Flipping the Metabolic Switch: Understanding and Applying the Health Benefits of Fasting

Summary

This review formalized the term "metabolic switch" — the transition from carbohydrate-derived energy to fatty-acid- and ketone-derived energy that occurs after liver glycogen stores are depleted, typically beyond about twelve hours of fasting depending on prior carbohydrate intake and activity. The authors synthesize the mechanistic literature on intermittent fasting protocols (alternate-day fasting, time-restricted feeding, periodic multi-day fasts) and argue that repeated engagement of this metabolic switch is what produces the adaptations associated with intermittent fasting: improvements in insulin sensitivity, lipid profile, blood pressure, inflammatory markers, and stress resistance. The review is positioned as a translational document for clinicians beginning to recommend intermittent fasting and emphasizes that the frequency of switching, not just the duration of any single fast, is plausibly the parameter that drives adaptation.