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The short answer: Hill's Science Diet and Pro Plan Senior tie at 58 apiece. Both are vet-recommended C-grade cat foods with real chicken as the first ingredient but heavy reliance on plant proteins and fillers after that. The differences are marginal — Hill's avoids caramel color, Pro Plan includes probiotics. Neither is a standout.

The scores

Purina Pro Plan Senior Cat: C (58/100)
Hill's Science Diet Cat: C (58/100)

Identical scores 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 Protein Meal, Powdered Cellulose, Chicken Fat

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 protein meal. Pro Plan includes poultry by-product meal (a vague protein source), while Hill's includes chicken meal (a named, concentrated protein) further down the list, at #7.

What Hill's Science Diet does well

Chicken meal at #7. A second named animal protein 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 →

What Pro Plan does well

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 a tie on score. Hill's Science Diet has 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/78) or Blue Buffalo (B/75) 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.

Corrected 2026-10-19: we removed claims we could not support; we corrected factual errors, and grades and score comparisons that did not match our current scores.