As a faithful companion to humans for some 10,000 years, the trend to humanize our companion dogs comes as no surprise. Yet despite his long and close association with humans, the dog remains closest genetically to the gray wolf, with whom he shares over 99% of his mitochondrial DNA. The close genetic relationship between dog and wolf led the Smithsonian Institution to reclassify the dog from its previous separate species designation of Canis familiaris to Canis lupus familiaris. In other words, the Timber wolf, the Tundra wolf, and our beloved companion dog, all fall under the genetic umbrella of the gray wolf: Canis lupus. Just like wolves, all dogs are evolved as carnivores, with anatomical features that clearly adapt them for meat-based diets.  Understanding the anatomical differences between carnivores, omnivores and herbivores helps understand why dogs and cats are classified as carnivores, and what foods best match their anatomy.

To understand the nutritional needs of dogs and cats, it is useful to begin with a basic understanding of their anatomical features, and how they differ from herbivores and omnivores. By understanding which anatomical features are associated with each kind of diet, we are able to classify an animal as:
CARNIVORE (meat eaters), 
HERBIVORE (plant-eaters), or
OMNIVORE (both meat and plant eaters) 
This classification helps us understand which foods the animal is actually adapted to eat.

HERBIVORES (cows, sheep)
Herbivores eat plants, not meat. So it’s no surprise that their anatomical features are adapted to process carbohydrates and other nutrients produced by plants.

Anatomical features common to herbivores confirm their adaptation for a plant-based diet.
LONG DIGESTIVE TRACTS up to 10 times their body length are needed due to the relative difficulty with which plant foods are broken down. Herbivores have significantly longer and much more elaborate guts than do carnivores.

SQUARE FLAT MOLARS provide an ideal grinding surface to crush and grind plants (but not meats). A lower jaw with a pronounced sideways motion facilitates the grinding motion needed to chew plants. 

CARBOHYDRATE-DIGESTING ENZYMES IN SALIVA. AMYLASE is a digestive enzyme in saliva that helps in digesting carbohydrates. Herbivores methodically chew their food to ensure the thorough mixing with amylase.

OMNIVORES (pigs, bears, humans)
Omnivores (from Latin: omne all, everything; vorare to devour) are evolved to eat both plants and animals.As general feeders, omnivores are not specifically adapted to eat meat or plant material exclusively.
Anatomical features common to omnivores include:
MEDIUM LENGTH DIGESTIVE TRACTS that provide the flexibility to digest both vegetation and animal proteins. 
FLAT MOLARS AND SHARP TEETH developed for some grinding and some tearing.
SALIVA CONTAINS CARBOHYDRATE DIGESTING ENZYME AMYLASE which is responsible for the majority of starch digestion.

CARNIVORES (wolves, dogs, cats)
Carnivore means 'meat eater' (Latin carne meaning 'flesh' and vorare meaning 'to devour') and classifies animals whose diets consist mainly of meat – such as dogs and cats.  

The anatomical features of carnivores are:
SHORT, SIMPLE & ACIDIC DIGESTIVE TRACTS. Protein and fat from animal source are quickly and easily digested – hence the short digestive system of dogs and cats. The ability of dogs and cats to secrete hydrochloric acid is also exceptional. To facilitate protein breakdown and kill the bacteria found in decaying meats, dogs and cats are able to keep their gastric pH around 1-2.
SHARP TEETH (designed for slicing meat, not grinding plants). Carnivores have elongated teeth designed for tearing and killing prey. Their molars are triangular with jagged edges that function like serrated-edged blades that give a smooth cutting motion like the blades on a pair of shears.
JAWS MOVE VERTICALLY unlike herbivores and omnivores that grind their food by side to side chewing, the jaws of dogs and cats operate vertically to provide a smooth cutting motion, and open widely to swallow large chunks of meat.
NO AMYLASE IN SALIVA. Amylase in saliva is something omnivorous and herbivorous animals possess, but not carnivorous animals like dogs or cats.  As amylase is not present in saliva, the burden is entirely on the pancreas to produce the amylase needed to digest carbohydrates.  Feeding dogs as though they were omnivores or herbivores makes the pancreas work harder in order to digest the carbohydrate-filled foods (instead of just producing normal amounts of the enzymes needed to digest proteins and fats).

Several key anatomical features separate dogs and cats from omnivores and herbivores, while clearly classifying them as carnivores with an adaptation for an almost exclusively meat based diet.
 Dogs & cats posses a short, simple gastro-intestinal tract. Because meat is easily digested (relative to plants) their small intestines are short. A high concentration of stomach acid helps quickly break down proteins (Carnivores have a stomach acidity of about pH 1 -compared to humans at pH 4 to 5).
A large mouth opening with a single hinge joint adapted for swallowing whole chunks of meat.
Short and pointed teeth designed for grasping, ripping and shredding meat (not grinding grains).
Teeth and jaws designed to swallow food whole (not for chewing or crushing plants).
Adapted to break down protein and fat from meat (not plants or grains), the saliva of dogs and cats does not contain the digestive enzyme amylase. Carnivores don’t chew their food. Unlike carbohydrate-digesting enzymes, protein-digesting enzymes cannot be released in the mouth due to the potential of damaging the oral cavity (auto digestion).  Therefore, carnivores do not mix their food with saliva—they simply bite off huge chunks of meat and swallow them whole.

The dog is, by all scientific standards and by evolutionary history, a domesticated wolf. This raises the question of which foods are appropriate for their carnivorous anatomy, and which are not?
As the dog's internal physiology does not differ from the wolf, dogs have the same physiological and nutritional needs as their wild predator cousins, whose diets are almost exclusively proteins and fats.  Modern dogs of all breeds are not only capable of eating the food of their wild ancestors, but actually require it for maximum health.  This is because their basic physiology has changed very little with domestication—despite the obvious differences in their physical appearances.
Protein is the staff of life for dogs and cats – essential to basic body functions, including cellular regeneration, tissue maintenance, hormone and enzyme production, and the provision of energy.  Although protein is essential, not all proteins function equally, with protein qualities varying enormously between various sources.

Three factors affecting protein quality include:
Due to the different amino acid profiles contained in animal and plant proteins, ANIMAL PROTEINS are considered ‘complete proteins’ for dogs and cats, while PLANT PROTEINS are considered ‘incomplete proteins’.
ANIMAL PROTEINS contain all of the amino acids essential to dogs and cats in quantities that match the requirements needed for their overall health, maintenance and growth.
PLANT PROTEINS such as corn gluten, soybean meal or plant protein isolates, do not contain all of the amino acids in the right proportions that a dog or cat needs.  Amino acids essential to dogs and cats often missing in plant proteins, include arginine, taurine, methionine, lysine and tryptophan.
Protein digestibility is a key quality measure. After all, what good is it to have a food made with a higher quality protein if it’s not also easy to digest? A food with high protein digestibility is one that can be broken down into smaller easy-to-absorb components easier and quicker than others. In the short digestive systems of dogs and cats, plant proteins are far less digestible than meat proteins. Animal source protein is the best choice -it is easily digested and contains the amino acids essential for dogs and cats.
While often viewed negatively by health conscious people, fat is an essential dietary requirement for dogs and cats.As many people are concerned with reducing our fat intake, we often fail to realize the essential role that fat plays in the diets of our dogs and cats.
Just as with protein, fats are also not created equally and differ greatly in their component structure and quality.Dogs and cats don’t suffer from cholesterol problems or heart disease caused by higher levels of animal fats, and it comes as no surprise that cats and dogs need fat from animals, rather than plant source.

Two key roles of dietary fat are:
1. Providing a concentrated source of energy.
2. Supplying the Essential Fatty Acids (Omega-3, for example) that dogs and cats cannot produce within their own body.
Both dogs and cats require a fairly high amount of animal fat in their diets. As companion dogs and cats enjoy a more sedentary lifestyle than their wolf relatives, moderation of fat is important, with an optimum range of 15-18% fat.
While both fats and carbohydrates provide energy, they function very differently in the body of a dog or cat. Fats are essential in the diets of dogs and cats, carbohydrates are not. Carbohydrates provide energy more rapidly compared to fats. In humans, a high intake of carbohydrates increases muscle-glycogen, which increases stamina. The same carbohydrate loading in dogs creates an excess accumulation of lactic acid in the muscles, leading to a condition called hypoglycemia, which causes weakness and fatigue. Animal fats are clearly the choice of energy for dogs and cats.
Essential fatty acids are the fatty acids present in fats that are required by the body. Because they cannot be produced in the body, Essential Fatty Acids must come from foods. The most important are linoleic and arachidonic4 (Omega-6), and DHA and EPA (Omega-3). An appropriate balance of omega-6 and omega3 is important as these two fats work together. A ratio of 2:1 to 5:1 is generally accepted as ideal for dogs and cats.  A lack of Omega-6 is extremely rare, most pet foods have too much Omega-6 and not enough Omega-3.
Omega-3 quality varies dramatically between plant and animal sources.  Of the 3 kinds of Omega-3: ALA (alphalinolenic acid) is from plants, while DHA (docosahexaenoic acid) and EPA (epicosapentaenoic acid) are from fish.

Dogs and cats require DHA and EPA, not ALA.
Plant source Omega-3 is ALA, a short-chain omega-3 found in soy, canola oil and flax. ALA must be converted to EPA and DHA to be of any nutritional benefit to a dog or cat. As cats and dogs are not adapted to create this conversion, ALA Omega-3 from plants is considered ‘inactive’ and not Biologically Appropriate for dogs and cats. While dogs are able to produce arachidonic acid from linoleic acid, cats cannot synthesize arachidonic acid and require it in their diet.
Omega-3 from fish:  Animal Omega-3s (EPA and DHA) are long-chain omega-3s that are absorbed readily and directly within the body. Naturally present in oily fish such as salmon, herring and lake whitefish, EPA & DHA and are by far the best Omega-3 choice for dogs and cats.
Carbohydrates are usually the first source of energy available to the body. Proteins and fats also provide energy  but carbohydrates are called upon first. According to the NRC guidelines, “Carbohydrates provide an economical source of energy in the diet of dogs.”

Carbohydrates are divided into two broad groups:
1). SIMPLE CARBOHYDRATES or sugars, and
Simple carbohydrates are made up of single sugars, or two sugars joined together and are found in grains such as corn, wheat and rice.  Simple sugars are quickly absorbed into the blood stream, causing a rapid rise in blood sugar levels. This rapid rise causes the body to produce a sharp rise in insulin levels and results in the sugars being converted into fat. The rapid rise in blood sugar levels is usually followed by a rapid drop, leading to feelings of hunger and weakness.
Complex carbohydrates have more than two units of sugar joined together and are found in potatoes, beans, as well as many other vegetables and fruits.  Complex carbohydrates can take a long time to break down in the stomach or pass through undigested, creating voluminous stool.
Dogs and cats have no nutritional need for carbohydrates and are evolved to use protein and fat as energy sources.


    The natural diet contains almost no carbohydrate at all, and the small predigested grains, fruits & vegetables in the stomach of a prey animal make up a very small fraction of the total diet.
    Today’s high carbohydrate pet foods lead to blood sugar fluctuations, insulin resistance, and are widely considered as a leading cause of obesity, diabetes and a host of other health problems in cats and dogs.  Conventional dry dog foods have a very high carbohydrate content, with most foods exceeding 40-50% in total carbohydrate content.
    Almost half of typical dry dog foods is nonessential, simple sugars! This important fact is often lost on consumers as pet food makers are not required to claim carbohydrate content on their packages.
    Carbohydrate intake above the daily needs of the dog (which regularly occurs with conventional pet foods) prompts internal enzyme factors to store the extra carbohydrate as body fat.
    The Association of American Feed Control Officials’ (AAFCO) nutrient profiles show that carbohydrates are not essential for dogs and cats, and that no minimum level of carbohydrate is needed in their diets. According to Dr. David S. Kronfeld, carbohydrates need not be supplied to adult dogs, even those working hard as the liver is easily able to synthesize sufficient glucose (from protein and fats).

Cats and dogs evolved as hunters and despite a modern lifestyle, their digestive systems and internal anatomical features have remained unchanged for hundreds of years. Despite obvious and dramatic changes in their physical appearance, the basic physiology of the modern dog has changed very little with domestication. 
Today’s modern dogs are not only capable of eating the food of their wild ancestors, but actually require it for maximum health. A biologically appropriate diet mirrors the same balance and variety of ingredients that would be consumed in the wild, including muscle meat, bone, fat, organ meat and vegetable materials.
Applied to dry foods, the Biologically Appropriate concept means higher-protein, moderate fat, low carbohydrate foods that contain high and various inclusions of animal ingredients that are processed as little as possible. Cereal grains are excluded and carbohydrates are reduced.

Despite obvious changes in their physical appearance, the basic physiology of the modern dog and cat has changed very little with domestication. Conclusions are that dogs and cats are evolved as carnivores, and that despite obvious differences in appearance from their wild cousins, their internal anatomy remains unchanged.
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.

    With short digestive tracts and gastrointestinal systems, dogs and cats are adapted to metabolize animal flesh and fat, not grains and carbohydrates.
    Today’s modern dogs (of any breed) are not only capable of eating the food of their wild ancestors, but actually require it for maximum health.
    Biologically Appropriate foods are designed to match the digestive capability of dogs and cats. Just like the natural diet, they are high in protein and low in carbohydrates, with a concentration and variety of minimally processed meats and fats from animal source.

Although these findings are well accepted within the scientific community, conventional dry dog and cat foods appear to be created on the premise that the digestive system of the dog and cat is similar to humans - with a heavy emphasis on inappropriate grains and carbohydrates.