Through inquiry, some doctors know that a low body mass index means that the human body is thinner, and the body mass index is the body mass index, which is the square number calculated by dividing the weight in kilograms by the height in meters. The normal value is 18.5-23.9. If the body mass index is low, the weight of the human body is relatively low relative to the height. Low body mass index often leads to malnutrition, which may include throat diseases, gastrointestinal diseases, hepatobiliary and pancreatic system diseases, all of which will affect energy intake and utilization, resulting in low weight. There are also chronic wasting diseases, such as hepatitis, tuberculosis, tumors and other diseases, or digestive system diseases, which will also lead to increased human consumption and weight loss. In addition, there are endocrine diseases such as hyperthyroidism. The human body is in a state of high metabolism, with symptoms such as overeating, emaciation and sweating. In addition, anorexia nervosa can also lead to a decrease in body mass index. In short, insufficient intake or increased consumption will lead to low body mass index, so it is necessary to find the reasons and targeted treatment.