Carbs begin to digest in the mouth, where enzymes in saliva start breaking complex molecules down into simpler sugars. Chewed-up carbs then pass through the esophagus and stomach with little additional digestion. In the small intestine, they are broken down into the simplest sugar molecules, which are then absorbed through the small intestine walls into the bloodstream and used by the body as fuel or sent for storage in the liver for use at a later time. Fiber, the indigestible cell walls found in carb plant foods such as beans, brown rice and whole wheat, passes through the digestive tract essentially undigested.