What Happens to Undigested Food? The Role of Fiber and Waste

Undigestible carbohydrates, known as dietary fiber, absorb water in the digestive tract, promoting the formation of soft, bulky stools that are easy to pass. In the colon, certain types of fiber are fermented by bacteria to produce short-chain fatty acids, such as acetate, propionate, and butyrate. Acetate and propionate are absorbed into the bloodstream, while butyrate provides energy for the cells in the colon. Bile acids, which help digest fats, are produced in the liver and released into the small intestine. Some bile acids are reabsorbed into the bloodstream in the ileum, and the remaining bile acids are eliminated with the feces in the colon. Water, electrolytes, and some undigested food residues continue to move through the colon and are eventually eliminated in the form of feces.