That part of food that can not be digested in the gastrointestinal tract, although it can be metabolized in the colon by the micro-organisms that exist there.

Fiber roughage consists of four categories: cellulose, hemicelluloses, lignins and pectins; found in unrefined foods; such as, wholemeal cereals and flour, root vegetables, nuts and fruit.

It is known to affect bowel functions, probably because of its capacity to hold water in a gel-like form and it is makes an important contribution to the prevention of constipation, diverticulosis (presence of small bulging sacs pushing outward from the colon wall), irritable bowel syndrome (IBS) or recurrent abdominal pain and diarrhea often alternating with periods of constipation, appendicitis, diabetes mellitus (disease associated with abnormally high levels of the sugar glucose in the blood), and cancer of the colon.

Many western diets do not contain enough fiber roughage to satisfy the needs in human bodies.