Undigested

Undigested is an adjective used to describe material that remains intact after the digestive processes have acted on it. In humans, digestion normally breaks down most carbohydrates, proteins, and fats into smaller, absorbable units in the mouth, stomach, and small intestine. Substances that human digestive enzymes cannot fully break down may pass through the gastrointestinal tract largely unchanged and appear in stool as undigested matter.

Common sources of visibly undigested material include plant fibers such as cellulose, seeds, skins, and other components of fruits, vegetables, and grains. Some foods contain resistant starch or intact structures that are not readily broken down by human enzymes, so portions may be seen in stool after meals rich in fiber or whole grains. In many cases, small amounts of undigested material are normal and reflect normal chewing or rapid intestinal transit.

Undigested matter can be more concerning when it is persistent, widespread, or accompanied by other symptoms. Conditions that can reduce digestion or absorption, such as pancreatic insufficiency, celiac disease, inflammatory bowel disease, infections, or certain motility disorders, may lead to increased or abnormal amounts of undigested material in stool. Medical evaluation may include dietary assessment, stool testing, imaging, or endoscopy, depending on accompanying signs and duration.

In addition to human digestion, some species and their gut microbiota can partially break down substances that are undigested by humans, illustrating the role of microbial digestion in processing dietary components.