ZOE Science & Nutrition Recap: Is it time for you to try fasting? | Dr Valter Longo & Tim Spector
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Dec 9, 2025 
Dr. Valter Longo, a pioneer in fasting research, and Tim Spector, a genetic epidemiologist, delve into the transformative powers of fasting. They explain how even short fasts can trigger beneficial cellular processes like autophagy and manage metabolism. Longo highlights his fasting-mimicking diet, which replicates fasting benefits without total deprivation, especially for type 2 diabetes. Spector shares insights from a massive study on time-restricted eating, emphasizing the importance of sustainable fasting over extreme measures for lasting health advantages.
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Rapid Systemwide Metabolic Switch
- Fasting triggers large-scale gene expression and metabolic shifts within days.
- After ~3–5 days the body switches to burning fat, producing ketone bodies, and making glucose internally.
Autophagy Takes Days, Not Hours
- Autophagy is a cellular cleanup process that usually requires several days to appear in human blood markers.
- Longo's group found markers of autophagy become measurable around day five, not after brief fasts.
Stick To A 12-Hour Eating Window
- Prefer 12-hour time-restricted eating over extreme daily windows like 16–18 hours.
- Longo warns longer daily fasts may cause side effects and links skipping breakfast to higher mortality in some studies.