”Surgery can treat type 2 diabetes caused by obesity,” and many low-weight patients are excited to hear about it and come to the clinic. I remember an 18-year-old type 2 diabetic patient who came to me for surgery. Her diabetes was so severe that she needed insulin injections to control her blood sugar, and it was a double blow to her body and mind to need insulin control every day at such a young age. She wanted to control her blood sugar but was afraid that her classmates would laugh at her, so she had great hopes for the surgery. However, her weight was low with a BMI of about 23, which did not meet the criteria for surgery. I refused her request for surgery after a comprehensive consideration. It is a bad feeling to dash a patient’s hope, and her lost look could not be waved away for a long time. Since this is a surgery to treat type 2 diabetes, why is it not suitable for patients with low body weight? There are two types of glucose-lowering mechanisms of gastric bypass surgery: 1. Stimulation of insulin secretion: when the patient’s pancreatic beta cells are ≥ 1/2 of the lowest normal level, gastric bypass surgery will stimulate insulin secretion after reconstruction of the digestive tract; 2. Relief of insulin resistance: the higher the body weight the more severe the insulin resistance will be and the less efficient the insulin will be. Although gastric bypass surgery will affect the patient’s absorption, it is not dangerous for people with good natural absorption ability, and the weight loss will not cause underweight and malnutrition. In the case of two-way gain, even if insulin secretion remains insignificant after stimulation, insulin efficiency increases after weight loss, and there is naturally a significant glucose-lowering effect. Surgery for low weight patients can only stimulate insulin secretion, and whether the effect is significant varies from person to person. Korea pioneered gastric bypass surgery for low body weight, and the benefit of the surgery is only 50-50. They take more risks because the impact on absorption can easily cause malnutrition, lower than normal weight, and even symptoms such as anemia. Although gastric bypass is a reversible surgery, I still strictly grasp the indications for the surgery so that the surgical results can be guaranteed. I will recommend surgery only when the patient’s benefit outweighs the risk while ensuring the safety of the surgery. We cannot gamble with the health of the patient.