We all know that we are what we eat. But did you know the same goes for your pet?

Typically, in deciding what -- and how much -- we should feed our pets, we have been pretty much at the mercy of the nation's pet food companies, who take in more than $3 billion each year from the 53 percent of the nation's households who own at least one cat or dog.

Whether your pet is of the canine or feline persuasion, the most important thing to remember is that, unlike humans -- who need a variety of different foods to maintain good health -- dogs and cats are primarily carnivores (meat-eating) and need a significantly different diet. Also remember that no matter how smart you think your dog or cat is, they're not smart enough to know their own nutritional needs. If they did, they'd probably be owning us and not the other way around.

And no matter what the commercials say, "Your pet cannot maintain good health on red meat alone," claim husband and wife outdoor writers Joan and Arthur Cone Jr., who with veterinarian Robert George have put together a forthright new book called Feeding Fido (EPM Publications, $5.95). The spiral-bound publication discusses in laymen's terms your dog's nutritional needs, the benefits and risks of commercial dog foods, caloric requirements (including how to tell if your pooch is in need of their version of the Scarsdale diet) and even some recipes you can share with your pet.

The authors point out that wild canines and felines consume not only the meat of their prey, but also the organs, skin and bones.

Says Dr. Michael Fox, veterinarian and scientific director of The Humane Society of the United States, "It's criminal that pet food companies have been allowed to say that all a dog needs is a whole meat diet. Diet is the basis of your pet's health. It underlies many serious health problems our pets may have."

Okay, our pets cannot live on meat alone. What else do they need? Water, for one thing. It should be fresh, cool (not cold, as it can cause intestinal distress) and available at all times, especially if your pet eats a dry food. The next most important thing is high-quality protein, which means that it contains enough of the essential amino acids the animal needs to convert the protein into energy. Both cats and dogs also need fats, vitamins, and minerals.

How do we know if we're giving our pets the balanced diet they need? One way is to use a commercial pet food whose label indicates that it's nutritionally complete. (Of course, commercial foods aren't a panacea, but with the pet food companies buying up practically all the scrap meat available, you'd probably go broke trying to do it yourself.)

If the label doesn't bear the words, "a complete and balanced diet," go on to the next brand. From there, you pretty much have to rely on your common sense. Bloated or dented cans, torn bags, odd odors or food that doesn't look right portends trouble for your pet's digestive system.

Both dog and cat food come in three varieties: canned, semi-moist and dry. Dry foods are responsible for about two-thirds of the market, since they by far give you the most for your money. With both canned and semi-moist (soft) foods, you're paying for water as well as food. Because your finicky eater may not eat dry food, many people compromise by mixing the dry with the canned. This is advisable also because dry foods don't provide enough fat.

Once you find a brand that you can afford and your pet will eat, stick with it. After overfeeding -- the most common mistake people make in feeding their pets -- comes a lack of consistency, says Fox. Dogs and cats are creatures of habit, and they should be fed at the same time every day (twice daily, the experts recommend). Some variety is all right, of course, but drastic switches can cause digestive problems.

If you feed your dog or cat only dry food, Fox advises supplementing the diet with some vegetable oil: 1 tablespoon for every 30 pounds of body weight. He recommends feeding your pet a diet of 80 or 85 percent commercial food, supplemented with 15 to 20 percent unprocessed food, including raw or lightly cooked beef, lamb, chicken, fish, kidney or liver. Since his is a vegetarian table, he cooks his dogs and cats a special "stew" that he forms into patties and freezes.

So far as possible risks in commercial foods, "None have been proven," says Fox, "but there's an increasingly recognized fact among veterinarians that a number of problems in both dogs and cats, from skin problems to weird chronic diseases, disappear when the animal is taken off commercial food.

"Other risks are more long-term, from diseased parts of animals being used although canned foods are heat-treated, too low a cooking temperature can pose a serious threat of disease and too hot a temperature can destroy the food's protein ingredients to the problem of drug, pesticide and agri-chemical residues."

All of which brings us to the do-it-yourself pet diet. Be forewarned, you need large amounts of both energy and money to pull this off, and you should consult your veterinarian to make sure that the diet you've planned is properly balanced.

Feeding Fido provides, among others, balanced diets for dogs that are hyperactive, allergic or have heart or kidney ailments. Most of the book's recipes, with a little selective spicing, also are perfectly suitable for human consumption, so you might not have to spend as much time in the kitchen as you would think.

A few words of warning. Raw milk, for dogs or cats, is out; it can cause stomach upset. Yogurt or cottage cheese are better tolerated and provide more protein. You also should avoid feeding your pet "too much of any one thing," according to Fox, just as you would avoid that for yourself.