A long time ago before I knew any better, I used to think that dogs eating cat food, and cats eating dog food was a seriously harmful thing. I never fed my animals food inappropriate, but I'd overreact if I caught my cat nibbling some dog nuggets he harvested from the floor. "No! That's not good for you!". I didn't really understand why at the time. I just knew it wasn't the right food, and I really thought something bad may come from that sneaky snack. I didn't have an understanding of nutrition, and I didn't know the first thing about how their bodies were different from each other.
While there are many trends in holistic foods, natural foods, etc. the thing we merely need to understand is that cats are "true carnivores", and dogs.. well, aren't. Dogs are omnivores, like us. I've heard stories of people who thought nothing of putting their pets on vegetarian diets, conversely others who thought nothing of giving their dog nothing but meat. While opinions vary on the impact of vegetarian diets regarding dogs, on the whole I've never met a person in the flesh who agreed this was appropriate. Cats, on the other hand, will suffer severe consequences if they are not fed high protein. (Cats without the proper nutrients they need from meat may suffer heart disease and blindness.)
We grow up with movies depicting the outside family dog, receiving his daily bone or steak, no worse for the wear. When we understand they are omnivores, we can start to better understand why an unbalanced diet is not good. Imagine if you ate nothing but steak! Good times, but your body craves carbohydrates, sugars, veggies, fruits, etc. and you know how you start to feel if you've been eating nothing but chicken wings all week.
If you are feeding your dog great dog food, and the correct amount, they should be getting all the nutrients they require. What more dog owners should be doing, is considering treats that are fresh and healthy to supplement that diet. Dog treats are not regulated like dog food is, and their ingredients don't need to be healthy per se. Knowing your dog isn't too unlike you, start to trade out that snack for a fresh carrot stick, celery stalk, or apple. (Be sure not to give your dog fruits with the seeds still in tact. Many seeds from fruit contain arsenic, so peel your fruit and give them slices. Apple seeds have arsenic, and so do cherry seeds).
The question often then arises, what about allergies? If your dog has allergies, ask your vet about any snacks you are giving. Every vet I've talked to, however, point out that most allergies are to certain meats, and not to fruits and veggies. Remember "table scraps" are only bad if they're processed, full of salt, or stuff we know we shouldn't even be eating. There's a huge difference in sharing a sprig of broccoli versus your mac n' cheese! Opinions vary on feeding our dogs raw meat in general,
so ask your vet. Many people rave about the benefits, others really disagree. Some have told me it's fine as long as you freeze it first, to kill off any parasites and then defrost it before feeding. The same goes for bones; pretty much everyone agrees that passing Fido a bone from cooked meat is bad; it breaks and shards into pieces which can lodge in their system. Some say raw bones are fine. Do your own research!
So back to the fear I had about my cat sneaking dog food and vice versa. No, it's not awful when they are just sneaking nibbles. What is awful, however, is choosing to feed your animal food that is not species specific. If a dog ate cat food, he'd get far too much protein in his diet. If your cat ate the dog's food, they'd suffer from not enough protein. You think this would be rare, but I ran into a couple of people who joked and said "Well, the cats just always seem to want the dog's food more.. so I just feed them all dog food". Animals seem to inherently want what they can't have, so remember to never let their deep, pouty eyes dissuade you from what you know is best for them!