This review considers fermentable carbohydrates and their role in maintaining health through their availability as fuel for the gut microbiota. The microbiota possesses remarkably diverse function, and is likely modifiable by diet. Therefore a diet rich in varied fermentable carbohydrates such as dietary fibre, glycosylated polyphenolics, glucosinolates and other plant glycans, applied in a sustained fashion may promote microbial diversity leading to improved health. This may be achieved by increasing the flexibility of the microbiota's capability to interact with diverse dietary environments, or via increasing production of short chain fatty acids (SCFAs) from the fermentation of carbohydrates. A higher functional modular complexity is indicative of gut health, whilst SCFAs may reduce the risk of developing gastrointestinal disorders, cancer, and cardiovascular disease.