There are three major factors responsible for weight loss in cats. Inadequate food consumption may be due to dietary deficiency of protein, fat or amount of food. Also, the cat may experienced an oropharyngeal disease, regurgitation, vomiting, nausea, anorexia, or psychologic or neurologic symptoms (For example, the stressed cat may starve itself to death). Maldigestion and malabsorption can be be due to deficiency of some enzymes or bile salts, villous atrophy, histoplasmosis, giardiasis, problems with pancrea or liver, chemotherapy. Excessive caloric loss may occur under some physiologic conditions (cold weather, heavy exercise, pregnancy, lactation) and in diseases that increase metabolic rate (fever, infection, neoplasia, congestive heart failure, trauma) or cause nutrient loss from the body (diabetes mellitus, kidney or intestinal disease, vomiting, diarrhea, burns, tumor).
So, when the cat is loosing weight, the first step should be a visit to a veterinarian, to make all tests and to understand what a cause of weight loss.
The prognosis for the cat with weight loss depends on how quickly the underlying cause of the weight loss is identified and managed and how rapidly a positive caloric balance is re-established in the body. Weight loss of 30-50% of body weight is usually fatal for a cat.