Human milk oligosaccharides are resistant to enzymatic hydrolysis in the upper gastrointestinal tract

Background: Human milk oligosaccharides (HMOs) show a complexity and variet y not found in milk of any other species. Although progress has been made i n the past 3 decades with regard to identification and structural character ization of HMOs, not much is known about the physiologic functions of HMOs. Objective: As a prerequisite for biological activity in infant metabolism, HMOs have to resist enzymatic hydrolysis in the gastrointestinal tract. To assess the extent to which selected HMOs are hydrolyzed, we carried out in vitro digestion studies using enzyme preparations of human and porcine panc reas and intestinal brush border membranes (BBMs). Design: Fractions of HMOs, including structurally defined isolated oligosac charides, were digested for up to 20 h with human pancreatic juice and BBMs prepared from human or porcine intestinal tissue samples. HMOs were incuba ted by using a porcine pancreatic homogenate and BBMs as enzyme sources. HM Os and digestion products were identified by mass spectrometry and anion-ex change chromatography. Additionally, free D-glucose, L-fucose, and N-acetyl neuraminic acid were determined enzymatically. Results: Whereas maltodextrin (control) was rapidly and completely hydrolyz ed, neutral and acidic HMOs showed a profound resistance against pancreatic juice and BBM hydrolases. However, cleavage of most of the HMOs was achiev ed by using a pancreatic homogenate containing intracellular, including lys osomal, enzymes in addition to secreted enzymes. Conclusions: The results of this study strongly suggest that HMOs are not h ydrolyzed by enzymes in the upper small intestine. Although intact HMOs may be absorbed, we postulate that a majority of HMOs reach the large intestin e, where they serve as substrates for bacterial metabolism. Therefore, HMOs might be considered the soluble fiber fraction of human milk.