Changing food preferences brought about by westernization that have deleterious health effects1,2—combined with myriad forces that are contributing to increased food insecurity—are catalysing efforts to identify more nutritious and affordable foods3. Consumption of dietary fibre can help to prevent cardiovascular disease, type 2 diabetes and obesity4–6. A substantial number of reports have explored the effects of dietary fibre on the gut microbial community7–9. However, the microbiome is complex, dynamic and exhibits considerable intra- and interpersonal variation in its composition and functions. The large number of potential interactions between the components of the microbiome makes it challenging to define the mechanisms by which food ingredients affect community properties. Here we address the question of how foods containing different fibre preparations can be designed to alter functions associated with specific components of the microbiome. Because a marked increase in snack consumption is associated with westernization, we formulated snack prototypes using plant fibres from different sustainable sources that targeted distinct features of the gut microbiomes of individuals with obesity when transplanted into gnotobiotic mice. We used these snacks to supplement controlled diets that were consumed by adult individuals with obesity or who were overweight. Fibre-specific changes in their microbiomes were linked to changes in their plasma proteomes indicative of an altered physiological state.