The Complete Guide to Fasting: Heal Your Body Through Intermittent, Alternate-Day, and Extended Fasting

by Jason Fung

The transition from the fed state to the fasted state occurs in several stages, as classically described by George Cahill, one of the leading experts in fasting physiology: 1. Feeding: Blood sugar levels rise as we absorb the incoming food, and insulin levels rise in response to move glucose into cells, which use it for energy. Excess glucose is stored as glycogen in the liver or converted to fat. 2. The postabsorptive phase (six to twenty-four hours after beginning fasting): At this point, blood sugar and insulin levels begin to fall. To supply energy, the liver starts to break down glycogen, releasing glucose. Glycogen stores last for approximately twenty-four to thirty-six hours. 3. Gluconeogenesis (twenty-four hours to two days after beginning fasting): At this point, glycogen stores have run out.