Fasting Timeline Hour by Hour: The Metabolic Switch

What really happens inside your body when you start fasting — hour by hour, from the first missed meal through a 72-hour fast. In this video, I break down the fasting timeline in detail: when your body flips the metabolic switch, what ketone bodies actually do, when autophagy is strongly activated, and when some of fasting's most powerful effects — including activated longevity-related pathways — begin. Why This Matters: When you fast, your body triggers autophagy — a cellular cleanup process that recycles damaged components. Insulin drops. AMPK activates. mTOR — a growth signal — quiets down. These are the same pathways scientists study for healthy aging, which is why fasting has become one of the most studied tools in longevity research. Your Daily Takeaway: ✅ Fasting isn't one-size-fits-all — it's not safe for everyone. If fasting interests you, the most important first step is talking to your doctor before making any changes. This video is a summary of peer-reviewed research. 📌 Our website includes a full list of references—largely peer-reviewed journal articles—so you can explore the science behind this video in more depth. For this specific video, check out the references here: https://thebiologyofwellness.com/25-fasting-timeline-hour-by-hour-the-metabolic-switch/ Video Chapters: 00:00 - How to Flip the Metabolic Switch? 00:50 - Fasting Timeline 01:04 - The Fed State 01:29 - Burning Glycogen Reserves 01:53 - The Metabolic Switch 02:49 - Ketogenesis 03:28 - Deeper Biological Changes 05:03 - The Beneficial Biological Changes Occurring from Fasting 07:39 - Important Considerations for Fasting Dr. Peterson holds a Ph.D. in biomedical sciences and trained at leading universities in the United States, the Netherlands, and Germany. This video is for informational purposes only. It is not intended as medical advice, nor should it be used as a substitute for professional medical diagnosis, treatment, or advice. Always seek the guidance of a qualified healthcare provider with any questions you may have regarding your health. References: Anton, S. D., Moehl, K., Donahoo, W. T., Marosi, K., Lee, S. A., Mainous, A. G., 3rd, Leeuwenburgh, C., & Mattson, M. P. (2018). Flipping the Metabolic Switch: Understanding and Applying the Health Benefits of Fasting. Obesity (Silver Spring, Md.), 26(2), 254–268. https://doi.org/10.1002/oby.22065 Burkewitz, K., Zhang, Y., & Mair, W. B. (2014). AMPK at the nexus of energetics and aging. Cell metabolism, 20(1), 10–25. https://doi.org/10.1016/j.cmet.2014.03.002 Ciastek, B., Kapłon, K., & Domaszewski, P. (2025). A Comprehensive Perspective on the Biological Effects of Intermittent Fasting and Periodic Short-Term Fasting: A Promising Strategy for Optimizing Metabolic Health. Nutrients, 17(13), 2061. https://doi.org/10.3390/nu17132061 Curry, D. W., Stutz, B., Andrews, Z. B., & Elsworth, J. D. (2018). Targeting AMPK Signaling as a Neuroprotective Strategy in Parkinson's Disease. Journal of Parkinson's disease, 8(2), 161–181. https://doi.org/10.3233/JPD-171296 de Cabo, R., & Mattson, M. P. (2019). Effects of Intermittent Fasting on Health, Aging, and Disease. The New England journal of medicine, 381(26), 2541–2551. https://doi.org/10.1056/NEJMra1905136 de Ciutiis, I., Djakovic, S., Cagigas, M. L., Masedunskas, A., Smith, L., Franceschi, C., & Fontana, L. (2025). Long-term fasting and its influence on inflammatory biomarkers: A comprehensive scoping review. Ageing research reviews, 110, 102797. https://doi.org/10.1016/j.arr.2025.102797 Fazeli, P. K., & Steinhauser, M. L. (2025). A Critical Assessment of Fasting to Promote Metabolic Health and Longevity. Endocrine reviews, 46(6), 856–876. https://doi.org/10.1210/endrev/bnaf021 Godos, J., Currenti, W., Ferri, R., Lanza, G., Caraci, F., Frias-Toral, E., Guglielmetti, M., Ferraris, C., Lipari, V., Carvajal Altamiranda, S., Galvano, F., Castellano, S., & Grosso, G. (2025). Chronotype and Cancer: Emerging Relation Between Chrononutrition and Oncology from Human Studies. Nutrients, 17(3), 529. https://doi.org/10.3390/nu17030529 Han, H. S., Kang, G., Kim, J. S., Choi, B. H., & Koo, S. H. (2016). Regulation of glucose metabolism from a liver-centric perspective. Experimental & molecular medicine, 48(3), e218. https://doi.org/10.1038/emm.2015.122 Henderson, C. G., Turner, D. L., & Swoap, S. J. (2021). Health Effects of Alternate Day Fasting Versus Pair-Fed Caloric Restriction in Diet-Induced Obese C57Bl/6J Male Mice. Frontiers in physiology, 12, 641532. https://doi.org/10.3389/fphys.2021.641532 Hofer, S. J., Carmona-Gutierrez, D., Mueller, M. I., & Madeo, F. (2022). The ups and downs of caloric restriction and fasting: from molecular effects to clinical application. EMBO molecular medicine, 14(1), e14418. https://doi.org/10.15252/emmm.202114418 & more.