Food that is not properly digested in the stomach is eventually pushed through the rest of the gastrointestinal tract and exits the body via the rectum as stool. The body treats undigested food like any other substance the stomach fails to break down.
The inability of the body to fully extract nutrients from food is known as malabsorption. This may occur due to a shortage or interruption of the digestive enzymes. Symptoms of malabsorption include diarrhea, weight loss and uncomfortably large and offensive-smelling stools. Doctors test for possible malabsorption by examining stools for fat. In some cases, a biopsy of the small intestine is necessary.