A diet low in microbiota-accessible carbohydrates (MACs), found in dietary fiber, leads to a progressive loss of gut microbiota diversity over generations, which is not recoverable by reintroducing MACs. This study shows that while a single generation of mice on a low-MAC diet can recover its microbiota when returned to a high-MAC diet, over multiple generations, the microbiota becomes increasingly distinct and less diverse. The loss of certain bacterial taxa is not efficiently transferred to the next generation and is at risk of extinction. The gut microbiota of hunter-gatherers and rural populations is more diverse than that of Westerners, likely due to higher dietary fiber intake. The study used humanized mice to investigate how a low-MAC diet affects microbiota diversity over generations. After four generations, the microbiota of mice on a low-MAC diet remained distinct and less diverse, even after switching back to a high-MAC diet. The loss of certain bacterial taxa, particularly from the Bacteroidales order, was significant. Reintroducing lost bacteria through fecal microbiota transplantation restored microbiota diversity and composition. The study highlights the importance of dietary MACs in maintaining a healthy gut microbiota and suggests that future therapeutic strategies may need to incorporate both dietary MACs and reintroduced bacterial taxa to restore microbiota function. The findings suggest that the Western diet, low in fiber, may contribute to the reduced diversity of the gut microbiota observed in industrialized societies compared to traditional populations.A diet low in microbiota-accessible carbohydrates (MACs), found in dietary fiber, leads to a progressive loss of gut microbiota diversity over generations, which is not recoverable by reintroducing MACs. This study shows that while a single generation of mice on a low-MAC diet can recover its microbiota when returned to a high-MAC diet, over multiple generations, the microbiota becomes increasingly distinct and less diverse. The loss of certain bacterial taxa is not efficiently transferred to the next generation and is at risk of extinction. The gut microbiota of hunter-gatherers and rural populations is more diverse than that of Westerners, likely due to higher dietary fiber intake. The study used humanized mice to investigate how a low-MAC diet affects microbiota diversity over generations. After four generations, the microbiota of mice on a low-MAC diet remained distinct and less diverse, even after switching back to a high-MAC diet. The loss of certain bacterial taxa, particularly from the Bacteroidales order, was significant. Reintroducing lost bacteria through fecal microbiota transplantation restored microbiota diversity and composition. The study highlights the importance of dietary MACs in maintaining a healthy gut microbiota and suggests that future therapeutic strategies may need to incorporate both dietary MACs and reintroduced bacterial taxa to restore microbiota function. The findings suggest that the Western diet, low in fiber, may contribute to the reduced diversity of the gut microbiota observed in industrialized societies compared to traditional populations.