The human intestine harbours a complex community of microbes, collectively called the microbiota, which is a key component of a healthy body. The microbiota composition and diversity changes when adults become elderly, a transition that is furthermore associated with various health and lifestyle changes. Both an altered composition and metabolic activity will impact age-related disease states. Although the link between ageing and a changing microbiota is evident, very little information is available on concomitant changes in functional capacity. To this end, we hypothesize that the impact of specific non-digestible carbohydrates on structure and function of the intestinal microbiota will be different in elderly versus adults, and that the extent of these differences will correlate with markers of frailty in the elderly. In the proposed project, we will characterize intestinal microbes and their fermentative capacity in colon and ileum of elderly and adults. In a first placebo-controlled GOS intervention study, the effect on faecal microbiota composition and activity will be assessed in both populations. Additionally, the effect on breath metabolites (VOCs) and host health factors relating to frailty will be included. A second study in subsets of each population will address microbiota composition and activity differences between distal ileum and proximal colon using GOS, and positioned catheters for in situ sampling. Subsequently, detailed characteristics of GOS fermentation by key microbes will be studied, which will lead to virtual microbiota models of the ileum and colon that can be used to predict carbohydrate fermentation kinetics and health outcome in elderly.