I think it's just a fiber filler. I don't think it will sabotage your Atkins efforts.
I found this info:
Cellulose is made of repeat units of the monomer glucose. This is the same glucose which your body metabolizes in order to live, but <b>you can't digest it in the form of cellulose.</b> Because cellulose is built out of a sugar monomer, it is called a polysaccharide.