The main reason you cannot eat or drink before surgery is to prevent aspiration, which is when stomach contents travel up the esophagus and enter the lungs. During anesthesia, normal reflexes like coughing to protect the airway are suppressed, so if there is food or liquid in the stomach, it could be inhaled into the lungs causing serious complications such as choking, pneumonia, or acute respiratory distress syndrome (ARDS). This risk of aspiration can be life-threatening, and fasting helps reduce it by keeping the stomach empty at the time of surgery.
Additionally, having an empty stomach reduces complications such as nausea, vomiting during surgery, and makes the procedure easier for the surgical team, especially in abdominal or endoscopic surgeries where food in the stomach could obstruct the view or working space.
In summary, not eating or drinking before surgery is a safety measure to protect the lungs and airways during anesthesia and to ensure the surgery proceeds smoothly without added risks from stomach contents.