Part 4/15:
The physiological journey of fasting is remarkable. Once glycogen stores in muscles and liver are depleted (typically within 24 hours), the body switches from using glucose to burning fat, producing ketone bodies—particularly beta-hydroxybutyrate—that serve as efficient fuel for the brain. This metabolic adaptation is a survival mechanism that allows humans to endure prolonged periods without food, theoretically up to 70 days in some cases.