The effects of carbohydrate structure on the composition and functionality of the human gut microbiota

Background: Human health depends on a population of microorganisms that inhabit the gut and contribute to homeostasis of the host, including nutrition, immunity and metabolism. The organisms are interactive and mutually dependent, generating much of their resources through substrate and metabolic cross-feeding, where the end-products for one organism become the fuel for another. To optimise the gut microbiota using diet, these interacting networks must be understood. Microbial cross-feeding is affected by the structure of the energy input, which is primarily dietary fibre, for the gut microbiota. The structure of dietary fibre has been reviewed by carbohydrate chemists, but its integration with the gut microbiota is limited. Scope and approach: The hierarchical structures of dietary fibre are reviewed, encompassing mesostructure, microstructure and molecular structure, and how they are impacted by food processing and digestion. These factors are considered in relation to their effects on microbial cross-feeding, to provide insight on the interactions between diet, the microbiota, and human health. Key findings and conclusions: Food processing and digestion impact food structure, primarily the removal of soluble fractions and increased solubilisation of insoluble fractions. The provision of insoluble carbohydrates to the colon appears important for the sustenance of species known to be primary degraders and keystone species that play a crucial role in stabilising the gut community. Further work is needed at the microbial strain level to understand the impact of increasing fibre solubility. This should be done in studies using well-characterised carbohydrates that consider the impact of food processing and digestion.