Poultry, beef, pork and fish are all excellent sources of protein that are common in dog foods. But, as any of your vegetarian friends will tell you, there are many other ways to get protein, including eggs, wheat, corn and legumes. A complete, balanced dog food does all the work for you. Whatever the protein source, this tells you it's the right amount to meet your pet's needs. If your dog is an athlete, meaning they complete field trials on a regular basis or run in agility, a high-protein dog food might be beneficial. If your dog is older, however, you'll want to back off on the protein since old kidneys aren't as forgiving as young ones. Likewise, if your dog has been diagnosed with kidney failure, your veterinarian will likely recommend controlled protein foods, among other nutrients. Age, breed size, activity level and lifestyle are all important factors to consider when choosing your pup's food. Your dog is unique, so ask your vet which dog food they suggest. Rather than focusing on just one nutrient, aim to feed your dog a high-quality food that's complete and balanced with all the nutrients they need to thrive.