Gut bacteria live mainly in the large intestine, where they ferment leftover fiber and other carbs. This process produces helpful compounds, including short-chain fatty acids, which can support the gut lining.
Your gut bacteria also interact with your immune system. They help train immune responses and can reduce overreaction when the gut barrier is healthy.
Because gut bacteria rely on what you eat and how your gut environment changes, their activity can shift quickly. A diet change can change which microbes grow, and that can change how you digest and respond to food.