Differences between Dog and Cat Food
The major difference between dogs and cats is that dogs are scavanging carnivores (they do best when eating animals, but can digest plant material as well) and cats are obligate carnivores (they can only digest animals). Cats can eat plants, but lack the digestive enzymes to break down the nutrients into a usable form. Because of this difference, cat food has more supplimentation (or supplimentation of a different form) to ensure that the cats get enough of the nutrients that they need. For example:
- Taurine. Taurine is a essential nutrient for cats, and "Dog food does not contain enough taurine to meet the normal requirements for a cat." (Cheryl Yuill, DVM, MSc, CVH, VCA Hospitals)
- Vitamin A. "Dogs have the ability to convert beta-kerotene to vitamin A. Cats on the other hand need pre-formed vitamin A in their diet, which can only be found in animal tissues. " (Joseph Hahn, College of Veterinary Medicine - University of Illinois). Beta-kerotene is easily/cheaply found in carrots and other vegetables, so it is likely that is how dog food is formulated to include Vitamin A. Since cats cannot use beta-kerotene, they will become deficient.
- "Arachidonic acid, a necessary fatty acid, can be synthesized by a dog using linoleic acid. The cat is unable to do this and needs to ingest arachidonic acid in their diet. This, too, can only be found in animal tissues." (Joseph Hahn, College of Veterinary Medicine - University of Illinois).
- Higher protein content. Cats need more protein than dogs. While less protein won't cause any illness, it may make them lethargic and loose muscle mass.
Is Your Cat Eating Enough Dog Food to Be a Problem?
One recommendation is that treats should not exceed 10% of a pet's caloric intake (Pets.webmd). You can look up the calorie counts of your foods, then figure out how much dog food (and other treats you may be feeding) is 10% of your cat's cat food intake to determine if it's just a harmless habit or something you should seriously be worried about.
If you need to stop your cat's dog food intake, I would recommend switching both animals to meal feeding.
Why is the cat eating dog food
I can speculate, but honestly, only the cat really knows.
My best guess is that if you're feeding dry food, cat food manufacturer's coat it with animal digest to make it smell more palatable, and I suspect that dog food manufacturers do the same. Animal digest is irresistable like an open bag of potato chips to a human, so since it smells like food, he eats it.