Not Medical Advice: Humans are incapable of digesting cellulose because the appropriate enzymes to break down the beta acetal linkages are lacking. Undigestible cellulose is the fiber which helps in the smooth working of the intestinal tract.
Most of the benefits you acquire from the foods you eat come from the nutrients your body is able to digest and absorb. You wouldn't be able to function fully if your body couldn't digest the fruits and vegetables you eat to absorb the vitamins and minerals they contain. However, though some parts of the carbohydrates you eat are not digested in the human body, these compounds can still have a beneficial effect on your health from your intestines.
Sugars, grains, starchy vegetables, fruits and some dairy provide most of the calories the majority of Americans consume through the carbohydrates they contain. Carbohydrates include different types of starches, sugars, and fibers.
The body can digest starches and sugars very well by using enzymes to break down these carbohydrates into very small molecules of sugar, called glucose. The glucose obtained from the digestion of sugary and starchy foods is easily absorbed into your blood and available to your cells as a source of energy. Fiber, on the other hand, cannot be digested by the human body. Because fiber is undigested, it stays in your gastrointestinal tract until it is eliminated in your feces.
Indigestible carbohydrates, or fiber, are gotten in whole grains, legumes, fruits, vegetables, nuts and seeds. There are two different kinds of fiber present in these foods, which are called soluble or insoluble.
Muscle building can be a difficult exercise, if you are a vegetarian. Find a list of muscle building foods for vegetarians here.