Efforts are under way to require manufacturers to include calories on all pet food packaging in coming years, but Wakshlag said listing the calorie content won't help pet owners much.
"There's really no way, based on how the label is structured now, to accurately give the calories," he said. Unless a company has done a pricey and lengthy digestibility study of their food, "they really don't know how many calories are in a can," he said.
Pets don't need many calories to maintain a healthy weight. Ward said an adult 10-pound, indoor, spayed cat only needs 180 to 200 calories per day while an older 20-pound, neutered dog needs 340 to 380.
To figure out your pet's daily caloric needs, Ward suggests this formula as a starting point: Divide your pet's weight by 2.2. Multiply this figure by 30, then add 70, and you'll have an idea of how many calories to feed a typical, inactive spayed or neutered pet.
"Of course, each pet's metabolism is different, so be sure to consult your veterinarian before starting a diet," he said.
There's more on helping your pet stay healthy at the U.S. National Library of Medicine.
SOURCES: Joe Wakshlag, D.V.M., assistant professor, clinical nutrition, College of Veterinary Medicine, Cornell University, Ithaca, N.Y.; Lisa Freeman, D.V.M., professor, nutrition, Tufts University Cummings School of Veterinary Medicine, North Grafton, Mass.; Ernie Ward, D.V.M., veterinarian, Calabash, N.C., and author, Chow Hounds: Why Our Dogs Are Getting Fatter -- A Vet's Plan to Save Their Lives
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