Proteins are the most critical component of food for your canine carnivore. They are also the most abundant component of your dog’s body. Your dog needs proteins to produce hair, nails, tendons, cartilage, and all the connective tissues that support the rest of the tissues and organs in her body. Adequate protein is important for your dog’s growth and proper development, her muscle development and strength, a functioning immune system, the production of functioning hormones, the proper volume of blood, injury repair and prevention, and much, much more.

Your dog’s body can also use proteins to produce energy, if necessary. Fats and carbohydrates are much more readily available sources of energy, but dogs can break down proteins and convert them to energy when necessary, such as when food supply is low.

Proteins are made up of amino acids linked in a chain. When your dog eats protein, enzymes that the pancreas secretes into the intestines break them down into shorter chains of amino acids called polypeptides, which are small enough for the intestines to absorb. A dog’s body makes 20 different amino acids — some are essential amino acids and others are nonessential amino acids. As the name implies, your dog requires essential amino acids in his food. Food that contains all the essential amino acids is called a complete protein source. The nonessential amino acids are … drum roll, please … not essential; if your dog doesn’t get them in his diet, he can convert other amino acids into those that he’s missing.

Your dog can get proteins from both animal and plant sources. But only animal-source proteins are complete protein sources, and not all of them are complete. Examples of complete protein sources that come from animals are eggs, whole milk, and lean meat. Grains are another important source of proteins in dog foods, but they are incomplete protein sources because they don’t contain some of the essential amino acids your dog needs. Plant protein sources frequently used in dog foods include soybeans, wheat, and corn.

Your dog’s major source of protein should be animal products, not grain. Don’t buy a dog food in which soybean meal, soy flour, or corn gluten meal is the primary, or even the secondary, source of protein (see the post “Reading a Dog Food Label” for more on this). Dogs don’t have the enzymes to use grains properly as main sources of protein.

The American Association of Feed Control Officials (AAFCO) is the organization that sets guidelines for the types and amounts of nutrients dogs need in their foods. The AAFCO has determined that foods for adult dogs should contain no less than 18 percent protein, and that foods for lactating females or puppies should have a minimum of 22 percent protein. Military or police dogs, mushing dogs, and other dogs who work hard every day or who are under stress may need more. Dogs recuperating from injuries or surgery may need more protein as well, to repair muscles, tendons, and ligaments.

Not all complete protein sources are created equal. A cow’s hoof and a filet mignon may both have all the essential and nonessential amino acids, but your dog can get the amino acids he needs more easily from the filet mignon than from the cow’s hoof. Some proteins are just more digestible than others. So how do we know which protein sources are digestible and which aren’t? Nutritionists measure the amount of protein in a food, feed it to dogs, and then measure the amount of protein in the dogs’ feces. The difference between how much was in the food to begin with and how much the dog excretes reveals how much of it the dog absorbed, and that is the digestible protein. A protein isn’t very useful to your dog if it ends up on your lawn rather than in his body. Hair and feathers are a cheap source of protein, too — but they’re indigestible. On the other hand, eggs are highly digestible but expensive. Not surprisingly, the more digestible the protein, the more expensive the dog food. As with many things in life, you get what you pay for.

Beware of foods that advertise over 90 percent digestibility. The highest-quality dog foods are 82-86 percent digestible, whereas economy foods (inexpensive brands you get in grocery stores) are around 75 percent. The percent digestibility of a dog food is not stated on the label, but most dog food manufacturers provide that information on request.

If your dog’s feces are voluminous, it may be a sign that his food isn’t highly digestible.

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