There are 3 basic classifications of animals based on diet; carnivores (eat meat), omnivores (eat most anything edible) and herbivores (eat plants primarily). Dogs are classified taxonomically as carnivores. They clearly have teeth and jaws suited to a carnivorous lifestyle. However, in terms of a healthy dog diet, there are many other factors to take into consideration, including physiology, metabolism, digestion and even taste preference.
So doesn't the classification of dogs as carnivores settle the issue? Absolutely not. Bears and raccoons are carnivores, but they are clearly adapted to an omnivorous lifestyle. Giant pandas are also classified as carnivores, despite the fact that they have a diet consisting of bamboo. Evolution can do funny things with animals, so classification won’t help settle this issue.
Another notable dog nutrition issue revolves around the ability of dogs to digest grains and vegetables. Some websites I have read say that dogs do not digest plant material. I have read through a number of scientific studies on this issue and they invariably report that vegetable proteins, carbohydrates and fats are all very digestible by dogs. The digestive tracts of animals give clues as to what kind of diet they can eat. The shorter the length of small intestine, the less capable the animals are of digesting plant materials. Herbivores have very complex and long digestive tracts, whereas humans have somewhat simpler and shorter digestive tracts. If you compare the length of the small intestine in cats (obligate carnivores) with that of a dog, the dog’s small intestine is longer relative to the animal’s body length (4:1 intestine/body length ratio in cats, 6:1 in dogs). Based on digestive system anatomy, and plant digestibility, it would seem that dogs are adapted to eat a diet that includes vegetable material.
“Obligate carnivores” are those that require a diet almost exclusively of meat. Vegetable matter in the diet is probably of little nutritional value. Cats are obligate carnivores, and one of their dietary requirements is a compound found exclusively in meat and animal tissues known as taurine. Taurine is essential for all animals, but because it is absent in plant material, herbivores and omnivores must synthesize it from other amino acids in their diet. In order for obligate carnivores to get enough taurine, they must eat other animals that contain taurine in their meat and organs. Cats need taurine in their diet, and they are obligate carnivores.
So what about taurine in dogs? Dogs can synthesize their own taurine, indicating that they are not obligate carnivores in terms of physiology. But that is not the end of the story, because not all dog breeds are the same. Large dog breeds tend to have relatively shorter intestines than smaller dogs (relative to body length). In addition, larger dogs can suffer from taurine deficiency (due to higher excretion in their urine) more often than smaller dogs. As such, large dogs probably require more meat in their diets than small dogs to ensure good health. But including vegetables and grains is still a very good idea for well-rounded nutrition.
Feeding dogs only meat is also environmentally unfriendly, unless you hunt for your meat (which could contain parasites), or get it from a very small, local farmer. The meat industry is one of the more polluting industries in the world, fouling streams, rivers, lakes and even bays nearby and creating “dead zones” where little or no oxygen remains. Also, raising animals for meat requires extra grain production, whereby several pounds of grain are required to produce each pound of meat. If every dog owner only fed their dogs raw meat every day, it would drive up the cost of meat, and the meat industry would create even more pollution than it already does.
So the answer to the question posed at the outset, “are dogs carnivores?” is - taxonomically yes, dietarily no. Dogs not only can be healthy on a diet consisting of meat, grains and vegetables, they will do exceptionally well on one. In fact, many dogs love the taste of grains such as rice and oatmeal. Of course many dogs will choose meat over vegetables if given a choice, but don’t most people have the same preference? That doesn’t mean that eating only meat is the healthiest diet. Plants contain many compounds that are healthful, and even anti-carcinogenic, and these compounds are not found in meats. What is true about eating vegetables being good for the human diet is also true about the diet of dogs.
If you feed your dog only raw meat, they will probably be just fine, but I have seen no scientific studies that show that a raw meat-only diet is better (more “optimal”) than a balanced cooked diet. If you know of one in a science journal (rather than a web site), please pass it along to me. A raw meat diet is environmentally less friendly, more expensive, and more likely to contain pathogenic bacteria or parasites. On top of that, there is no convincing evidence I have seen that it is healthier than a balanced, cooked diet including meats, grains and other non-meat items. I should probably mention that non-pet owners would be right to point out that many people in the world rarely get regular meat-containing meals because meat production capacity is not able to provide sufficiently for all the people on earth.
Finally, you may ask, what do we feed our dogs? We feed our dogs a diet of about 60-70% cooked meat (usually chicken or beef) mixed with home-made baked kibble that we make mostly from grains (rice and oatmeal), with added eggs, cheese, olive oil and meat. So they do get a diet composed mostly of meat, but they love the kibble and will do tricks for it. They also get occasional multivitamins and calcium. I am sure that if we reduced the meat percentage in their diets to 50% or even lower, they would still be just as healthy, energetic and happy. Dogs and people have coevolved as partners on earth for the last 20,000 to 40,000 years, and we are both very adaptable in our lifestyles, and diets.