Many veterinarians recommend commercial weight loss diets, and some of these work so well that they are available only by prescription. Dogs and cats do lose weight with these diets if fed the proper amount, and if the animal's metabolism can handle the increased grain protein and fiber provided by the diet. One common problem that we see is a gradual degeneration in coat quality. A much more dangerous problem involves the animal's pancreas, months to years on low fat diets seem to make these animals more sensitive to extra fat in their diets. If they eat supplements of meat containing fat or even a maintenance pet food after becoming accustomed to low fat diets, they may develop a potentially serious disease called pancreatitis.
Pancreatitis is sometimes fatal in animals and humans, and can lead to diabetes.
There are quite a few pets, however, who don't seem to lose weight well on these diets. Some pets may simply need more meat, and less grain, in their diets. Perhaps the secret is that canned foods force the owner to turn to meal feeding, instead of allowing free choice feeding. In any case, try to find a canned diet that is high quality and free of preservatives. If you have a pet that is already on a weight loss diet, do not switch back to maintenance diets, simply use the canned form of a weight loss diet.