Variation in gastrointestinal metabolization of prebiotics: Learnings from human milk oligosaccharides and modified starch

Prebiotics and its close interactions with human gut microbiota is gaining attention in recent years, as it exerts multiple impacts on human health. Studying the metabolization of metabolic fates of indigestible oligo- and poly- saccharides in the gastro-intestinal tract is vital for understanding the mechanism of how the carbohydrates influence gut environment and immune system. The current thesis describes the metabolization of indigestible carbohydrates, human milk oligosaccharides (HMOs) and isomalto/malto-polysaccharides (IMMPs) by infant or adult gut microbiota using in vivo or in vitro study approaches.

HMOs are considered as the first natural prebiotic for life, providing essential benefits to support infant health. Concentrations of 19 major HMOs present in human milk and infant fecal samples were obtained by combining high performance anion exchange chromatography - pulsed amperometric detection (HPAEC-PAD) ans porous graphitized carbon - liquid chromatography mass spectrometry (PGC-LC-MS). Relative levels of fucosylated and sialylated HMO structural elements was obtained by one-dimensional 1H nuclear magnetic resonance (1D 1H NMR).  Mother milk and paired infant fecal samples at one-month postpartum from the KOALA cohort study and at two-, six-, and 12- weeks postpartum from the BINGO cohort study were analyzed for HMOs present. Fecal microbiota composition was characterized using Illumina HiSeq amplicon 16S rRNA sequencing. The results showed inter-individual variations regarding HMO synthesis by mothers and metabolism by infant gut microbiota during the first three months of life. HMOs with different structural elements were differentially consumed when passing through infant gut, and their metabolic fates depended on both linkage types and decoration of fucose/sialic acid moieties. Three distinct consumption patterns of HMOs were found among infants under three months of age, which correlated to three microbial community clusters found in the infant fecal samples. Caesarean section and early exposure to hospital/clinic associated surroundings were found to delay the gradual progression of infant gut development stages. Several phylotypes (OTUs) within specific genera were suggested to be the key taxa responding for HMO metabolization.

 Prebiotic fermentation behaviors of IMMPs were studied using a batchwise in vitro fermentation model with human fecal inoculum. Structural degradation on molecular levels, metabolites production, enzyme expression and microbiota composition changes during in vitro fermentation were linked together, pointed out a slow-fermenting behavior and structure-specific prebiotic effects of IMMPs.

Overall, these findings provide valuable evidence for researchers and industry to identify functional structures of prebiotics for humans at different developmental stages.