It's important to understand that food has the ability to heal or harm your pet, depending on the type and quality of nutrition you provide.
The first factor you should evaluate is the species-appropriateness of what your dog or cat is eating.
A species-appropriate diet contains lots of good quality protein as well as moisture. The protein is necessary because both dogs and cats are carnivores.
High moisture content is required in order to prevent organ dysfunction, including kidney failure. Dogs and cats are designed to eat food that is about 70 percent moisture, which is what a diet of mice and rabbits would provide if your pet hunted his own food.
If you feed your pet dry food only, he's getting only about 12 percent moisture instead of the 70 percent his body demands. This is especially unhealthy for cats, because they don't supplement their moisture intake by drinking large amounts of water like dogs do.
Pets on dry food diets (kibble or pelleted) live in a state of chronic, mild dehydration that over time can cause significant stress to their organs.
Species-appropriate nutrition does not contain much starch, also known as grains or carbohydrates. Corn, wheat, rice and soy are found in most commercial processed pet foods, but your dog or cat has no biological need for them.
I recommend you follow the laws of nature when it comes to your pet's diet, which is to feed everything his body needs and eliminate ingredients that provide no nourishment.
In addition to the species-appropriateness of your pet's diet, it also needs to be balanced. By balanced I mean food that contains all the vitamins, minerals and other nutrients your dog or cat needs.
This isn't something you can guess at – it should be guaranteed through testing.
Nutritional balance is vitally important because deficiencies will develop much faster in your dog or cat than they will in you. A poorly nourished puppy or kitten can end up with obvious signs of skeletal problems and organ degeneration before she's six months old.
An older animal can develop life-threatening organ degeneration, among many other not-so-obvious symptoms, over a one to three year period of eating an unbalanced, nutrient-deficient diet.