A human milk oligosaccharide (HMO) or a synthetic composition comprising said HMO, for use in increasing the abundance, particularly the relative abundance, of a Bifidobacterium of the B. adolescentis phylogenic group in the microbiota in the gastro-intestinal tract of a human, preferably a non-infant human. The HMO(s) and/or synthetic composition is useful in； increasing particularly the relative abundance of B. adolescentis and/or B. pseudocatenulatum； for treating or preventing in said non-infant human with type 2 diabetes and/or obesity； an enteropathogenic infection； impaired gut barrier function and/or； an inflammation related to gastro intestinal condition, preferably irritable bowel disease (IBD) or irritable bowel syndrome (IBS)； as a nutritional composition. The HMO is a fucosylated neutral HMO, a non-fucosylated neutral HMO, or a mixture of a fucosylated and a non-fucosylated neutral HMO. The HMOs comprises 2′-fucosyllactose (preferred), 2′-fucosyllactose (preferred), 3-fucosyllactose, di-fucosyllactose (preferred), lacto-N-fucopentaose, fucosyl-lacto-N-hexaose, fucosyl-para-lacto-N-hexaose, lacto-N-tetraose (pre-ferrel), lacto-N-neotetraose (preferred), lacto-N-hexaose, lacto-N-neohexaose, para-lacto-N-hexaose and para-lacto-N-neohexaose.
