Suddenly losing weight, we have to consider from several aspects: 1) difficulty or insufficiency of nutrient intake; 2) disorder of nutrient digestion, absorption and utilization; 3) increase of nutrient consumption. From the viewpoint of difficult and insufficient nutrient intake, if the patient shows symptoms of difficulty in swallowing, it should be considered whether there is the occurrence of esophageal cancer or cardia cancer, and possibly stomatitis. If it is because of decreased appetite, consider whether there is anorexia nervosa or depression. From the perspective of impaired digestion, absorption, and utilization of nutrients, it is possible to consider gastritis if it is of gastrointestinal origin, and diabetes mellitus if it is of endocrine metabolic disease. Because the metabolism of glucose, fat and protein is disturbed in patients with diabetes, the body’s fat will be used, which will produce the phenomenon of excessive weight loss. The increase in nutrient consumption can be considered from the following aspects: 1) endocrine metabolic diseases, like hyperthyroidism and type 1 diabetes; 2) chronic wasting diseases, such as severe tuberculosis, tumors and other chronic infectious diseases.