Human milk oligosaccharides are differentially metabolised in neonatal rats

Human milk oligosaccharides (HMO) are complex glycans that are highly abundant in human milk, but not in infant formula. Accumulating data, mostly from in vitro and animal studies, indicate that HMO benefit the breast-fed infant in multiple ways and in different target organs. In vitro incubation studies suggest that HMO can resist the low pH in the infant's stomach and enzymatic degradation in the small intestine and reach the colon in the same composition as in the mother's milk. The oligosaccharide composition in faeces of breast-fed infants is, however, very different from that in the mother's milk, raising questions on when, where and how HMO are metabolised between ingestion and excretion. To answer some of these questions, we established a pulse-chase model in neonatal rats and analysed HMO profiles to track their composition over time in five consecutive equal-length intestinal segments as well as in serum and urine. The relative abundance of individual HMO changed significantly within the first 2 h after feeding and already in the segments of the small intestine prior to reaching the colon. Only 30-sialyllactose, the major oligosaccharide in rat milk, and hardly any other HMO appeared in the serum and the urine of HMO-fed rats, indicating a selective absorption of rat milk-specific oligosaccharides. The present results challenge the paradigm that HMO reach the colon and other target organs in the same composition as originally secreted with the mother's milk. The present results also raise questions on whether rats and other animals represent suitable models to study the effects of HMO.