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 the food source, which may explain why some people respond well to certain diets while others do not.
Scientists focused on a bacterium called Bacteroides thetaiotaomicron, or B. theta, which makes up a large part of the gut microbiome in many people. This bacterium plays a role in immune function. The researchers studied B. theta by analyzing diet and gut samples from real people, testing mice given sugar water, and growing the bacteria with 190 different carbohydrates in a lab.
The study found that B. theta can change its genetic activity depending on what a person eats. This means the bacterium can reprogram itself based on food choices. The type of carbohydrate determines whether the bacteria help reduce inflammation or cause inflammatory reactions. Natural fruit sugars led to anti-inflammatory responses, while certain processed carbohydrates triggered pro-inflammatory effects.
People who regularly drank soft drinks with white sugar had B. theta bacteria that functioned differently than those who did not. The sugar did not just affect digestion. It weakened the gut’s protective barrier and lowered immune defenses. Researchers observed reduced levels of immune cells that help fight infections and poorer gut tissue repair. These changes started within a few weeks and continued over time.
The findings help explain why a one-size-fits-all diet does not work for everyone. The same food can produce different results in different people based on their unique gut bacteria composition. The good news is that these bacterial changes are not permanent. As a person adjusts their diet, gut bacteria quickly adapt their behavior.
To support gut bacteria and immune function, the researchers suggest rotating carbohydrate sources each week. Switching between foods like sweet potatoes, quinoa, oats, and different fruits exposes bacteria to varied nutrients. Paying attention to how the body feels after eating different carbohydrates can help guide food choices. 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 also help avoid negative effects on the gut microbiome.
Every meal offers a chance to guide gut bacteria toward better immune function. The carbohydrates eaten act like daily instructions for beneficial bacteria. The research shows that sugary drinks and processed carbohydrates tend to push gut bacteria toward inflammatory responses, while whole food carbohydrates encourage anti-inflammatory, health-supporting behaviors. A person does not need to eliminate entire food groups or follow extreme diets. Instead, focusing on giving gut bacteria a variety of fiber-rich, whole food carbohydrates can support the immune system and overall health.
