Ingredients must be listed on the label in decreasing order by weight. Some owners are concerned about using diets that contain any vegetable-based proteins, such as soybean or corn. These are NOT added as fillers and contain important nutrients. There is no reason why "grain free" foods are better for either dogs or cats (and the potatoes that are used in place of grains in these foods, in fact, have a lower nutritive value than grains).
While grains are useful sources of protein and other nutrients, it is wise to be sure an animal product is one of the first three ingredients in a food. Pet food ingredients have strict definitions so meat by-products, for example, are not allowed to contain the non-nutritious animal parts that people sometimes worry about (by-products refer to the non skeletal muscle meat in other words, the organs). Be aware though, that these legal definitions only define the composition of the ingredient, not its quality. There are good quality and poor quality meat by-products as well as good quality and poor quality meats.
The quality of meat by-products and other ingredients can be assured by using foods made by reputable companies (not just the ones with the most persuasive marketing). In general, pet owners place too much emphasis on the ingredients in a particular pet food (and the marketing that goes along with them), rather than relying on more important factors, such as nutritional adequacy (see next section) and the company's quality.
Be careful of what's in your food!
Next Section: Nutritional Adequacy