Dogs will eat just about anything, including people food, cat food, grass and other gross bodily substances (no need to discuss those here). Some dogs will even happily eat cat litter that their otherwise intelligent owner accidentally fed them. The owner, who shall remain nameless, should not be judged. (It looked like dog food, OK!)
The point is, life requires flexibility. That's what substitutes are for. They're especially helpful "in a pinch," like when you're cooking and run out of oil and use applesauce instead, or when a teacher is sick and has to call a substitute teacher.
It seems natural to be able to replace cat food for dog food. Cats and dogs are both furry, fun, have tails and make good pets. Plus, dog and cat food look the same. But are they?
The simple answer is no. That means it's not advisable to feed cat food to your dog or vice versa. "One meal of the others' food won't cause instant death, but an abrupt change of food is always a risk," explains Aaron Orr, D.V.M. of Orr Animal Hospital in northeast Cumming, a suburb of Atlanta. "Such a sudden change can cause both dogs and cats quite a bit of intestinal distress, especially smaller dogs and cats."
Even though dogs like cat food (and a lot of things they have no business eating), they should never eat it on a regular basis. Cats, on the other hand, are known to be much more finicky than dogs, and are less likely to eat dog food, but there are always exceptions.
For starters, cats are obligate carnivores, which means they need an especially high amount of protein to survive. In contrast, dogs are omnivores living on animals and plants. The high protein, calorie and fat content of a cat's diet (probably why dogs like it, calories + fat = tasty), cannot only cause a dog gastrointestinal issues, but also the high calorie count can cause obesity. And too much protein can cause kidney problems and even pancreatitis. An obese dog with kidney problems is not a healthy dog.
It's even more dangerous for cats to routinely eat dog food, even in the short term. Cats need the amino acid taurine in their food, whereas dogs make their own taurine. A lot of dog foods are deficient in taurine and a shortage of the amino acid can cause a cat to develop a specific type of heart disease, called hypertrophic cardiomyopathy.
Both cats and dogs need vitamin A, but dogs can convert beta carotene into the vitamin, so it's not necessary for it to be in their food. Cats have the same problem with the fatty acid arachidonic acid, which they need in their food, while dogs can make their own. Even though dog food may contain protein, most don't have the percentage of protein that cats need.
So if you run out of Fido or Felix's food, it's better to feed them a bowl of bland "people" food like boiled plain chicken, white rice and even oatmeal because they are easiest on the digestive system. (And never, ever feed them cat litter. Just trust me.)
And if you're a cat and dog family, keep the cat's bowl out of the dog's reach, especially if Fido appears to be letting his physique go. Weight gain is a sure bet he's finding dessert somewhere. Not only is it unhealthy to eat the cat's food, but forgiveness is not generally a cat's strong point.