Diet

• A change in diet may cause weight loss in cats. This may be due to lower calorie food or to food allergy or intolerance. If the cat is allergic or intolerant to ingredients in the new food, she will often develop other symptoms, such as itchiness and skin rash, in addition to losing weight.
  
  Feeding a cat a new food she doesn't like can also lead to unintended weight loss. Cats can be very picky about food sometimes and may refuse to eat.
  
  

Parasites

• Some parasites, such as roundworm, can cause weight loss. Fatigue, fever, and digestive upset are other symptoms that may appear if this is the cause.
  
  

Disease and Illness

• Cancer, diabetes, kidney disease and liver disease are among diseases that may cause a cat to lose weight. Often, weight loss is the first symptom the owner notices.
  Any illness, even minor ones like a cold, can make a cat lose its appetite for a while, resulting in weight loss. Monitor an ill cat closely and feed him a special high-calorie diet (available from veterinarians), if this is the case.
  
  

Complications

• Even if the cause of weight loss is a temporary, minor illness, sudden weight loss can be harmful to a cat. According to marvistavet.com, sudden weight loss in obese cats may cause a condition known as hepatic lipidosis (fatty liver), which can lead to liver failure if not treated. If your cat loses weight you should take her to the veterinarian to make sure she has not developed fatty liver and to treat the cause of the weight loss.
  
  

Intentional Weight Loss

• In some circumstances, a veterinarian may recommend a weight loss plan for obese cats.
  
  Because cats can develop fatty liver if they lose weight too quickly, diets should be handled carefully. Cats should lose weight gradually until he is at his ideal weight. You should take your cat for a physical before starting him on any weight loss diet plan.