The scores
Iams: C (62/100)
Purina ONE: D (52/100)
A 6-point gap that crosses the C/D grade boundary. Iams holds onto the low end of C territory while Purina ONE falls into D — the difference between "average" and "below average." At similar price points, that grade difference makes the decision straightforward.
How the ingredients compare
Here are the first five ingredients side by side:
Iams: Chicken, Corn Grits, Chicken By-Product Meal, Corn Gluten Meal, Dried Beet Pulp
Purina ONE: Chicken, Corn Gluten Meal, Soybean Meal, Brewers Rice, Wheat Flour
Both start with chicken, which is the baseline expectation for any cat food worth considering. After that, the formulas split. Iams follows with corn grits (a filler, but at least straightforward), chicken by-product meal (concentrated animal protein from lower-quality parts), corn gluten meal (plant protein booster), and dried beet pulp (a solid prebiotic fiber). Purina ONE follows chicken with corn gluten meal immediately at position two (plant protein right away), soybean meal at three (another plant protein), brewers rice at four (processed rice fragments), and wheat flour at five (grain filler). Purina ONE front-loads plant proteins far more aggressively.
Where Iams pulls ahead
No artificial colors. The most damning ingredient in Purina ONE's formula is caramel color — a purely cosmetic additive that makes kibble look a certain way for human shoppers. Cats don't care what color their food is. Iams avoids all artificial color additives, keeping the formula focused on nutrition rather than shelf appeal.
Functional extras. Iams includes fructooligosaccharides (FOS), a prebiotic fiber that feeds beneficial gut bacteria. It also has L-Carnitine for fat metabolism support, which matters for indoor cats prone to weight gain. Purina ONE has none of these functional additions — the formula is stripped to the bare minimum.
Less plant protein loading. Both formulas use corn gluten meal, but Purina ONE doubles down with soybean meal at three and soy protein isolate at ten — two soy-based protein sources inflating the label. Iams has corn gluten meal at four but avoids soy entirely. For an obligate carnivore, fewer plant protein fillers means the protein percentage is more honest about what's coming from animal sources.
Dried beet pulp. Iams includes dried beet pulp at position five — a genuinely useful prebiotic fiber that supports digestive health. It's not glamorous, but it's functional. Purina ONE has wheat flour in the same position, which contributes nothing beyond cheap carbohydrates. Shop on Amazon →
Where Purina ONE holds its own
Fish meal. Purina ONE includes fish meal further down the list, providing some animal-sourced protein and a small amount of omega-3s. Iams has no dedicated omega-3 source, so neither formula excels here — but Purina ONE at least has a trace contribution.
Price accessibility. Purina ONE is often the cheapest name-brand cat food on the shelf, sometimes undercutting even Iams by a dollar or two per bag. For owners on the tightest budget, that difference matters — though the 6-point quality gap suggests the savings aren't worth it.
Availability. Both brands are everywhere — grocery stores, big box retailers, pharmacies — but Purina ONE tends to have slightly wider shelf presence in convenience and dollar stores. If accessibility is the top priority, both deliver, though Iams is nearly as easy to find. Shop on Amazon →
The bottom line
Iams is the clear winner in this budget cat food matchup. A C/58 beating a D/52 is a full letter grade difference, and the reasons are concrete: no caramel color, FOS prebiotics, L-Carnitine, and less aggressive plant protein loading. Purina ONE's cosmetic additives and double soy formula drag it into D territory where it doesn't belong on any cat's plate if there's an alternative at the same price — and Iams is exactly that alternative. Neither formula is going to compete with B-tier brands like Blue Buffalo (B/76) or Wellness (B/80), but if your budget tops out at the grocery store aisle, Iams is the smarter pick.
Read our full reviews of Iams and Purina ONE for the complete ingredient breakdowns.