Science Diet is considered a "premium" cat food and most vets recommend it. I'm sure whatever byproducts it has are far better than the by products in Purina One. Tubby was 18 1/2 when he died, and the only food he got his entire life was Science Diet. Peanut is almost 18 and the only food she has ever gotten her entire life is Science Diet. How bad can the by products be?