From Small Animal Clinical Nutrition, 4th Ed., pp. 216-219:
DOGS AS OMNIVORES
The word carnivore can be used to indicate either a taxonomic classification or a type of feeding behavior. The order Carnivora is quite diverse and consists of 12 families containing more than 260 species. Omnivorous and carnivorous feeding behaviors are most common among members of the order Carnivora; however, the order also includes spcies that are herbivores (e.g. pandas).
EATING BEHAVIOR
Several researchers have examined the eating habits of wolves (Canis lupus), the nearest ancestors of our domestic dogs, and close relatives such as coyotes (Canis latrans). Both are opportunistic predators and scavengers, hunting and eating what is available regionally. Coyotes eat carrion and hunt rodents, other small mammals, birds, amphibians, and other species. In addition, they have been reported to consume droppings of herbivorous prey; domestic dogs will also readily consume herbivore feces. Regional ungulates such as buffalo, deer, elk, moose, wildebeest, antelope, and zebra are the natural prey of wolves. Viscera are considered the choicest part; therefore, partially digested vegetable material is a normal part of the wolf's diet. Both coyotes and wolves also eat plant matter such as fruits, berries, persimmons, mushrooms, and melons. Similarly, dogs are opportunistic eaters and have developed anatomic and physiologic characteristics that permit digestion and usage of a varied diet.
ANATOMY AND PHYSIOLOGY
ORAL CAVITY
The oral cavity functions to decrease the physical size of food for introduction into the rest of the alimentary tract. Decreasing the physical size of food creates particles small enough to pass through the esophagus and increases the surface area of the food, which enhances enzymatic digestion in the stomach and small intestine. Dogs have cutting canine teeth for ripping and tearing and molar teeth with large occlusional tables for crushing, which are associated teleologically with the capacity to use plant material. Dogs may fix large pieces of food with their paws in order to tear off small pieces with their cutting canine teeth, after which the food particle is advanced to the back of the oral cavity where it may be crushed by the molar teeth and mixed with saliva before being swallowed.
STOMACH
Wild canids typically eat large meals, usually infrequently, due to intermittent food availability. Dogs may consume their daily energy requirement (DER) in one or two large meals, ingested rapidly. This eating pattern means that the stomach must be able to expand markedly. On average, a medium-sized, adult domestic dog has the capacity to ingest 30 to 35 g of dry matter per kg body weight per day. However, the canine stomach can adjust, within limits, to accommodate the amount of food ingested and can hold 1 to 9 liters depending on the breed.
SMALL AND LARGE INTESTINE
The characteristics of the canine small intesting are consistent with those of animals that digest an omnivorous diet. The small intestine composes approximately 23% of the total gastrointestinal (GI) volume of dogs, vs. 15% for cats. The ratio of GI tract length to total body length is 6:1 for dogs, 4:1 for cats, 10:1 for rabbits, and as high as 20:1 for some herbivores. This anatomic relationship is consistent with ingestion of an omnivorous diet with intermediate digestibility (i.e., between low digestible herbaceous forages and highly digestible animal flesh). Dogs digest starch efficiently via pancreatic enzymes and mucosal disaccharidases.
NUTRIENT REQUIREMENTS AND METABOLISM
Much can be learned about an animal's nutritional requirements simply by analyzing its natural food source. True carnivores are limited to what is available from prey tissues such as skeletal muschle and liver to provide energy and nutrients, including protein, taurine, arginine, arachadonic acid, and niacin. Consequently, carnivorous animals (e.g. cats) developed more efficient pathways to use these nutrients, and have lost the ability or have a decreased ability to synthesize them from precursors. Being omnivorous and receiving a varied diet of plant and animal tissue, dogs maintained or improved the ability to synthesize nutrients from precursors. The differences lend more evidence to early evolutionary divergence and further support the premise that dogs are omnivores.