This is false. Dogs are carnivores, not omnivores.
Dogs ARE very adaptable, but just because they can survive on an
omnivorous diet does not mean it is the best diet for them. The
assumption that dogs are natural omnivores remains to be proven,
whereas the truth about dogs being natural carnivores is very
well-supported by the evidence available to us.
Look into your dog or cat's mouth. Those huge impressive teeth (or tiny needle sharp teeth) are designed for grabbing, ripping, tearing, shredding, and shearing meat (Feldhamer, G.A. 1999. Mammology: Adaptation, Diversity, and Ecology. McGraw-Hill. pg 258.). They are not equipped with large flat molars for grinding up plant matter. Their molars are pointed and situated in a scissors bite (along with the rest of their teeth) that powerfully disposes of meat, bone, and hide. Carnivores are equipped with a peculiar set of teeth that includes the presence of carnassial teeth: the fourth upper premolar and first lower molar.
Contrast this with your own teeth or the teeth of a black bear. A black bear is a true omnivore, as are we. We have nice, large, flat molars that can grind up veggies. Black bears, while having impressive canine teeth, also have large flat molars in the back of their mouth to assist in grinding up plant matter. Dogs and most canids lack these kinds of molars. Why? Because they don't eat plant matter. Teeth are highly specialized and are structured specifically for the diet the animal eats, and the difference between a bear's teeth and a dog's teeth (both species are in Order Carnivora) demonstrates how this can be (Feldhamer, G.A. 1999. Mammology: Adaptation, Diversity, and Ecology. McGraw-Hill. pgs 260.). To see a visual comparison of the teeth of a dog to the teeth of a black bear, please click here. One can logically ask: If a dog (or cat or ferret) has the dentition of a carnivorous animal, why do we feed it pelleted, grain-based food?
Dogs (and cats) are equipped with powerful jaw muscles and neck muscles that assist in pulling down prey and chewing meat, bone, and hide. Their jaws hinge open widely, allowing them to gulp large chunks of meat and bone. Their skulls are heavy, and are shaped to prevent lateral movement of the lower jaw when captured prey struggles (the mandibular fossa is deep and C-shaped); this shape permits only an up-and-down crushing motion, whereas herbivores and omnivores have flatter mandibular fossa that allows for the lateral motion necessary to grind plant matter (Feldhamer, G.A. 1999. Mammology: Adaptation, Diversity, and Ecology. McGraw-Hill. pgs 258-259.). Consider this quote from the previously-cited Mammology text:
This translates to a simple fact: everything about a dog or cat's body design says they were designed for a carnivorous, hunting lifestyle geared toward killing prey. However, humans have done some major tinkering with this body design (resulting in varying sizes and conformations), but we have done nothing to change the internal anatomy and physiology of our carnivorous canines.
Dogs and cats have the internal anatomy and physiology of a carnivore (Feldhamer, G.A. 1999. Mammology: Adaptation, Diversity, and Ecology. McGraw-Hill. pg 260.). They have a highly elastic stomach designed to hold large quantities of meat, bone, organs, and hide. Their stomachs are simple, with an undeveloped caecum (Feldhamer, G.A. 1999. Mammology: Adaptation, Diversity, and Ecology. McGraw-Hill. pg 260.). They have a relatively short foregut and a short, smooth, unsacculated colon. This means food passes through quickly. Vegetable and plant matter, however, needs time to sit and ferment. This equates to longer, sacculated colons, larger and longer small intestines, and occasionally the presence of a caecum. Dogs have none of these, but have the shorter foregut and hindgut consistent with carnivorous animals. This explains why plant matter comes out the same way it came in; there was no time for it to be broken down and digested (among other things). People know this; this is why they tell you that vegetables and grains have to be preprocessed for your dog to get anything out of them. But even then, feeding vegetables and grains to a carnivorous animal is a questionable practice.
Dogs do not normally produce the necessary enzymes in their saliva (amylase, for example) to start the break-down of carbohydrates and starches; amylase in saliva is something omnivorous and herbivorous animals possess, but not carnivorous animals. This places the burden entirely on the pancreas, forcing it to produce large amounts of amylase to deal with the starch, cellulose, and carbohydrates in plant matter. Thus, feeding dogs as though they were omnivores taxes the pancreas and places extra strain on it, as it must work harder for the dog to digest the starchy, carbohydrate-filled food instead of just producing normal amounts of the enzymes needed to digest proteins and fats (which, when fed raw, begin to "self-digest" when the cells are crushed through chewing and tearing and their enzymes are released).
Nor do dogs have the kinds of friendly bacteria that break down cellulose and starch for them. As a result, most of the nutrients contained in plant matter—even preprocessed plant matter—are unavailable to dogs. This is why dog food manufacturers have to add such high amounts of synthetic vitamins and minerals (the fact that cooking destroys all the vitamins and minerals and thus creates the need for supplementation aside) to their dog foods. If a dog can only digest 40-60% of its grain-based food, then it will only be receiving 40-60% (ideally!) of the vitamins and minerals it needs. To compensate for this, the manufacturer must add a higher concentration of vitamins and minerals than the dog actually needs.
Is the dog an omnivore? Its dentition, internal and external anatomy, and physiology say it is not. Even its evolutionary history (discussed later) says the dog is a carnivore. So when people tell you the dog is an omnivore, ask: "What about this animal makes you think it is an omnivore?" Make them explain their position to you before you explain yours. Chances are they'll cite this next myth as "proof".