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

You should do more website searches regarding "soluble" vs. "insoluble" 
fiber and why dietary natural fiber from fruits, vegetables and grains are 
healthful.This is not due to management of obesity. Sawdust, or wood pulp, 
is an insoluble fiber that is not hydrolysed since it is insoluble in 
gastric juice and humans have no intestinal enzymes to degrade it. It is 
essentially cellulose that has been used in the  past at very low levels 
(1-6 grams), made into derivatives, and used as a bulk-forming laxative. 
Due to osmotic effects in the lower gut, it will take up water. However, it 
can deplete electrolytes(sodium and potassium) and can be quite 
dehydrating. With a relatively large amount of intake (a slice of sawdust 
bread) it can be quite irritating and painful within the human intestinal 
tract, may cause very painful gas formation due to some bacterial action in 
the lower bowel, and can produce partial or complete intestinal 
obstruction.

Textbooks on human physiology/pharmacology can give additional information. 
There is some information in "Toxic Hazards in Food", D.M. Conning and 
A.B.G. Lansdown, 1983. Croom Helm Ltd, Beckenhem, Kent.