The digestive system breaks down the carbohydrates in food ( like cereal products, rice, corn, fruits and potatoes) to glucose, which is then absorbed into the blood through the intestinal wall and spread throughout the entire body.
The pancreas produces the hormone insulin in the β cells of the Langerhans. As a messenger, insulin hormone helps the transport of glucose into the body cells, where it is utilized for energy purpose. In addition, insulin also helps in the storage of glucose in the form of glycogen in the liver as well as in the muscle cells. Because of its actions on cells, liver and muscles, insulin maintain the blood glucose level within narrow limits (about 80-120 mg/dl or 4.5-6.7 mmol/l ).
Even during fasting, the blood sugar level remains within normal levels, because liver converts the stored glycogen back to glucose and releases it into blood.