The book begins by explaining the difference between the fed and fasted state, insulin's role in storage and fat-burning, and how autophagy works as the body's cellular "spring cleaning." Fasting is compared to running a hybrid car - switching between glucose and fat as energy sources.
Importantly, it highlights that women and men respond differently to fasting. Women's bodies are more sensitive to energy signals, so fasting must be gentler and adapted to hormonal rhythms.