The scores
Purina Pro Plan Senior Cat: C (58/100)
Hill's Science Diet Cat: C (60/100)
A 2-point difference at C grade. Both foods land in the same quality tier and share similar ingredient strategies — real chicken first, followed by grains and plant protein boosters.
How the ingredients compare
Here are the first five ingredients side by side:
Pro Plan Senior: Chicken, Rice, Corn Protein Meal, Poultry By-Product Meal, Wheat Flour
Hill's Science Diet: Chicken, Whole Grain Wheat, Corn Gluten Meal, Pork Fat, Chicken Meal
Both start with chicken — a good sign. After that, both pivot to grains and plant proteins. Pro Plan uses rice and corn protein meal; Hill's uses whole grain wheat and corn gluten meal. Pro Plan includes poultry by-product meal (a vague protein source), while Hill's includes chicken meal (a named, concentrated protein). Hill's slight edge comes from having a second named animal protein in the top five.
Where Hill's Science Diet pulls ahead
Chicken meal at #5. A second named animal protein in the top five gives Hill's a slight protein quality advantage. "Chicken meal" is specific and concentrated. "Poultry by-product meal" is vague and lower quality.
No caramel color. Pro Plan Senior includes caramel color — an artificial coloring that serves zero nutritional purpose. Hill's avoids this unnecessary additive.
Whole grain wheat. While still a grain filler, "whole grain" wheat retains more nutritional value than Pro Plan's wheat flour, which is refined and stripped of fiber and nutrients. Shop on Amazon →
Where Pro Plan holds its own
Probiotics. Pro Plan includes Bacillus coagulans, a probiotic strain that supports digestive and immune health. Particularly relevant for senior cats whose gut health may be declining. Hill's doesn't include live probiotics.
Dried chicory root. A natural prebiotic that feeds beneficial gut bacteria. Combined with the probiotic, Pro Plan offers better digestive support than Hill's for aging cats.
Fish meal. Provides omega-3 fatty acids for joint and cognitive support — both important for senior cats. Hill's relies on fish oil but in a lower concentration. Shop on Amazon →
The bottom line
This is essentially a tie. Hill's Science Diet earns a marginally higher score thanks to better protein sourcing (chicken meal vs. poultry by-product meal) and no artificial coloring. Pro Plan counters with probiotics and prebiotics that matter for senior digestive health. If your vet recommends one of these, the ingredient quality difference is too small to override that advice. If you're choosing on your own, both are average C-grade foods — and brands like Wellness (B/80) or Blue Buffalo (B/76) offer meaningfully better ingredient quality at comparable prices.
Read our full reviews of Purina Pro Plan Senior Cat and Hill's Science Diet Cat for the complete ingredient breakdowns.