New research shows that gut bacteria can change how they interact with the immune system based on the types of carbohydrates a person eats. The findings suggest that the same bacteria can act differently depending on dietary choices, which may explain why some people respond well to certain foods while others do not.
Scientists focused on a bacterium called Bacteroides thetaiotaomicron, or B. theta, which is common in the human gut and plays a role in immune function. They studied this bacterium in three ways: by analyzing diet and gut samples from people, by testing mice given sugar water, and by growing the bacteria with 190 different carbohydrates in a lab setting.
The researchers found that B. theta can change its genetic activity based on what a person eats. This means the same bacterium can either help reduce inflammation or promote it, depending on the type of carbohydrate it receives. Natural fruit sugars led to anti-inflammatory responses, while some processed carbs triggered pro-inflammatory reactions.
The study also looked at people who regularly drank soft drinks with white sugar. In those cases, B. theta functioned differently. The sugar did not just affect digestion. It weakened the gut’s protective barrier and lowered immune defenses. Levels of immune cells that help fight infections dropped, and gut tissue repair was poorer. These changes appeared within a few weeks and continued over time.
The findings suggest that one-size-fits-all diets may not work well because each person’s gut bacteria respond differently to the same foods. However, the changes are not permanent. As a person adjusts their diet, gut bacteria can quickly adapt their behavior.
To support gut bacteria and immune function, researchers suggest rotating carbohydrate sources weekly, such as switching between sweet potatoes, quinoa, oats, and different fruits. Paying attention to how the body feels after eating certain carbs can also help. Combining fiber-rich carbs with meals, such as adding berries to yogurt or vegetables to grains, provides diverse fuel for bacteria. Limiting sugary drinks and replacing them with water infused with fruit or herbal teas may also help avoid negative effects on the gut microbiome.
The research indicates that whole food carbohydrates encourage anti-inflammatory, health-supporting behaviors from gut bacteria, while sugary drinks and processed carbs tend to push them toward inflammatory responses. The study was published in Nature Communications.
