Shopping for pet food can be confusing. With so many options—wet, dry, organic, grain-free—how can you be sure what's best for your cat or dog? There's a lot of persuasive marketing of pet food, and that means pet owners can lose sight of what's most important when it comes to feeding their animal companions. "Having the right nutrient profile is most important," says Dr. Adronie Verbrugghe of the Ontario Veterinary College at the University of Guelph. That profile, according to the National Research Council of the National Academy of Sciences, a U.S. research organization, includes 38 essential nutrients required for dogs and 40 for cats.

Each pet's dietary needs depends on many factors—including age, breed and
medical conditions—and your vet can best advise on proper feeding. But assuming you have a healthy adult cat or dog, here are the five must-have pet-food ingredients.

Protein

Why it's important: Protein (and the essential amino acids it brings) is necessary to build, maintain and repair cells, tissues and organs.

What cats need: Look for nutrient-dense whole meat ingredients (meat "byproducts" are lower quality). Organ meat—heart, liver or kidney—is especially beneficial because it's a natural source of many essential vitamins and minerals, including the must-have amino acid taurine (also added to commercial pet food to meet required levels).

What dogs need: Roughly 15 to 30 percent of your dog's diet should be protein, ideally from whole meat, such as chicken, lamb or turkey, instead of byproducts. Pure "meat meal" is also fine; it's a concentrated protein consisting of fat-free, nutrient-dense powdered meat.

Fat

Why it's important: Fat helps your pet absorb fat-soluble vitamins A, D, E and K and delivers energy along with omega-3 and omega-6 fatty acids, which keep your pet's skin and coat healthy.

What cats need: Essential fatty acids, including linoleic and arachidonic acids, are vital to your cat's overall health and best absorbed from meat-based sources, such as chicken fat.
 
What dogs need: Fat is your dog's top source of energy, but it shouldn't make up more than about 10 to 15 percent of its diet since too much can lead to obesity. In dog food, fats typically come from pork, poultry or vegetable oils.

Water

Why it's important: Water is vital for both species and usually accounts for 60 to 70 percent of your adult pet's weight. It regulates body temperature, transports oxygen and nutrients throughout the body, aids digestion and flushes the urinary tract.

What cats need: Cats generally have low thirst drives and can get what they need through wet food, which is typically between 68 to 78 percent water (compared with an average of 10 percent moisture in kibble). Always have a bowl of fresh water available, as well.

What dogs need: Dog kibble is about five to 10 percent water, while wet food has much more. Dogs will readily drink water, so getting it through food isn't as big a concern. A healthy adult dog needs to consume roughly 30 millilitres of water for every 4 1/2 kilograms (that's about an ounce of water for every 10 pounds) in body weight daily, so keep a bowl of fresh water on the go for your pooch.

Vitamins and Minerals

Why they're important: Vitamins and minerals assist with chemical reactions in the body, provide nutrients and help build strong muscles and bones. Pet food that's labelled "complete and balanced" meets the Association of American Feed Control Officials nutritional requirements, so unless your vet says so, there's no need for further supplements. That said, if you're feeding your pet a raw-food or homemade diet, check with your vet or a pet nutritionist to determine if it contains all the essential vitamins and minerals, and supplement accordingly, if needed.

What cats need: Fat-soluble vitamins A, D, E and K, as well as B vitamins niacin and thiamine. Minerals, especially calcium and phosphorus, need to be consumed in specific proportions, which "balanced/complete" food provides correctly.  

What dogs need: Vitamins A, D, E and K, along with water-soluble vitamin C
and B-complex vitamins. Again, supplements aren't necessary; in fact, too much of certain vitamins or minerals can cause damage. For example, excess vitamin A can damage a dog's blood vessels.

Regardless of which food you buy, its overall nutritional makeup is what's key. Look for the words "complete" or "balanced" on the label and talk to your vet if you have questions. "With good nutrition, you can prevent a lot of diseases later in life," says Dr. Verbrugghe. And that means a longer, healthier life for your furry friend.

We've got all the answers to your pet nutrition questions here.