Plants contain cellulose, and plant cell walls are composed of cellulose and pectin. Mammals have no cell walls and certainly no cellulose.
Most meat, milk, etc. do not contain cellulose, as long as they are not plants.
Dietary fiber is a kind of polysaccharide, which can neither be digested and absorbed by gastrointestinal tract nor produce energy. Therefore, it was once considered as a "non-nutritive substance" and has not been paid enough attention for a long time.
However, with the in-depth development of nutrition and related sciences, people gradually found that dietary fiber has very important physiological functions. So, today, dietary fiber has become a concern of academic circles and ordinary people, and it is recognized as the seventh kind of nutrient by the nutrition academic circles, which ranks with the traditional six nutrients-protein, fat, carbohydrates, vitamins, minerals and water.