Most domestic animals are capable of eating and digesting roughage in considerable amounts. A horse and cow eat hay and get nourishment from it, but a human being cannot. The cellulose of hay cannot be digested properly by the human digestive tract, and the material is passed rapidly through the intestines.
Most vegetables and fruits and whole-grain cereals contain cellulose, which serves to give bulk to the material in the bowel and in that way to give the intestines something to work on.
Cellulose may be sufficiently tender to be partially digested, as, for example, in the form of lettuce, fruits, and cooked vegetables, but in general it is not digested. Potatoes, beans, nuts, and olives have some cellulose, which may be utilized to a certain extent in the body, but in the majority of instances the cellulose is not properly utilized, except for roughage.
Cellulose is found in paper obtained from wood; cotton is practically pure cellulose, and the substance is also found in large amounts in bran. When water acts on cellulose it may swell it up somewhat, increasing its bulk still further.
Few people realize the danger of a diet containing too much cellulose. Such a diet interferes with digestion of the useful material, and it may irritate a sensitive intestinal tract, to the extent of causing an erosion or inflammation.
The bran of rice and wheat contains vitamins which may be of great value to the human body. The bran contains about 22 per cent of the protein of the wheat kernel. Bran proteins are relatively rich in those nutritionally-essential amino acids that are deficient in the endosperm of wheat. However, nature has seen to it that the vitamins are available in many forms, and it is not necessary to overload the intestines with roughage to secure a sufficient amount of any one vitamin.
The various vitamin B components are found in the bran of cereals, and in the embryo of cereals as well. Incidentally, the fiber of the green and yellow vegetables serves as a cleanser of the bowel and is not harsh enough to irritate. Too much bran can easily irritate the bowel.