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Fasting Index

24 hours · intermediate

24-hour fast

Dinner to dinner, once or twice a week. Long enough to draw liver glycogen down substantially and shift toward fat oxidation. Over a year, the closest well-studied relative of this schedule performed no better than simply eating less every day.

Who should not do a 24 hours fast

  • Anyone taking insulin or a sulfonylurea, without supervision from the prescribing clinician.
  • Anyone who is pregnant or breastfeeding.
  • Anyone with a current or past eating disorder.
  • Children and adolescents.
  • Anyone underweight, or with a condition requiring regular food intake.
  • Anyone with a history of fainting, low blood pressure, or a kidney condition affecting salt handling, without medical advice.

This is not medical advice and no clinician has reviewed it. If any of the above applies to you, speak to a doctor before fasting rather than after.

A schematic timeline of a fast from 0 to 72 hours. A tone band along the top darkens as the body moves from fed to deeply fasted. Beneath it, liver glycogen is drawn as a stipple field that is largely spent within the first 24 hours. Below that, three traces on independent scales: glucose falls then levels, insulin falls steeply and stays low, ketones rise from about 12 hours and climb through the second and third day. The pattern is well described but individuals vary widely, and the horizontal scale breaks at 24 hours.
The hours of a fast. Above, the phases as a tone that darkens as the body moves from fed to deeply fasted. In the middle, liver glycogen as stipple, largely spent inside the first day. Below, three traces on independent scales: glucose, insulin, ketones. A schematic of a well-described pattern. Individuals vary widely, and the scale breaks at 24 hours.

What is actually happening

Across 24 hours the liver moves from releasing stored glycogen to making glucose from lactate, glycerol and amino acids. Liver glycogen is largely spent within roughly the first day, though the exact point varies with the last meal and with activity, and ketone production rises as fat oxidation increases. This is the shortest protocol that reliably crosses out of the fed state into something a reader would recognise as fasting physiology. It is also the point at which sodium loss begins to be worth thinking about: falling insulin promotes sodium excretion by the kidney, which is the mechanism behind the headache and lightheadedness that many people attribute to low blood sugar.

What the evidence shows

  • In a one-year randomised trial of 100 metabolically healthy adults with obesity, alternate-day fasting, the closest well-studied relative of a repeated 24-hour fast, produced no greater weight loss or cardiovascular improvement than daily calorie restriction, and had the highest dropout rate of the three study arms.
  • In the 2025 BMJ network meta-analysis, alternate-day fasting was the only fasting schedule to beat continuous calorie restriction on weight, by 1.29 kg, at moderate certainty of evidence.
  • That same network meta-analysis found no diet strategy outperformed any other among the 17 trials lasting 24 weeks or longer, so the alternate-day advantage is a short-trial finding rather than a durable one.
  • Sodium excretion rises during fasting as insulin falls, a mechanism described in controlled human fasting studies from the 1970s and 1980s, which is why symptoms at the 24-hour mark are often electrolyte-related rather than glucose-related.

What the evidence does not support

  • The evidence does not support a repeated 24-hour fast as superior to eating fewer calories every day. The one-year trial that tested the closest equivalent found no advantage, and higher dropout.
  • Adherence is a real finding, not a footnote. The fasting arm of that trial was the one people left most.
  • Claims about autophagy at 24 hours are not established in humans. The human measurements that exist were taken at 36 hours, in muscle, and are markers rather than autophagy itself.
  • The alternate-day results should not be read as applying to a once-weekly 24-hour fast. Nobody has run the long trial on that schedule specifically.

How it is run

  • One meal to the same meal the following day. Dinner to dinner is the usual form, because it puts the hardest hours in sleep.
  • Water, black coffee and plain tea throughout. Salt in water is reasonable if lightheadedness appears.
  • Run once or twice a week rather than daily. Daily 24-hour fasting is OMAD, which is a different protocol with different problems.
  • Break it with an ordinary meal. A 24-hour fast does not require a refeeding protocol, and treating it as though it does tends to produce a larger meal than intended.

What breaks a 24 hours fast

The rules differ by protocol. Check any food, drink or supplement against this protocol below.

See the full index

Sources

No clinician reviews this site. Sources on this page were verified against PubMed by the author, who is not a doctor or a dietitian. What that means.

Last updated 2026-08-16.

The phases of a fast covers the physiology behind every protocol on this site, hour by hour.