Celluase in Humans

As we should all be aware, humans can't digest wood, nor can any mammal (as far as I know) without the aid of symbiotic gut bacteria. Ruminants get around this by having four stomachs, one of which houses all of the cellulasing bacteria. 2-stomach mammals also have a dedicated chamber for cellulase-bacteria (e.g. horses, pigs), although I can't recall their specific digestion processes at the moment. The only reason why cellulose can't be digested by glycolysis enzymes is that cellulose molecules are linked by two positions on the glucose molecule that can't be accessed by glycolysis enzymes, so another enzyme is required. One problem with cellulase bacteria is that they tend to eat all the good stuff (e.g. the glucose from the cellulose) and leave behind only their by-products (this is how four-chambered ruminants live). So, one must wonder, would it be beneficial for mammals to produce cellulase themselves? The enzyme is not substantially different from starch glycolysis enzymes in structure, and I would be fairly certain that mammal cells could be easily transformed to incorporate it into the genome.

One problem is that rather tough teeth are required for chewing plant matter, but many mammals already possess these (and humans could just drink pulped plant/eat soft plants like we already do).
Another reason that mammals might not have intrinsic cellulase is the effect of non-soluble fibre: we all have heard that it is good for our digestive tract. But perhaps another solution could be found? For humans, at least, increased water in the diet would alleviate some of the problems, suggesting that tropical humans would be able to cope with the loss of fibre (also keeping in mind that it is likely that not all fibre would be broken down by the body, since there is so much cellulose: we would need bigger guts for it to take longer to leave the body, and then a new gut might not require fibre). How do other animals cope with the fibre problem?

If humans could digest cellulose, would that substantially change our energy levels? Perhaps it would result in sugar overloads.

Lactase is another of these sugar enzymes that most mammals do not possess (except for most humans). If humans have lactase, why can we not have cellulase too? Does anyone know of any reasons why adult mammals lose their lactases?

Also, if humans could express cellulase, where would the best place to express it be? Obviously, we don't want bacteria eating all of the released glucose. Presumably, if it was expressed in the same region as glucose is taken up, we could avoid this problem. Thus, we could test the effect of including cellulase by using tablets... a rather uninvasive treatment (so long as they dissolve at the right spot). This could be useful as a short term ability, for soldiers in the field, for example.

Josh Harbort