When blood-sugar (glucose) levels rise, the pancreas secretes insulin in response, which then signals the muscle cells to take up and burn more glucose. Insulin also signals the fat cells to take up fat and hold on to it. Only when the rising tide of blood sugar begins to ebb will insulin levels ebb as well, at which point the fat cells will release their stored fuel into the circulation (in the form of fatty acids); the cells of muscles and organs now burn this fat rather than glucose.
