Of all the different issues related to the care of dogs and cats, I think the issue of pet food is plagued with the most misconceptions. Here are some important things to know:
1. Unlike dogs, cats are obligate carnivores. They need food higher in protein. That's why we frequently recommend canned food instead of dry food for overweight cats. Canned food tends to be higher in protein than dry food, which can be too carbohydrate rich for cats. Also, cats' mouths open and close, but they don't masticate their food like dogs (and people), so they don't get the dental benefit of the crunchy food.
2. "Grain-free" is a marketing scheme for the most part. It's not something veterinarians worry about. That doesn't mean those diet are bad, but there's nothing special about grain-free diets. When dogs have food allergies, they tend to be to protein sources such as beef, poultry, lamb, etc.
3. Certain protein sources, like chicken and beef, have developed a bad name, but we don't feel that they are a problem if your dog is not allergic or intolerant to them. There is probably no real benefit in normal dogs to feeding fish, venison, rabbit and other meat sources. In fact, we need those protein sources for dietary allergy testing and every time they're used heavily, that's one more protein source we can't use as an elimination diet!
4. On a pet food label, probably only the first five ingredients are present in enough amounts to matter. Also, many terms, like "by-products," are neutral terms. There are good by-products and bad by-products.
5. Just because it's good for people, that doesn't mean it's good for pets! You can create nutritional deficiencies (serious ones!) in dogs and cats by feeding diets of only people food.
6. Fad diets can be dangerous. There is currently an FDA study investigating a possible link between heart disease in dogs and diets that use ingredients such as lentils, potatoes, peas, and other legume seeds as their main ingredients.
7. Obesity is bad. Very bad. There is a Purina study that dogs with lean body condition may live 15% longer than overweight dogs. That's 1.8 years for a Labrador Retriever! And, since cats get diabetes analogous to Type II diabetes in people, obesity increases their risk of diabetes mellitus.
8. We ask two questions when investigating pet foods we aren't familiar with: A. Is it formulated by a VETERINARY NUTRITIONIST? B. Is it made in a dedicated facility? Some foods may sound good on the surface but may be made in a contracted mill, so their purity and consistency is determined by the mill, not the company.
9. All pet foods will list fat, fiber, and protein concentrations, but the quality of the actual ingredients can vary greatly. At the minimum, make sure the food has an AAFCO (The Association of American Feed Control Officials) label.
Some of this information is borrowed from: Ernest Ward, DVM. "Chow Hounds: Why Our Dogs Are Getting Fatter-A Vet's Plan To Save Their Lives." HCl, March, 2010.