There's nothing that's really documented. In 1998, the Animal Protection Institute conducted a survey of 700 veterinarian and veterinary student members of the Association of Veterinarians for Animal Rights. The 150 respondents felt that obesity, inflammatory bowel disease, and developmental bone disease in dogs, and obesity, inflammatory bowel disease, and lower urinary tract disease in cats were the health problems most influenced by nutrition. Twenty percent frequently recommend home-cooked foods for healthy dogs and cats, while about a third recommended them for animals with specific health issues. The more educated consumers become about the pet food industry, the more confused they probably get.
Experts can, however, offer some guidance.
Avoid pet foods that have by-product meal or meat and bone meal as one of the first ingredients listed. Preferably, your pet food of choice won't use them at all. You'll also want to steer clear of foods that don't list any meat products near the top of the ingredient lists. That means they're relying more on filler, like vegetable products, and are less nutritious. She also recommends gradually switching foods at least every three to four months, even if it's just within a particular brand name.