I saw mention in a recent post of powdered cellulose. In this case it was an ingredient in a dog chew, but you'll also commonly find it as an ingredient in commercial dog food.
For anyone not aware of it, powdered cellulose is woodpulp. Sawdust. A cheap and nasty source of fibre without any nutritional value whatsoever. And I do mean zero nutritional value. That's how much commercial dog food manufacturers care about your best friend.
Next time you fill your dog's bowl with kibble, ask yourself if your dog doesn't deserve better!