Subject: Why can't sawdust be used instead of flour in low calorie diets?

In doing some websearching on diet foods it is uniformely stressd that Dietary 
Fiber is not digested.  Fruits, vegetables and grain are all inclusive of such 
fiber and part of all diets, both for weight loss and normal meals.  As such it 
seems that vegitation fiber could include wood "sawdust" as it could serve as a 
substitute for flour and produce a slice of bread at considerably less calories 
than even diet breads. Some animals are provided it in their mix for low cal 
food. . .why can't us humans also use it for feeling filled with very low 
caloric intake?

Re: Why can't sawdust be used instead of flour in low calorie diets?