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The short answer: Iams and Purina ONE now tie at C/58 within the same C tier, after Purina ONE's 2026 reformulation closed what used to be a clear gap. Both are grocery-store staples at similar prices. Iams puts chicken first, includes FOS prebiotics, and avoids artificial colors; Purina ONE still has caramel color and double-soy protein loading. Same score, but Iams delivers the cleaner formula for a similar price.

The scores

Iams: C (58/100)
Purina ONE: C (58/100)

A dead heat within the C tier. Both sit in the mid-C range — Purina ONE's 2026 reformulation lifted it out of D (adding dried chicory root, soy protein concentrate, and replacing "animal digest" with named natural flavor) right up to Iams's level. Both are "average" ingredient quality, with Iams holding a slim qualitative edge on the strength of FOS, L-carnitine, and the absence of caramel color.

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 Iams's cleaner formula at the same C/58 score suggests the savings aren't worth much.

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 and Purina ONE land in a dead heat after Purina ONE's 2026 reformulation — both at C/58. The score is tied, but the reasons still lean Iams: no caramel color, FOS prebiotics, L-Carnitine, and less aggressive plant protein loading. Purina ONE's improved chicory-root and named-flavor changes pulled it out of D territory and up to Iams's level, but caramel color and double-soy loading keep both firmly in mid-C. Neither formula is going to compete with B-tier brands like Blue Buffalo (B/75) or Wellness (B/78), but if your budget tops out at the grocery store aisle, Iams is the marginally cleaner pick.

Read our full reviews of Iams and Purina ONE for the complete ingredient breakdowns.