In healthy adults, baseline gut microbiota diversity predicts the direction of immune response to a high-fiber diet, with lower baseline diversity associated with increased immune activation and higher baseline diversity associated with decreased immune activation.

People with low gut bacteria diversity have fewer microbes that can break down fiber, so when they eat more fiber, their gut microbes produce different chemicals that trigger immune cells to become more active. People with high gut bacteria diversity already have microbes that break down fiber efficiently, so adding more fiber leads to chemicals that calm immune cells instead.