Sawdust of Pine Tree!
I won a bet on this when somebody wouldn't believe they could put this stuff as the 2nd Main ingredient in "prescription" scam food.
(Cross-posting:)
"There are various forms of powdered cellulose available from trees like pine and beech to bamboo and cotton. By and large, the cellulose used in petfood applications is derived from pine trees. The ingredient starts its journey in the pulping mills, the same mills used to produce paper. The pulp is made into long continuous sheets and rolled just like paper stock going to the local newspaper. However, cellulose intended for food and feed is ground through specially-designed hammer mills, then sized to certain particle lengths in giant "ball-mills."
http://www.petfoodindustry.com/Columns/Ingredient_Issues/2376.htmlWebsite is a resource FOR pet food industry people BY pet food industry people.