Volunteers with healthy infants acted as controls. Inhibitory milk contained significantly more nonesterified fatty acids (total, palmitic, and oleic) than did controls. Fat and protein concentrations and bile salt-stimulated lipase activities were similar in the two groups. Unstimulated lipase activity was higher in the inhibitory milks (11.9 +/- 0.8 mM x min-1 x ml-1) than in the controls (6.0 +/- 0.1 mM x min-1 x ml-1) (P less than 0.01). The specific activity (mM x min-1 x mg protein) of unstimulated lipase was also significantly higher in the inhibitory milks (P less than 0.0001). The high nonesterified fatty acid levels in inhibitory milks is accounted for by the elevated unstimulated lipase activities. How these circumstances lead to jaundice in the infants remains to be shown.