By Dr. Becker
Many pet parents – especially those with both a canine and feline in the family – wonder if there's really a difference between dog and cat food. This question often comes up when a pet owner runs out of one type of food and wonders if there's any harm in feeding Fido a little of Fluffy's food, or vice versa.
Another time the question arises is when a particularly finicky dog turns up his nose at his own meal, but dives head first into the cat's food bowl.
The answer? Generally speaking, a healthy dog or cat will not suffer one iota from eating a meal intended for the other species. If healthy Fido gobbles up a bowl of cat food while your back is turned, or you need to offer Fluffy some of Fido's dog food in a pinch, there's no need for concern.
Obligate Carnivore (Cat) Versus Scavenging Carnivore (Dog)
The reason dog food differs from cat food is because each species requires its own nutrient profile for optimal health. Felines and canines are both carnivores (meat eaters), but with a very important distinction. Cats are obligate carnivores, whereas dogs are scavenging carnivores.
The definition of an obligate carnivore:
An obligate carnivore (or true carnivore) is an animal that must eat meat in order to thrive (Syufy 2008). They may eat other foods, such as fruits, honey, grains, and so forth, but meat must be included in their diet.
True carnivores lack the physiology required for the efficient digestion of vegetable matter, and, in fact, some carnivorous mammals eat vegetation specifically as an emetic.
The domestic cat is a prime example of an obligate carnivore, as are all of the other felids (Pierson 2008).1
Dogs are scavenging, or facultative carnivores, which in general terms means they are primarily meat-eaters, but can survive on plant material alone if necessary. The key word here is "survive." To survive is not to thrive. To thrive is to grow vigorously. To survive means simply to stay alive.
One of the arguments for feeding dogs grain or plant-based or even vegetarian diets seems to be the distinction between obligate and scavenging carnivores. It's assumed, since dogs aren't strict carnivores like cats are, they can easily transition to a meatless diet. This is a dangerous misconception.
In fact, I often see dogs referred to as omnivores rather than carnivores. I strongly disagree with this assumption. Just because dogs fed plant-based diets are able to stay alive doesn't make them omnivores.
Taxonomically, dogs are in the Order Carnivora and the family Canidae along with other carnivorous mammals.
Cats Have a Unique Requirement for Animal Protein
Cats must eat animal meat and organs to meet their nutritional needs, and plant-based proteins (grains and vegetables) simply aren't a good substitute. Cats lack the specific enzymes necessary to use plant proteins as efficiently as animal proteins.
The proteins derived from animal tissue contain a complete amino acid profile. Amino acids are the building blocks of protein. Plant-based proteins don't contain all the amino acids critical for the health of an obligate carnivore.
Humans, who are omnivores, have the physiological ability to turn plant proteins into the missing pieces needed for a complete amino acid profile. To a very limited extent dogs can do this, but a cat's body isn't equipped for it whatsoever.
Cats also need much more protein in their diet than other animals. Kittens require 1.5 times more protein than puppies. Adult cats need 2 to 3 times the amount adult dogs require.
One of the reasons for this is because while other mammal species use most of the protein they consume for growth and body maintenance, cats use protein for those purposes and also as a source of energy.
When other species of animals are fed a low-protein diet, their bodies make adjustments to conserve amino acids to manage the deficit.
But a cat's body must continue to use protein even when there's not enough in the diet, which is why protein malnutrition happens quickly in sick or injured cats, and cats suffering from anorexia.
In addition to their increased need for protein, cats also have a higher requirement for certain specific amino acids found naturally in animal tissue.
One of the amino acids missing in plants is taurine, which is found in animal muscle meat, in particular the heart and liver. Taurine deficiency causes serious health problems in cats, including cardiovascular disease and blindness. Dogs can make their own taurine.
Cats Also Have a Unique Dietary Requirement for Certain Vitamins
Cats evolved hunting a different set of prey species than dogs did, so their dietary requirements are different than dogs. Cats have a special requirement for vitamin A, which is available naturally only in animal tissue.
They lack the intestinal enzymes necessary to convert B-carotene in plants to the active form of vitamin A. Vitamin A is essential for maintenance of vision, growth of bone and muscle, reproduction, and the health of epithelial tissues.
Cats also require 5 times more dietary thiamine (vitamin B1) than dogs do. A thiamine deficiency can result in a poor quality coat, loss of appetite, hunched posture, neurologic problems including seizures, and even death.
Unfortunately, thiamine isn't stable in commercial pet foods and levels drop significantly the longer the food is stored, so many cats may be deficient unless they are eating very fresh food.
Vitamin D is also essential in the diets of all mammals. Cats (and dogs) must consume vitamin D in their diet (they can't synthesize it through their skin). The liver and fatty tissue of prey animals is rich in vitamin D.
Arachidonic acid is an omega-6 fatty acid that dogs can make themselves, but cats must get from their diet.
Cats Also Need a Moisture-Dense Diet
Another distinctive biological feature of cats is their need to get most of their water intake from the food they eat.
Domestic kitties — who evolved from desert-dwelling ancestors, after all — are not as responsive as other animals to sensations of thirst or dehydration. Unlike dogs who drink frequently from their water bowls, when fed a diet devoid of moisture (e.g., kibble), cats aren't driven to search for another source of water to make up the difference between what their bodies require and what their diet provides.
This can result in chronic mild dehydration, a condition that will ultimately result in disease, especially of the feline lower urinary tract and kidneys.
Species-Appropriate Diets Are the Best Option for Both Dogs and Cats
Obviously, cats can't thrive on a diet designed for dogs. And while dogs may be able to survive on cat food, it's certainly not an optimal diet for them.
Diets designed for kitties are significantly higher in calories, protein, and fat than dogs require. A steady diet of cat food fed to even a very healthy dog may ultimately result in an overweight pet who suffers bouts of diarrhea and vomiting, and is at increased risk for pancreatitis, which can be life-threatening.
So as I said earlier, in a pinch, a healthy dog can eat a meal of cat food, or a healthy cat can eat a meal of dog food.
A better option, of course, is to offer your dog or cat species-appropriate safe human food until you can home prepare or purchase more of his regular food.