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, difucosyllactose (preferred), lacto-N-fucopentaose, fucosyl-lacto-N-hexaose, fucosyl-para-lacto-N-hexaose, lacto-N-tetraose (preferred), lacto-N-neotetraose (preferred), lacto-N-hexaose, lacto-N-neohexaose, para-lacto-N-hexaose and para-lacto-N-neohexaose.