Prebiotic foods — produce, whole grains, pulses, nuts and seeds — can help “fuel” friendly gut bacteria in your GI tract, London explains. By stimulating that “microbiota,” prebiotics boost intestinal immunity and prevent inflammation, diarrhea, and other GI problems, according to 2013 research by the Institute of Food Technologists. Stock up on tomatoes, chicory, onions, asparagus, and wheat for the best benefits.