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Extended10 studies in people cited

24-Hour Fast

1 day without food · One day at a time, up to three days a week in the trials

Written by Reviewed Sep 2026

Also called: One day fast, Dinner to dinner, Full day fast

One full day with no calories. It is the shortest fast with a study where the same people tried both fasting and eating. That study found growth hormone went up, blood counts went up because the blood got less watery (not because of new red cells), and blood fats dropped and were still down a day later.

Overview

One day is the shortest fast on this site that puts you in a different state, not just a longer overnight fast. Here is what exists for it. One study where 30 healthy adults tried a fast day and a normal day. One study of 20 people in a sealed room that measures energy burned. Two studies that took muscle samples at exactly this hour. And hour-by-hour tracer studies on either side of it.

The useful thing about 24 hours is how much has not happened yet. Ketones (beta-hydroxybutyrate), which the body makes when burning fat, barely moved through the first 18 hours. At 24 hours they are only partway up their steepest climb. The liver has been making most of the body's sugar from scratch since the first day; it does not switch on at a set hour. On the other hand, 70% of the insulin drop seen across a 72-hour fast has already happened. And the biggest single jump in fat release across three days comes between hours 18 and 24.

One thing to know before you start: mild ketosis after a day without food is normal, not an achievement. In 1,535 people without diabetes who fasted only to get ready for surgery, 59.4% had ketones in their urine afterwards. Of the 929 who had blood gas tests during surgery, 29.0% had too much acid in the blood.

How it works

Dinner to dinner, or waking to waking, drinking only things with no calories. The study where people tried both used water only for 24 hours, against a day of normal eating. The 8-week muscle sample study had 50 women fast for 24 hours on three days a week, not back to back. The shortest group in a chemotherapy study fasted for 24 hours around treatment.

The medicine review is the part of the setup that really matters, and there is a drug-by-drug guide written for exactly this length of fast. It tells doctors to stop a sulfonylurea on a 24-hour fast day, stop mealtime insulin when no carb meal follows, and stop an SGLT2 inhibitor on a fast day, especially if dehydration is a risk. It also lowers long-acting insulin on fast days, by an amount that depends on how well the diabetes is controlled. That amount is for your prescriber to set, not something to work out from this page.

The same paper has two lines worth reading twice. In one study, people cut long-acting insulin by 50% on fasting days and still often had low blood sugar. In another, people cut mealtime insulin by 70% and still often had low blood sugar.

At this length, fasting communities use salt if a headache shows up, and nothing else. Most say potassium and magnesium are not needed. That is common practice, not evidence. No study has tested a salt and mineral plan at any fasting length.

What 1 day actually reaches

In the study where people tried both a fast day and a normal day, growth hormone was clearly higher after fasting. Haemoglobin, red cell count and haematocrit all rose, and so did total cholesterol and HDL cholesterol. Blood fats (triglycerides), bicarbonate and weight fell. In the 16 people who fasted first, most markers, including growth hormone and cholesterol, were back to normal by 48 hours. Weight and blood fats were not. When all three blood count measures rise together, it means the watery part of the blood shrank. It does not mean the body made new red cells. The paper does not print how much growth hormone rose in the part we could read, so no percentage is given here. The insulin and fat picture comes from 6 healthy men who gave blood samples over and over. By 24 hours, 70% of the insulin drop seen across a 72-hour fast had already happened. Fat release was climbing toward about double its 12-hour level. 60% of that rise came between 12 and 24 hours, and the biggest single jump came between 18 and 24. Autophagy (the cell's clean-up process) at 24 hours has been measured twice in human muscle. This is the fast length where the popular claim is most directly contradicted by studies in people. In thigh muscle from 50 women, a 24-hour fast raised a marker called p62/SQSTM1. That points toward less clean-up, not more. The only study in people with several time points during a fast found three clean-up markers (LC3I, LC3II and p62) all fell at 24 and 36 hours, and only in people who did not exercise. Neither study measured how fast the clean-up was actually running. Snapshot markers count the cell's clean-up bags; they do not measure how fast material moves through them. So this page does not say a 24-hour fast reaches autophagy. No plan on this site claims mitophagy (clean-up of the cell's power plants), because no study in people has measured it on any fast.

Fig. 1 · what has been measured, hour by hour
  1. 0h to 4h

    Still digesting

    Burning
    Sugar from the meal you just ate. Insulin is high enough to keep fat locked away.
    • Insulin is at its after-meal peak. While it is up there, the enzyme that releases fat from fat cells stays switched off, so fat is being stored, not released.
    • The hour count on most fasting charts starts at the last bite. But the body is still digesting a mixed meal here, so the clock and what the body is doing are not yet the same thing.

    Read the biologyInsulin and insulin sensitivity during a fast

    Measured in people
    The hour-by-hour fasting studies in people start later than this. Klein 1993 took its first sample at 12 hours, in 6 healthy men. Rothman 1991 gives its first figure for new sugar making as an average across the first 22 hours. This row is the fed state those measurements are compared against. No measurement from a long fast falls inside it.

  2. 4h to 12h

    Meal digested, overnight fast

    Burning
    A mix of leftover sugar and fat. The liver keeps the brain supplied with sugar.
    Blood sugar
    5.58 mmol/L (100.5 mg/dL) at the 12 hour mark, in 6 healthy men
    Ketones
    Total ketones around 0.20 mmol/L after an overnight fast. That figure is acetoacetate and BHB added together, not BHB on its own.
    Insulin
    64.6 pmol/L (9.3 microunits/mL) at 12 hours, in the same 6 men
    • At 12 hours, glycerol is released at 2.08 and palmitic acid at 1.63 micromol/kg/min. These are two markers of fat being released. They are the starting numbers everything later in the fast is compared against.
    • Making new sugar from scratch already supplies most of the body's sugar at this point. It is not a backup waiting to be called on. This is the first place the popular "switch" story breaks down.

    Read the biologyInsulin and insulin sensitivity during a fastGlycogen depletion and gluconeogenesis

    Measured in people
    Klein 1993 measured blood sugar, insulin and fat release with tracers at 12 hours, in 6 healthy men. Féry 1983 measured total ketones in people after an overnight fast and put the starting level at 0.20 mmol/L. Rothman 1991 measured liver glycogen again and again with a special MRI scan through a 68 hour fast in healthy adults. It worked out that new sugar making supplied 64% of all the body's sugar across the first 22 hours.

  3. 12h to 18h

    Ketones have not moved yet

    Burning
    Fat use rising. More and more of the body's sugar is being made fresh, not pulled from storage.
    Ketones
    A middle change of 0% between 12 and 18 hours, measured every 6 hours in 34 adults
    • BHB, the main ketone, does not move in this window. In 34 adults sampled every 6 hours through the standard 72 hour fast doctors use for testing, the middle change from 12 to 18 hours was zero.
    • Making new sugar supplies 64% of the body's sugar across the first 22 hours. A second, unrelated method put it at about 47% by 14 hours. The body does not switch from stored sugar to fat at a set hour.
    • In 11 overweight adults, activity of the autophagy gene LC3A in blood cells was 22% higher after an 18 hour fast than after a 12 hour fast. In the same people, activity of the MTOR gene also rose, by 9%.

    Read the biologyGlycogen depletion and gluconeogenesisKetogenesis and beta-hydroxybutyrateAutophagy

    Measured in people
    Service 2005 measured BHB every 6 hours in 34 adults whose 72 hour test fasts came back clear for a rare insulin-making tumour. Rothman 1991 measured the share of new sugar making with a special MRI scan. Jamshed 2019 measured gene activity in blood cells in an 11-person, 4 day study where each person tried two eating windows: 8am to 2pm, and 8am to 8pm. That is one gene's activity in blood cells. It is not the real speed of autophagy, and it is not muscle. No study in any species has tested a 16 hour time point for autophagy.

  4. 18h to 1d

    The biggest hours for fat release

    Burning
    Fat and glycerol, with ketone making starting its climb
    Ketones
    Climbing. The middle rise from 18 to 36 hours was +333%, the steepest stretch of a 72 hour fast.
    Insulin
    70% of the whole drop seen across a 72 hour fast has already happened by 24 hours
    • The single biggest jump in fat release across a whole 72 hour fast comes between 18 and 24 hours. 60% of the total rise in fat release happens between 12 and 24 hours. The authors put that down to falling insulin, not to any change in blood sugar.
    • In a study of 30 healthy adults, growth hormone was clearly higher after a 24 hour water-only fast than after a normal eating day. Each person did both. The size of that rise is not in the published summary, so it is not given here.
    • In the same study, haemoglobin, red blood cell count and haematocrit all rose together at 24 hours. That pattern means the fluid part of the blood shrank. It does not mean new red blood cells were made.
    • In 20 volunteers, energy burned over 24 hours, measured in a sealed room, fell during a 24 hour fast. How much it fell varied from person to person.

    Read the biologyInsulin and insulin sensitivity during a fastThe growth hormone responseKetogenesis and beta-hydroxybutyrateAutophagy

    Measured in people
    Horne 2013 had 30 seemingly healthy adults do both a 24 hour water-only fast and a normal eating day, in random order. It measured growth hormone, blood counts, cholesterol, triglycerides, bicarbonate and weight. Klein 1993 measured fat release with tracers. Hollstein 2020 measured energy burned over 24 hours in a sealed room in 20 volunteers. The exact percentage drop is not in the published summary, so it is not given here. On autophagy, a 24 hour fast raised p62 (also called SQSTM1) in the thigh muscle of 50 women. That points toward less recycling in muscle, not more.

  5. 1d to 36h

    Thyroid drops before metabolic rate does

    past this window
    Burning
    Fat. The remaining sugar is built from glycerol, lactate and protein building blocks.
    Ketones
    1.308 mmol/L on average after day 1, in 13 men starting a 10 day water fast, up from 0.177 at the start. People varied a lot: the spread around that average (1.053) was nearly as big as the average itself.
    • Making new sugar supplies 82% of the body's sugar between 22 and 36 hours.
    • In 8 healthy men, the thyroid hormone T3 fell from 2.30 to 1.84 nmol/L by 30 hours. TSH, the hormone that tells the thyroid to work, fell below 1 mU/L, and its usual night-time peak disappeared. Blood cortisol did not change.
    • So the thyroid slows down within the first 30 hours, well before anyone can measure a drop in resting metabolic rate.
    • Salt loss has not turned yet. In 9 women with obesity who kept eating their usual amount of salt while fasting, salt in the urine started to rise after about 48 hours and then went above what they took in.

    Read the biologyGlycogen depletion and gluconeogenesisKetogenesis and beta-hydroxybutyrateAutophagy

    Measured in people
    Rothman 1991 measured the share of new sugar making with a special MRI scan in healthy adults. Hugues 1984 fasted 8 healthy men for 30 hours and tested their thyroid hormones again and again. Dai 2022 measured blood BHB daily in 13 men fasting in a supervised facility. Sigler 1975 collected urine in 3 hour blocks from 9 women with obesity on a hospital ward. On autophagy, the only study in people with several time points in one fast took thigh muscle samples at 2, 12, 24 and 36 hours. The markers LC3I, LC3II and p62 all fell, but only in untrained people. The authors sum it up as muscle recycling being only mildly affected by 36 hours of fasting.

What studies in people found

There is one study where people tried both a fast day and a normal day, with a full set of blood tests. There is one study in a sealed room that measures energy burned. There are two muscle sample studies at exactly 24 hours, tracer studies on either side of that hour, and a large surgery study that shows how common ketosis is when people simply skip food.

  • Lim 2024, 1,535 people without diabetes who fasted only to get ready for surgery. 59.4% had ketones in their urine after surgery. Of the 929 who had blood gas tests during surgery, 29.0% had too much acid in the blood. Younger people with a lower BMI were at higher risk.
  • Chaudhary 2022, 50 women with an average age of 51. For 8 weeks they fasted 24 hours on three days a week, not back to back, eating either 70% or 100% of their energy needs. Researchers took thigh muscle samples after 12-hour and 24-hour fasts. The 24-hour fast raised the marker SQSTM1. The paper's own title gives its conclusion: clean-up markers went up in mouse liver, but not in mouse or human muscle.
  • Service 2005 measured ketones (beta-hydroxybutyrate) every 6 hours through the standard 72-hour hospital test fast in 34 adults. The typical change from 12 to 18 hours was 0%. From 18 to 36 hours it was a 333% rise. A 24-hour fast sits partway up that second stretch.
  • Horne 2013, 30 healthy people who each tried a one-day water-only fast and a day of normal eating. Growth hormone rose. Haemoglobin, red cell count, haematocrit, total cholesterol and HDL cholesterol rose. Blood fats, bicarbonate and weight fell. In the 16 who fasted first, most markers, including growth hormone and cholesterol, were back to normal at 48 hours. Weight and blood fats were not.
  • Hollstein 2020 measured energy burned over 24 hours in a sealed room while 20 people fasted for 24 hours. It went down. How much it went down varied from person to person, and tracked how much heat each person made in mild cold and changes in a hormone called FGF21. The exact percentage is not in the part we could read, so it is not given here.
  • Rothman 1991 scanned the liver's stored sugar over and over through a 68-hour fast in healthy adults. It worked out how much of the body's sugar the liver was making from scratch. That share was 64% across the first 22 hours, 82% from 22 to 36 hours, and 96% after that. Landau 1996 used a different method and got the same picture: about 47% at 14 hours and 67% at 22 hours. There is no hour when the body flips from stored sugar to fat.
  • Dethlefsen 2018 took thigh muscle samples at 2, 12, 24 and 36 hours of fasting. It is the only study in people with several time points for muscle clean-up markers. LC3I, LC3II and p62 all went down, only in people who did not exercise. The authors sum it up as muscle clean-up being only a little affected by 36 hours of fasting. We could not confirm how many people took part, because the full paper is behind a paywall.

What this does not tell you: The study where people tried both days had 30 people, lasted a single day, and looked at blood tests, not health outcomes. The exact result of the sealed-room study is not in the part we could read. Both muscle studies used snapshot markers, not a measure of how fast clean-up was running, and one did not report a size we could confirm. No study has tested repeated 24-hour water fasts against a comparison group for real health outcomes. No study of any fasting plan has tracked heart attacks, strokes or deaths.

Reading the research record

The argument that starts at this length, and never stops, is whether fasting slows your metabolism. It belongs here because the shortest whole-day measurement is at 24 hours.

The popular claim comes from Zauner 2000, which is a real study. It measured 11 healthy lean people through an 84-hour fast, using a clear hood over the head to track energy burned at rest. Resting energy burn rose 14.1% by day 3. Noradrenaline, a stress hormone, roughly doubled by day 4.

Mansell 1990 is often cited next to it, but it does not measure the same thing. It was a 48-hour study in 6 men without diabetes, using a lab test that pumps in insulin. It measured heat made during that test, not resting energy burn during fasting. And its result points the other way: the heat effect was clear when people were fed, and not clear after starving.

A stronger study disagrees with the popular version. Andriessen 2023 kept 12 healthy lean men in a sealed room for 60 hours straight. Each man did this once fasting and once eating just enough to hold his weight. Total energy burned over 24 hours was clearly lower while fasting: 10.9% lower on day 2 and 5.5% lower on day 3. Hollstein found a drop at exactly 24 hours using the same kind of room.

Both sets of numbers are true, and they measure different things. Resting energy burn can rise while whole-day energy burn falls, because you are no longer burning energy digesting food, and you move around less. The honest summary: in lean people, resting energy burn rises a little through about day 3, while total energy burned across the day falls over the same time. Resting burn does fall eventually: by 20.3% across a 21-day fast in 13 people.

What happens to muscle

Nobody has measured body make-up across a single 24-hour fast. It is not clear what a scan would even be measuring if they did.

Longer studies show that most of the early "lean" loss on a fast is not real tissue. Lean mass means the non-fat part of you, mostly muscle and water. In 13 men on a 10-day water fast, scanned at the start, on fasting day 6, and on day 5 of eating again, lean mass was down 9.2% at day 6 and back to normal by day 5 of eating again. Fat was down 17.2%.

In 16 men on a 10-day low-calorie fast, they lost 3.53 kg of scan lean mass. Of that, 44% was water outside the cells, 14% was stored sugar with its water, and 42% was working lean tissue. That came to 1.5 kg, or 25% of total weight lost. The lean part of a body scan is mostly water, and stored sugar holds about three times its own weight in water. So a scan at the end of a fast makes lean loss look bigger than it is.

Nobody has published how much protein people lose per day on days 1 to 3 of a total fast, in any open study of people. You will often see figures credited to a 1970 paper by Cahill: roughly 12 g of nitrogen a day early on, falling to 3 to 4 g later. We could not confirm them against that paper, so they are not used here. The numbers we could confirm come from longer fasts. In people with obesity at 5 to 6 weeks, protein loss was 4.66 g of nitrogen a day, and the paper says about 3 to 6 g a day after week 4. In 10 women with moderate obesity at 3 weeks, the body was losing 5.9 g of nitrogen a day overall.

Doing it

At this length, the medicine review is the whole safety story
The published guide for doctors says to stop a sulfonylurea on a 24-hour fast day, stop mealtime insulin when no carb meal follows, and stop an SGLT2 inhibitor. It also lowers long-acting insulin on fast days by an amount your prescriber sets. In the same paper, a 50% cut in long-acting insulin and a 70% cut in mealtime insulin each still led to a lot of low blood sugar.
Ketosis at 24 hours is normal
Of 1,535 people without diabetes who fasted only for surgery, 59.4% had ketones in their urine afterwards. 29.0% of those with blood gas tests during surgery had too much acid in the blood. A positive urine strip after a day without food does not mean anything unusual has happened.
Salt, and nothing established beyond it
No published plan checks salts and minerals for a fast under 24 hours. You pass more salt during a fast, because falling insulin stops holding it in. But in the hospital ward study, that shift came after about 48 hours. Fasting communities use salt if they get symptoms. No study has set a salt and mineral plan for any fast done by choice.
A blood sugar test taken now is not a normal one
Blood sugar falls about 25% between 12 and 72 hours in healthy men, and at 24 hours you are on that slope. In healthy people the drop mostly causes no symptoms, because the brain is running on ketones. That also means symptoms are a poor warning sign.
Expect the scale to move more than the tissue
In the study where people tried both days, weight clearly fell and was still down at 48 hours. At the same time, haemoglobin, red cells and haematocrit rose, a sign the watery part of the blood shrank. Early weight loss on a fast is mostly salt, water and stored sugar.
Standing up
Feeling faint or light-headed was recorded at 217 of 768 visits in the biggest review of supervised water-only fasting, all of it rated medium. That comes from longer fasts. It is the most steadily reported physical sign at any fast longer than a day.

Breaking the fast

Refeeding syndrome is not a worry after one day in a well-fed person. The UK NICE risk list is built around BMI, recent weight loss, past salt and mineral levels, and eating little or nothing. More than 5 days of little or no food counts as one of two warning signs, and more than 10 days is enough on its own. A 24-hour fast is nowhere near either. Fasting communities say the trouble on longer fasts starts with the first meal, not the fast. Their advice to each other after a day is simple: make the first meal high in protein, eat it slowly, and do not break the fast with something sugary. On protein, a big first meal is not wasted. In 36 active young men, 100 g of milk protein after a full-body weight workout gave a bigger and longer muscle-building response than 25 g. It lasted more than 12 hours. Muscle building was about 20% higher over the first 4 hours and about 40% higher from 4 to 12 hours, and very little of the protein was burned off.

Sources

  1. Study in people2013
    Randomized cross-over trial of short-term water-only fasting: metabolic and cardiovascular consequences

    Nutrition, Metabolism and Cardiovascular Diseases

    30 healthy volunteers each did a 24-hour water-only fast and a normal eating day. Fasting raised growth hormone, red blood cell counts and cholesterol, including HDL, and lowered blood fats, bicarbonate and weight. In the 16 who fasted first, most of these, including growth hormone and cholesterol, were back to normal by 48 hours, but weight and blood fats were not. The size of the growth hormone rise was not in the record we could check.

    30 apparently healthy volunteers, randomised crossover of a 24 hour water-only fast against a day of usual eating. Growth hormone rose (p = 1.1 x 10^-4); haemoglobin, red cell count, haematocrit, total cholesterol and HDL rose; triglycerides, bicarbonate and weight fell. In the 16 randomised to fast first, most markers including growth hormone and cholesterol returned to baseline by 48 hours while weight and triglycerides did not. The magnitude of the growth hormone change is not printed in the accessible record.

  2. Study in people2020
    Metabolic Responses to 24-Hour Fasting and Mild Cold Exposure in Overweight Individuals Are Correlated and Accompanied by Changes in FGF21 Concentration

    Diabetes

    20 people spent a 24-hour fast in a sealed room that measures calories burned. Calories burned over the day went down, by different amounts in different people. How much it dropped tracked how much extra heat each person made in mild cold, and changes in the hormone FGF21. The exact percentage drop was not in the record we could check.

    24 hour energy expenditure measured in a whole-room calorimeter during 24 hour fasting in 20 volunteers decreased, with the magnitude varying between individuals and correlating with the thermogenic response to mild cold and with changes in FGF21. The exact percentage decrease is not in the accessible record.

  3. Study in people1993
    Progressive alterations in lipid and glucose metabolism during short-term fasting in young adult men

    American Journal of Physiology

    6 healthy young men were tested at points from 12 to 72 hours of fasting. Insulin fell from 64.6 to 30.1 pmol/L, with 70% of that drop in the first 24 hours. Fat release from body stores rose sharply, with 60% of the rise between 12 and 24 hours and the biggest jump between 18 and 24. The authors say falling insulin, not blood sugar, drives this early fat release.

    6 healthy men sampled at 12, 18, 24, 30, 42, 54 and 72 hours with stable isotope tracers and indirect calorimetry. Insulin 64.6 ± 12.9 to 30.1 ± 7.9 pmol/L (p < 0.001), with 70% of the decline complete inside the first 24 hours. Glycerol rate of appearance 2.08 ± 0.22 to 4.36 ± 0.36 and palmitic acid 1.63 ± 0.20 to 3.26 ± 0.40 micromol/kg/min, with 60% of that rise between 12 and 24 hours and the largest single interval change between 18 and 24. The authors attribute early lipid mobilisation to falling insulin, not to glucose kinetics.

  4. Study in people1991
    Quantitation of hepatic glycogenolysis and gluconeogenesis in fasting humans with 13C NMR

    Science

    Liver scans during a 68-hour fast in healthy adults tracked where blood sugar came from. Sugar made fresh by the body supplied 64% of it in the first 22 hours, 82% from 22 to 36 hours and 96% after that, with stored sugar (glycogen) making up the rest. There is no single hour when the body flips from stored sugar to fat.

    Hepatic glycogen measured serially by 13C nuclear magnetic resonance through a 68 hour fast in healthy adults, with gluconeogenesis calculated by subtraction: 64 ± 5% of total glucose production across the first 22 hours, 82 ± 5% from 22 to 36 hours, 96 ± 1% thereafter. There is no hour at which glucose production switches from glycogen to fat.

  5. Study in people1996
    Contributions of gluconeogenesis to glucose production in the fasted state

    Journal of Clinical Investigation

    A different method checked the same thing. Healthy people drank heavy water, a tracer, so researchers could tell fresh-made sugar from stored sugar in the blood. The share made fresh was about 47% at 14 hours, 67% at 22 hours and 93% at 42 hours. Some spread figures in the summary did not add up, so they are left out.

    Independent confirmation of the gluconeogenic share by a completely different method. Healthy subjects ingested deuterated water and deuterium enrichment at specific carbons of blood glucose was used to partition gluconeogenesis from glycogenolysis: about 47% at 14 hours, 67% at 22 hours and 93% at 42 hours. The dispersion figures printed in the indexed abstract are internally inconsistent and are not reproduced here.

  6. Study in people2022
    Intermittent fasting activates markers of autophagy in mouse liver, but not muscle from mouse or humans

    Nutrition

    50 women, average age 51, fasted 24 hours on 3 days a week for 8 weeks, eating 70% or 100% of their calorie needs, with thigh muscle samples taken. One cell clean-up (autophagy) marker rose after a 24-hour fast, but others fell in the 70% group. The authors concluded that clean-up markers in muscle went down, and put it down to weight loss.

    50 women, mean age 51, randomised to 24 hour fasts on three non-consecutive days a week at 70% or 100% of energy requirements for 8 weeks, with vastus lateralis biopsies after 12 and 24 hour fasts. A 24 hour fast raised SQSTM1; BECLIN1, SQSTM1 and LAMP2 mRNA fell in the 70% arm after an overnight fast. The authors conclude that autophagy markers in human muscle were reduced and attribute it to weight loss.

  7. Study in people2018
    Training state and skeletal muscle autophagy in response to 36 h of fasting

    Journal of Applied Physiology

    Thigh muscle samples were taken at 2, 12, 24 and 36 hours of fasting, the only study in people with several checks during one fast. Three cell clean-up (autophagy) markers fell, but only in people who did not train. The authors said 36 hours of fasting only mildly affected muscle clean-up. We could not check how many people took part.

    Vastus lateralis biopsies at 2, 12, 24 and 36 hours, the only human study with several within-fast timepoints for muscle autophagy markers. LC3I, LC3II and p62 all decreased (p < 0.05), in untrained subjects only. The authors' own summary is that skeletal muscle autophagy was only modestly affected by 36 hours of fasting. The exact N could not be verified from the paywalled full text.

  8. Study in people2024
    Perioperative urinary ketosis and metabolic acidosis in patients fasted for undergoing gynecologic surgery

    Acta Anaesthesiologica Scandinavica

    1,535 patients without diabetes fasted only to get ready for gynecologic surgery. After surgery, 59.4% had ketones in their urine, and 29.0% of the 929 tested during surgery had too much acid in their blood. Younger, slimmer patients were more at risk. This shows how common ketosis is from ordinary fasting, not from a planned fasting plan.

    1,535 non-diabetic patients fasted only for preoperative preparation. 59.4% had postoperative ketonuria, and 29.0% of the 929 with intraoperative blood gases showed metabolic acidosis. Risk was higher in younger patients with lower BMI. This is the base rate of fasting ketosis rather than an effect of a deliberate protocol.

  9. Study in people2005
    Increasing serum betahydroxybutyrate concentrations during the 72-hour fast: evidence against hyperinsulinemic hypoglycemia

    Journal of Clinical Endocrinology and Metabolism

    34 adults did the standard 72-hour hospital fast used to test for an insulin-making tumour, and all tested negative. Ketones were checked every 6 hours. They did not move from 12 to 18 hours, then rose 333% from 18 to 36 hours, 210% from 36 to 54 and 167% from 54 to 72. 74% passed 2.7 mmol/L before 72 hours.

    Beta-hydroxybutyrate measured every 6 hours through the standard 72 hour diagnostic fast in 34 adults whose fasts were negative for insulinoma. Median changes: 0% from 12 to 18 hours, +333% from 18 to 36, +210% from 36 to 54, +167% from 54 to 72. 74% of those with a negative fast exceeded 2.7 mmol/L before the 72 hour mark.

  10. Review of studies2019
    Clinical Management of Intermittent Fasting in Patients with Diabetes Mellitus

    Nutrients

    A guide for doctors on diabetes drugs during fasting done for non-religious reasons. For a 24-hour fast it says to skip sulfonylurea pills, lower long-acting insulin by an amount the doctor sets, skip mealtime insulin when no carb meal follows, and skip SGLT2 inhibitor pills on fast days. It also warns that studies cutting long-acting insulin by 50%, or mealtime insulin by 70%, still saw a lot of low blood sugar.

    The drug-by-drug adjustment table for non-religious intermittent fasting: skip a sulfonylurea for a 24 hour fast, one third of usual basal insulin if controlled and half if not, skip prandial insulin when no carbohydrate meal follows, skip an SGLT2 inhibitor on a fast day. Two lines from its own text matter most: one study cut basal insulin 50% on fasting days and still had significant hypoglycaemia rates, and another cut prandial insulin 70% and still had significant hypoglycaemia rates.

  11. Study in people2026
    Hypoglycaemia and other risks of ramadan fasting in patients with primary adrenal insufficiency: A prospective controlled trial using 24-hour glucose monitoring

    Journal of Endocrinological Investigation

    30 people with primary adrenal insufficiency, on hydrocortisone and fludrocortisone and taught how to manage their condition, wore glucose monitors in and out of Ramadan fasting. Problems hit 67% while fasting against 23% outside it, and 11 people (37%) broke the fast because of one. 37% felt low blood sugar symptoms against 3%, but the monitors caught low blood sugar about equally often, 71% against 63%.

    Prospective randomised crossover case-control study in 30 patients with primary adrenal insufficiency on hydrocortisone and fludrocortisone, all of whom had received therapeutic education, with 24 hour continuous glucose monitoring. Complications occurred in 67% during fasting against 23% outside it (P = 0.001) and 11 patients (37%) broke the fast because of one. Symptoms of hypoglycaemia were reported by 37% against 3% (P = 0.002), while monitor-detected hypoglycaemia did not differ (71% against 63%, P = 1.000).

What people report

These are things people say about their own fasting. Nobody checked them, so they are not evidence. They are here because they tell you what to expect and what to watch for, which the studies do not. They cannot tell you whether anything works.

  • Hunger coming in waves at old meal times, not building steadily. People say hours 20 to 24 are easier than hours 14 to 18.
  • Salt in water as the usual first fix for a headache, at about a quarter to half a teaspoon.
  • A flat, low afternoon, then a second wind in the evening.
  • Feeling cold in the last few hours.
  • On one meal a day in particular, hunger at all three old meal times for the first three to seven days, easing over about two weeks.

Sources: These are self-reports from public fasting communities. There is no comparison group, no count of how many people tried, and no follow-up on people who stopped posting. They are here because they describe what people go through, which is a different kind of information from a study. Only hunger has a measured match. In the biggest supervised group, eating 200 to 250 calories a day, 93.2% said they had no hunger and 6.81% said hunger kept coming back mildly.

Who this is wrong for

  • Anyone taking an SGLT2 inhibitor, unless a prescriber has stopped it first. Together with fasting, it can cause ketoacidosis (a dangerous acid build-up) while blood sugar looks normal, and a finger-prick test will not warn you. The FDA says to stop canagliflozin, dapagliflozin and empagliflozin at least 3 days, and ertugliflozin at least 4 days, before planned surgery for exactly this reason. One published case shows the risk can last after the drug is stopped.
  • Anyone on insulin, a sulfonylurea or a meglitinide, unless a prescriber changes the dose first. In CREED, a study of 3,250 people with type 2 diabetes fasting for Ramadan, low blood sugar hit 16.8% of people on insulin and 5.3% on pills. The biggest warning sign was having had low blood sugar before: those people were 7.80 times as likely to have it again.
  • Anyone with type 1 diabetes outside a set program with glucose monitoring and an insulin plan. Injected insulin keeps working on its own schedule whatever your blood sugar is doing. Cut it too far and nothing stops ketone build-up. So the risk runs both ways.
  • Anyone with primary adrenal insufficiency. In a study of 30 treated patients who had all been taught how to manage it, 67% had problems while fasting against 23% when not fasting. 11 of the 30 broke the fast because of a problem, even though glucose monitors showed no clear difference in low blood sugar.
  • Pregnancy and breastfeeding. Making milk pulls about 30 to 50 g of sugar a day out of the mother. In 18 reported cases of ketoacidosis while breastfeeding, dieting or exercise to lose weight set off 76% of them.
  • Anyone with a past eating disorder, or who now has episodes of eating they cannot control. A plan judged by how well you keep a food rule sets you up to keep tightening the rule. Surveys find the same link in women, men and people who do not fit either label.
  • Children and teens. There are no long-term studies on growth or development. It is also the age group where the link with eating disorders is strongest.
  • Anyone with a BMI under 18.5, or who already meets the UK NICE risk list for refeeding problems.
  • Anyone with chronic kidney disease who takes an ACE inhibitor or ARB. In 131 people with kidney disease, a blood marker of kidney strain (creatinine) rose during fasting in 60.4% by day 7. People on this family of blood pressure drugs were twice as likely to have the rise.
  • Anyone on lithium who fasts without water. The kidneys take lithium back in along with salt, so losing fluid raises lithium levels, and the gap between a safe and a toxic level is small. The risk comes from skipping water, not from skipping food.

Questions

Does a 24 hour fast turn on autophagy?
The two measurements in people at exactly this length point the other way in the tissue they tested. A 24-hour fast raised the marker p62/SQSTM1 in the thigh muscle of 50 women, which points toward less clean-up. The only study with several time points during a fast found three clean-up markers (LC3I, LC3II and p62) all fell at 24 and 36 hours, in people who did not exercise. Neither measured how fast clean-up was running. Snapshot markers count the cell's clean-up bags; they do not measure how fast material moves through them. Blood cell measurements at 18 hours point up while muscle measurements point down. That is the most useful thing to know about this research.
Does one day of fasting slow my metabolism?
Measuring the whole day says it falls. Measuring at rest under a hood says it rises. Both are real. In a sealed room, energy burned over 24 hours of fasting went down in 20 people. In a sealed room over 60 hours, total energy burned was 10.9% lower on day 2 and 5.5% lower on day 3 than when people ate enough to hold their weight. Resting energy burn measured under a hood rose 14.1% by day 3 in 11 lean people. Resting burn can rise while the whole day falls, because you are no longer burning energy digesting food, and you move around less.
Is the growth hormone rise the one quoted as 1,300% and 2,000%?
No. Those numbers come from a 2011 press release about work shown at a meeting, describing a 24-hour fast. The same release says LDL cholesterol rose 14% during the fast. The checked, published study at exactly 24 hours found growth hormone clearly higher than on a normal eating day, but did not print how much. The best-measured figure is a five-fold rise in growth hormone over 24 hours on the second day of a fast, in 9 men.
Will one day of fasting cost me muscle?
Nobody has measured body make-up across a single 24-hour fast. Longer studies show the early "lean" loss is mostly water and stored sugar. In a 10-day water fast, lean mass was down 9.2% at day 6 and back to normal after eating again. In a 10-day low-calorie fast, 58% of the scan's lean loss was water outside the cells plus stored sugar and its water. Nobody has ever published how much protein people lose per day in the first three days of a total fast.