Because the calorie counts of diet petfoods vary widely among brands, pet owners may be unknowingly overfeeding their pets, according to a study conducted at the  Cummings School of Veterinary Medicine at Tufts University .

The  study  measured the calories in 93 commercial diet petfoods (44 canine diets and 49 feline diets) and found that a pet owner serving one cup or can of diet petfood could potentially be adding more than 200 calories a day to their pet's diet by switching brands.