Autophagy is the process by which a cell dismantles its own damaged components — misfolded proteins, worn-out mitochondria, fragments of debris — and reuses the parts. Yoshinori Ohsumi won the 2016 Nobel Prize for working out how it operates. It is not speculative biology.
The switch
Two sensors decide. mTOR detects abundance — particularly amino acids and insulin — and when it is active the cell builds and suppresses autophagy. AMPK detects energy scarcity and does the opposite. Autophagy runs when mTOR is down and AMPK is up.
- Fasting lowers insulin and amino acid availability, releasing the brake
- Exercise activates AMPK directly, and does so in muscle and brain
- Low protein intake reduces mTOR signalling, especially leucine
- Ketones appear to signal in the same direction independently of the fasting that produced them
The sixteen-hour number is made up
You will see a specific hour at which autophagy is said to begin. There is no such number in the literature. Autophagy is not a switch that flips; it is a continuous process that varies in rate, differs by tissue, and cannot be measured non-invasively in a living human. Most of what is quoted comes from rodent studies, and mice metabolise several times faster than we do.
What can be said honestly: longer fasts increase autophagy more than short ones, the effect is graded rather than triggered, and nobody knows your personal number because nobody has measured it.
Why it declines
Autophagic capacity falls with age, which is one of the proposed explanations for why damaged proteins accumulate in ageing tissue — most visibly in neurodegenerative disease. It is also suppressed by the state most people live in: constant feeding, chronically elevated insulin, and never a stretch without incoming nutrients.
Practically
- Regular eating windows with a genuine overnight gap — the difference between eating for sixteen hours a day and eating for eight is real, and it is achievable
- Occasional longer fasts of 24 to 72 hours, if health and circumstances allow
- Training, particularly the kind that depletes glycogen
- Not eating late. Autophagy and sleep-related repair overlap, and a full stomach interferes with both
The other side
Autophagy is a recycling process, not a virtue. Suppressing it is what allows muscle and tissue to be built, and there are people for whom more fasting is the wrong prescription entirely: anyone underweight, pregnant, with a history of disordered eating, or already frail. Cycling between building and cleaning is the point. Living permanently in either mode is not.
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