AbstractGlobally increased life expectancy strongly triggered interest to delay the onset of frailty, which has been associated with alterations in compositional and functional characteristics of intestinal microbiota. In the current study, we used an in vitro batch incubation model to compare the metabolic capacity of the faecal microbiota of adults (n = 6) versus pre-frail elderly (n = 6) to degrade various glycosidic carbohydrates, including galacto-oligosaccharides, 2′-fucosyllactose, chicory fructo-oligosaccharides and inulin, and isomalto/malto-polysaccharides. The in vitro metabolic capacity was also compared with an in vivo GOS intervention study based on the same subjects. Analysis of 16S rRNA gene sequences and metabolites revealed distinct portions of variation in overall microbiota and metabolite composition during incubation being explained by individuality of the subjects and carbon source. In addition, the age group of the subjects also had significant impact on microbiota variation, carbohydrate degradation and metabolite production. This was accompanied by elevated increase in the relative abundance of Bifidobacterium in the microbiota of adults compared to that of pre-frail elderly and significantly decreased effectiveness to degrade galacto-oligosaccharides by the latter group. Altogether, the carbohydrate degradation in elderly was different compared to adults, with some carbohydrates showing decreased degradation rates. Longer interventions periods may be required to enhance bifidobacterial abundance in the microbiota of pre-frail elderly and thereby to obtain associated prebiotic health benefits.