There IS proof that certain types of cellulose are indigestible, and in fact, consumption of them will degrade the ability to digest OTHER types of cellulose later on when eaten!!!
For example, in the work of Van Soest's group10 11 in which healthy
volunteers were fed controlled diets with the addition of cellulose from
either cabbage, bran, or a purified cellulose (Solka Floc), average cellulose
digestibility was 74% on the control diet, 75% in the cabbage, about 53%
in the bran but only 25% from the Solka Floc. Moreover, the purified
cellulose depressed the breakdown of other cell wall polysaccharides and
reduced cellulose digestion in the subjects when they were changed to
other diets. The capacity of colonic microorganisms to digest cellulose in
vitro was also tested and in these studies the purified cellulose was virtually
indigestible, while that from cabbage was extensively degraded. Similar
findings were reported in 1936 by Williams and Olmsted' who fed three
medical students cellulose from a wide range of food sources and observed
that while 60-70% from carrot and cabbage was digested only 0-10% of a
purified cellulose was broken down and 3-25% from cotton seed hulls.
Once again, I contend that if it happens like that in the human gut, it's going to do the same thing to canine and feline guts. No vet is going to get me to believe that they're not like us in a lot of ways--otherwise they'd just not be used as lab-rats to study medicines intended for humans.
From:
http://gut.bmj.com/content/25/8/805.full.pdf
