Cellulose cannot be digested by the human gastrointestinal tract or the bacteria present in it. The ingested cellulose passes through the digestive system and is released through defecation. Cellulose helps prevent constipation by providing the required fibers in the diet.
Humans obtain cellulose by ingesting plant material. Cellulose is the chief component of the cell walls in plant cells. It is a complex carbohydrate called a polysaccharide and is composed of thousands of glucose molecules bound together. Due to the complexity and rigidity of its structure, cellulose cannot be broken down by the human digestive tract.
It can be digested in certain herbivores, such as cows and horses. Herbivores have bacteria in their intestines that are different from the bacteria found in the human gut. They produce an enzyme called celluse which is capable of breaking down the cellulose. Also, several herbivores are ruminants, that is, they regurgitate their food and chew it again to enhance the breakdown of nutrients. Since humans are not ruminants and do not harbor bacteria that are capable of breaking down the cellulose structure, the ingested cellulose tends to be excreted.
While cellulose is of very little nutritional value to humans, it provides the fiber necessary for the process of defecation. A low amount of cellulose can result in constipation. Cellulose is also thought to reduce the risk of colon cancer by reducing the harmful DNA-damaging effects of bile acids.