New research shows that gut bacteria can change how they interact with the immune system based on the type of carbohydrates a person eats. The study, published in Nature Communications, focused on a bacterium called Bacteroides thetaiotaomicron, or B. theta, which is common in many people’s digestive systems and plays a role in immune function.
Scientists analyzed diet and gut samples from people, tested mice given sugar water, and grew B. theta with 190 different carbohydrates in a lab. They found that the same bacteria can behave differently depending on what carbohydrates are available. Some carbohydrates caused B. theta to produce compounds that reduce inflammation and support gut health, while others led the bacteria to create inflammatory molecules linked to immune problems.
The type of carbohydrate determines whether gut bacteria become helpful or harmful to the immune system. Natural fruit sugars promoted anti-inflammatory responses, while certain processed carbohydrates triggered pro-inflammatory reactions. This suggests that the bacteria can flip genetic switches based on what a person eats.
Effects of sugary drinks
The researchers also studied people who regularly drank soft drinks with white sugar. They found that B. theta bacteria in these individuals functioned differently compared to those who did not consume such drinks. The sugar compromised the gut’s protective barrier and weakened immune defenses. People who drank sugary drinks had lower levels of immune cells that help fight infections and showed poorer gut tissue repair. These changes appeared within a few weeks and continued over time.
Why diets work differently for different people
The findings help explain why the same diet can work well for one person but not another. Gut bacteria respond differently to the same foods based on each person’s unique microbiome composition. These bacterial changes are not permanent. As a person adjusts their diet, gut bacteria quickly adapt their behavior. This means people can choose carbohydrates that encourage beneficial bacterial responses without making drastic dietary changes.
Practical steps for gut health
To support gut bacteria’s immune function, people can rotate carbohydrate sources weekly. Switching between sweet potatoes, quinoa, oats, and different fruits exposes bacteria to varied nutrients. Paying attention to energy levels, mood, and digestion after eating different carbohydrates can help identify which foods work best. Combining fiber-rich carbohydrates 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 can avoid negative effects on the gut microbiome.
Key takeaway
The carbohydrates a person eats act like daily instructions for gut bacteria, guiding them toward better immune function. Sugary drinks and processed carbohydrates tend to push gut bacteria toward inflammatory responses, while whole food carbohydrates encourage anti-inflammatory, health-supporting behaviors. The research emphasizes that extreme diets are not necessary. Instead, focusing on a variety of fiber-rich, whole food carbohydrates can help support the immune system, mood, and overall health. The study was conducted by researchers and published in Nature Communications in May 2026.
