How do gut bacteria digest dietary fiber that the small intestine cannot digest?

Gut bacteria digest dietary fiber that the small intestine cannot process through a metabolic process known as fermentation.1

Because the human digestive tract lacks the specific enzymes required to break down certain complex carbohydrates and dietary fibers, these nutrients pass through the small intestine largely intact. Once they reach the large intestine, they become available to the resident gut microbiota.21

Certain bacteria within the gut produce specialized enzymes that degrade these complex polysaccharides into smaller sugar units. Through fermentation—an anaerobic process—these bacteria break down the fibers into short-chain fatty acids, such as acetate, butyrate, and propionate. This process is mutually beneficial: it provides energy for the cells lining the colon, helps reinforce the intestinal barrier, and aids in regulating systemic metabolism and inflammation.31