The gut is home to trillions of microbes that play a fundamental role in many aspects of human biology including immune function and metabolism 1,2. The reduced diversity of the Western microbiota compared to populations living traditional lifestyles presents the question of which factors have driven microbiota change during modernization. Microbiota accessible carbohydrates (MACs) found in dietary fiber, play a key role in shaping this microbial ecosystem, and are strikingly reduced in the Western diet relative to more traditional diets 3. Here we show that changes in the microbiota of mice consuming a low-MAC diet and harboring a human microbiota are largely reversible within a single generation, however over multiple generations a low-MAC diet results in a progressive loss of diversity, which is not recoverable upon the reintroduction of dietary MACs. To restore the microbiota to its original state requires the administration of missing taxa in combination with dietary MAC consumption. Our data illustrate that taxa driven to low abundance when dietary MACs are scarce are inefficiently transferred to the next generation and are at increased risk of becoming extinct within an isolated population. As more diseases are linked to the Western microbiota and the microbiota is targeted therapeutically, microbiota reprogramming may need to involve strategies that incorporate dietary MACs as well as taxa not currently present in the Western gut
