The scores
Iams ProActive Health: C grade, 63/100
Purina ONE SmartBlend: C grade, 58/100
A 5-point gap within the C tier — narrow enough that the formulas are remarkably similar in quality and approach. Both are mass-market chicken-first kibbles built to deliver baseline nutrition at a low price point. Iams gets the narrow win for its FOS prebiotic and slightly cleaner grain profile.
How the ingredients compare
Here are the top five ingredients side by side:
Iams: Chicken, Corn Grits, Ground Whole Grain Sorghum, Chicken By-Product Meal, Dried Beet Pulp
Purina ONE: Chicken, Rice Flour, Corn Protein Meal, Whole Grain Corn, Chicken By-Product Meal
Both lead with chicken — a real protein source — and both include chicken by-product meal for additional concentrated protein. The difference is in their carbohydrate strategy. Iams leans on corn grits and sorghum, while Purina ONE uses rice flour and whole grain corn. Neither approach is significantly better than the other. Both rely heavily on grains and refined carbohydrates to fill out the formula.
Where Iams holds its own
Iams includes FOS (fructooligosaccharides) prebiotics, which are a well-studied prebiotic fiber that supports gut health and beneficial bacteria. Ground whole grain sorghum in the third position is a slightly more nutritious grain than Purina ONE's corn protein meal — sorghum is gluten-free and provides decent fiber. Dried beet pulp at number five is another solid fiber source that aids digestion.
Iams also has a longer track record — the brand has been around since 1946. For owners who value brand stability and history, that may matter. Shop on Amazon →
Where Purina ONE holds its own
Purina ONE includes chicory root, a natural source of inulin (a prebiotic fiber), which serves essentially the same function as Iams' FOS — supporting digestive health through different means. Rice flour in the second position is a more digestible carbohydrate than Iams' corn grits, which may be gentler on sensitive stomachs.
Purina ONE is also part of the broader Purina family, which means if you decide to upgrade later, you can step up to Purina Pro Plan (C/62) within the same brand ecosystem. Shop on Amazon →
The bottom line
A narrow 5-point gap between two very similar budget foods. Iams earns a C grade (63/100) and Purina ONE earns a C grade (58/100) — both are functional budget foods that cover the basics without excelling anywhere. The prebiotic sources differ (FOS vs chicory root) but serve the same purpose. Neither food includes fish oil, probiotics, or glucosamine.
If your dog is doing well on either, there's no reason to switch between them. If you want a meaningful upgrade from this tier, consider Diamond Naturals (B/78) — a full grade higher for only a few dollars more per bag.