The scores
Purina Pro Plan: C (56/100)
Hill's Science Diet: C (60/100)
A 4-point difference within the same letter grade. Neither formula is going to wow you, but Hill's pulls ahead just enough to matter. Both land in the lower half of C territory — adequate nutrition with ingredient lists that don't match the premium branding or veterinary endorsements.
How the ingredients compare
Here are the first five ingredients side by side:
Purina Pro Plan: Chicken, Brewers Rice, Corn Gluten Meal, Poultry By-Product Meal, Wheat Flour
Hill's Science Diet: Chicken, Whole Grain Wheat, Corn Gluten Meal, Chicken Meal, Powdered Cellulose
Both start with chicken — a positive for any cat food. But after that, the formulas diverge. Pro Plan follows with brewers rice (a processed rice fragment), corn gluten meal (cheap plant protein), poultry by-product meal (unnamed, lower-quality parts), and wheat flour (a grain filler). Hill's follows with whole grain wheat (still a grain cats don't need, but less processed), corn gluten meal (same plant protein booster), chicken meal (named, concentrated animal protein), and powdered cellulose (wood pulp filler). Hill's fourth ingredient being chicken meal instead of poultry by-product meal is the key difference — it's a named animal protein rather than a vague by-product.
Where Hill's Science Diet pulls ahead
Fish oil for omega-3s. Hill's includes fish oil, providing DHA and EPA omega-3 fatty acids that support skin, coat, and brain health. Purina Pro Plan has no omega-3 source at all — a significant omission for an obligate carnivore formula marketed as premium nutrition.
Named protein throughout. Hill's uses chicken and chicken meal — both clearly identified animal proteins. Pro Plan relies on "poultry by-product meal" at position four, which could be chicken, turkey, or any combination. For cats with protein sensitivities, that vagueness is a real problem.
No soy protein isolate. Hill's avoids soy entirely. Pro Plan includes soy protein isolate at position six — yet another cheap plant protein that inflates the label's protein percentage without delivering the amino acid profile cats actually need. Between corn gluten meal, wheat flour, and soy protein isolate, Pro Plan has three plant-based protein fillers in its top six ingredients. Shop on Amazon →
Where Purina Pro Plan holds its own
No powdered cellulose. Hill's includes powdered cellulose (wood pulp) as a top-five ingredient — it's marketed as hairball and weight management fiber, but it's still a filler taking up valuable formula space. Pro Plan avoids this, relying on other fiber sources further down the list.
Research infrastructure. Purina operates the PetCare Center and conducts extensive feeding trials with decades of data behind their formulations. Hill's has a strong research program too, but Purina's scale in nutritional research is hard to match. Both brands can point to real clinical outcomes.
Price. Purina Pro Plan typically costs a few dollars less per bag than Hill's Science Diet. With both formulas landing in the same C grade, the price difference means Pro Plan delivers similar nutritional value for less money — even if "similar" means similarly mediocre. Shop on Amazon →
The bottom line
Hill's Science Diet wins this matchup, but calling it a "winner" overstates the case. A C/60 beating a C/56 is the ingredient equivalent of choosing the taller jockey — neither stands out. Hill's earns its edge with fish oil, named proteins, and no soy filler, but both formulas are built around grains and plant proteins that cats don't need. Both are outclassed by brands in the B tier like Blue Buffalo (B/76) and Wellness (B/80) at comparable price points. If your vet recommends one of these for a specific clinical reason, follow that advice. For everyday feeding, your cat can do better.
Read our full reviews of Purina Pro Plan and Hill's Science Diet for the complete ingredient breakdowns.