A fast of sixteen hours and a fast of seventy-two are not the same intervention scaled up. Somewhere past the second day the body stops treating the gap as a long night and starts running a different programme.
The switch
Liver glycogen is largely gone within twenty-four hours. After that, glucose must come from gluconeogenesis, and the body begins protecting protein by shifting more of its fuel demand onto ketones. Blood ketones that sat around 0.5 mmol/L on day one commonly reach two to three by day three.
Growth hormone rises sharply — several-fold over baseline in the classic studies — which is part of how muscle is spared. Autophagy, the clearing of damaged cellular components, increases; how much and how fast in humans is less precisely known than the popular accounts suggest, since most of the specific timing figures come from animal work.
What genuinely goes wrong
The risks here are real and they are almost all electrolytes.
- Sodium loss accelerates as insulin falls; without replacement it produces headache, weakness and light-headedness
- Potassium and magnesium follow, and low magnesium is where cramping and heart rhythm complaints come from
- Refeeding syndrome after longer fasts, where reintroducing carbohydrate drives phosphate into cells with dangerous speed — the reason a long fast is broken gently
- Medication that assumes you are eating: insulin, sulfonylureas and blood pressure drugs all become too strong
How to run one without incident
- Salt deliberately: several grams of sodium a day, from unrefined salt
- Magnesium 300–400 mg daily, potassium from a supplement designed for it if going beyond three days
- Water to thirst, not to a target — overdrinking while low on sodium is its own hazard
- Stop if you experience heart palpitations, confusion, or fainting on standing that salt does not fix
- Break it with protein and fat, in a small meal, and leave carbohydrate until the following day
Anything beyond about five days without food belongs under supervision, not under a web page. The failure modes at that length are not the kind you notice in time.
Who it suits
Extended fasting is a periodic intervention, not a lifestyle. It suits people who are already fat-adapted, who have run shorter fasts without difficulty, and who have a specific reason — a stall, a reset, a period of inflammation they want to interrupt. Starting here rather than working up to it is how people end up concluding that fasting does not agree with them.
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