Your cat snuck a bite of the dog's food and you're wondering if you need to worry. You don't — not for a single incident. The issue is when it becomes a habit, because cats and dogs have genuinely different nutritional needs and dog food doesn't meet a cat's requirements.

Why cats can't live on dog food

Cats are obligate carnivores, which is a more extreme designation than dogs. Dogs are omnivores — they can extract nutrition from plant and animal sources. Cats cannot synthesize certain nutrients from plant precursors the way dogs can. They have to get them preformed in their food.

The two biggest issues with dog food for cats:

  • Taurine. Cats cannot produce taurine on their own and must get it from their food. Cat food is specifically formulated with adequate taurine. Most dog foods are not. Taurine deficiency in cats causes dilated cardiomyopathy (heart disease) and retinal degeneration — both serious, both slow to develop, and both potentially irreversible by the time symptoms are obvious. The ASPCA explicitly flags taurine as a required dietary supplement for cats.
  • Arachidonic acid. Cats also can't synthesize this fatty acid from plant sources, while dogs can. Dog food often lacks adequate levels because dogs don't need it directly supplied.
  • Vitamin A. Cats can't convert beta-carotene to vitamin A — they need preformed vitamin A from animal tissue. Cat food accounts for this. Dog food may not.

On top of all that, dog food is generally lower in protein than cat food, and cats have higher protein requirements relative to body weight than dogs do.

In practice

If your cat steals a few kibbles from the dog's bowl occasionally, nothing bad is going to happen. The problems develop over months or years of primary feeding with dog food, not from incidental exposure.

If you have both cats and dogs, the cleanest solution is to feed them in separate rooms or at least separate spaces, and pick up the dog's bowl when they're done eating rather than leaving it out. This also stops the dog from eating the cat's food, which is a different problem — cat food is high in protein and fat, and dogs that graze on it regularly tend to gain weight.