Urinary excretion of in vivo 13C-labelled milk oligosaccharides in breastfed infants

Recent observations indicate that human milk oligosaccharides (HMO) are involved in a variety of physiological processes in infants. Their metabolic fate, however, is virtually unknown. We investigated metabolic aspects in infants after endogenous C-13-labelling of HMO. An oral bolus of natural and C-13-labelled galactose (Gal; 23 g Gal + 4 g C-13-Gal) was given to ten lactating women. Aliquots of milk at each nursing as well as breath samples from the mothers and urine from their infants were collected over 36 h. The C-13-enrichment of HMO and their renal excretion was determined by isotope ratio-MS; characterisation was achieved by fast atom bombardment-MS. After the Gal bolus was given, an immediate C-13-enrichment in milk and in infants' urine was observed which lasted 36 h. Mass spectrometric analysis of C-13-enriched urinary fractions confirmed the excretion of a variety of neutral and acidic HMO without metabolic modification of their structures. Components with glucose split off at the reducing end were also detectable. Quantitative data regarding the infants' intake of lacto-N-tetraose and its monofucosylated derivative lacto-N-fucopentaose II ranged from 50 to 160 mg with each suckling, respectively; renal excretion of both components varied between 1 and 3 mg/d. Since the intake of individual HMO by the infants was in the range of several hundred mg per suckling, i.e. several g/d, and some of these components were excreted in mg amounts as intact HMO with the infants' urine, not only local but also systemic effects might be expected.