I think konnyaku is low in everything - it's almost a non-food. Don't overdo it too fast, though, your digestive system might not recognize it as a food either.

Another thing that is good is koya-dofu, which is the freeze-dried tofu. You have to soak it in hot water for about five minutes and squeeze out the water until it runs clear (it's just like a sponge). Then if you cut it up into pretty small pieces and cook it in a liquid (stew or soup etc) it will absorb the flavor also. The longer you cook it the more flavor it gets (and the less like a sponge it tastes ), and I find it a great substitute for potatos in beef stew. The package lists a lot of carbs, but it's one of those "per 100g" things, and the amount you use in one block is nowhere near that.