If your pet is eating its usual amount of food but is losing weight, that's a signal that something may be wrong. Since it can be difficult to judge an animal's weight visually, it's good to weight your pet once a month and keep records. A loss of one pound in a cat or small dog or two to three pounds in larger dog is reason to consult your veterinarian.

If your pet is eating normally but gaining weight, that's a reason to call the doctor too. A sudden increase in weight may indicate fluid accumulation and may mean that your pet has underlying kidney, heart or liver disease. Or if your pet is gaining weight and seems especially lazy, it may have hypothyroidism, a common canine disease. Try to feed your pet only high-quality pet foods instead of high-calorie junk food.

Appetite loss, or anorexia, occurs in a wide variety of diseases. If your pet doesn't eat for more than a day, call your vet immediately.

Overeating can occur in diseases that prevent the absorption and utilization of nutrients. Diabetis and gastrointestinal problems are examples. An increase in appetite also occurs when an animal needs more food, such as in pregnancy or when intestinal parasites compete for food it has eaten. If your pet overeats for more than a day, consult your veterinarian.