Why cant humans digest cellulose?

Humans lack the enzymes needed to successfully digest cellulose as a whole. Only small amounts of cellulose, or what is commonly known as fiber, remain intact after leaving the digestive system.

Ruminants, such as cows, sheep, and goats, break down cellulose by the help of several stomachs containing helpful bacteria. This process is refined when the animals cough up and further break down cellulose in plants by chewing their cud. A diet containing fruit and vegetable matter with high fiber matter is recommended by doctors and other health professionals, as fiber passes through the human body quickly for optimal elimination. This cuts down health risks due to retained waste products.