A weight loss without a change in diet can be a sign of illness. Diabetes, renal disease, and hyperthyroidism are just three possibilities. Your cat should be examined to rule out diseases before just assuming a diet change will solve the problem. If your cat is healthy, then consider diet changes to help gain weight. Canned food may increase calorie intake, but add slowly to prevent digestive upset. Calorie supplements such as Stat or Nutrical can give additional nutritional support.