Introduction We've designed the Purina ONE Ingredient Tool as an easily accessible means of determining what's in your pet's food and, more important, why it's there. When you select an ingredient from the list at left, you'll get a description of what that ingredient contributes in the way of essential nutritional elements and how it benefits specific areas of the body. You can also click to see a list of every Purina ONE dog and cat formula that contains the ingredient in question. Guide to Understanding Labeling Click "Guide to Understanding Labeling" at any time for an explanation of the content of pet food labels and for what you should keep in mind when comparing different labels.