Science Plan Chicken Perfect Weight

Powdered cellulose is wood pulp used as a cheap bulking fibre, and the comparative research is unflattering. In Beagle feeding trials (Journal of Animal Science), apparent total tract digestibility was 11% for cellulose against 29% for beet pulp — less than half. Cellulose is insoluble and poorly fermentable, so it passes through largely untouched: it reduces overall dry matter and organic matter digestibility, increases stool volume, and produces lower concentrations of beneficial short-chain fatty acids with a less acidic fecal pH than fermentable fibres. It adds bulk and satiety without feeding the gut. Fermentable fibres — beet pulp, pumpkin, chicory — do the job better.

The problem here is transparency, not toxicity. AAFCO defines 'animal fat' as rendered fat from ANY mammalian species — the source is not named, need not be consistent, and can vary batch to batch. That's the lowest level of ingredient disclosure available on a label. Two practical consequences: a dog with a protein allergy cannot avoid the trigger if the species is unknown, and you have no way to judge quality. Rendering itself is a legitimate process, and named fats (chicken fat, beef fat) are perfectly good ingredients. The concern is specifically the anonymity — a manufacturer confident in the source usually names it.