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Extended9 studies in people cited

36-Hour Fast

36 hours without food · Once a week, or every other day in the alternate-day trials

Written by Reviewed Sep 2026

Also called: Monk fast, Dinner to breakfast the day after next, Alternate-day fasting window

A day and a half with no calories. The studies that took blood hour by hour skipped this exact hour, so what we know comes from studies on either side of it. The one thing measured at exactly 36 hours is a set of muscle clean-up markers, and they went down, not up.

Overview

36 hours is the fast inside alternate-day fasting, and the shape of the weekly fast some people call a "monk fast". It is also a gap in the research. The best hour-by-hour study took samples at 12, 18, 24, 30, 42, 54 and 72 hours, but not at 36. So what we know about this hour comes from studies on either side of it, and from two that landed right on it.

Here is what happens in this stretch. Ketones (beta-hydroxybutyrate), which the body makes when burning fat, make their steepest climb of the whole three days: a typical rise of 333% between 18 and 36 hours. The liver makes 82% of the body's sugar from scratch across the same stretch. Thyroid hormone has already dropped by 30 hours. And compared with a 12-hour fast, three things happen at once: the first burst of insulin after sugar is a little smaller, the liver responds to insulin better, and the body as a whole resists insulin more.

This is also the length where the growth hormone claim is best tested. The experiment that explains how growth hormone saves protein during a fast ran for 37 hours.

How it works

Dinner on day one, nothing on day two, breakfast on day three. In the studies, the comparison was usually a 12-hour fast. The insulin study compared 13 healthy young men at 36 hours against 12 hours. The muscle sample study took samples at 2, 12, 24 and 36 hours.

What makes this harder than 24 hours is a second missed dinner and a full working day without food. Community accounts say that is the hard part, more than the total time. The report that comes up again and again is that hours 30 to 40 are the worst mentally, and it gets easier after that.

Salts and minerals are where fasting communities say 24 to 36 hours is the point where they go from optional to routine. That line comes from habit, not measurement, and no study has tested a salt and mineral plan during a fast done by choice at any length. What has been measured is that salt loss has not peaked yet at this point. In 9 women with obesity who kept eating their usual salt while fasting, salt in the urine started rising after about 48 hours, and only then went above what they took in.

What 36 hours actually reaches

Three measurements define this stretch. First, the liver makes 82% of the body's sugar from scratch between 22 and 36 hours. This was measured by scanning the liver in healthy adults. Second, ketones rise by a typical 333% between 18 and 36 hours, in 34 adults tested every 6 hours. That is the steepest climb of a 72-hour fast. Third, at 30 hours in 8 healthy men, active thyroid hormone (T3) fell from 2.30 to 1.84 nmol/L. The brain's thyroid signal (TSH) fell below 1 mU/L, and its normal night-time peak disappeared. An inactive form of thyroid hormone (reverse T3) rose. The stress hormone cortisol did not change. Eating 800 calories did not measurably bring T3 back up. The thyroid result matters for the order of events. The drop in T3 starts within the first 30 hours, well before any measurable drop in resting energy burn. In 13 healthy young men compared at 36 hours against 12 hours, the first burst of insulin was a little smaller. The liver responded to insulin better. And the body as a whole resisted insulin more. The authors say the smaller insulin burst at 36 hours may be a healthy response to the liver working better, not a problem. On autophagy (the cell's clean-up process), this is the far end of the only study in people with several time points during a fast. Muscle samples at 2, 12, 24 and 36 hours found three clean-up markers (LC3I, LC3II and p62) all went down, and 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. Those are snapshot markers, not a measure of how fast clean-up is running. A falling marker does not prove clean-up fell. But it is not the direction popular timelines give this hour. This page does not claim a 36-hour fast reaches autophagy. It does not claim mitophagy (clean-up of the cell's power plants) at any length, 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

    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.

  6. 36h to 2d

    Growth hormone jumps, IGF-1 does not follow

    past this window
    Burning
    Fat, with ketones now carrying a real share of the load. Nearly all the body's sugar is now made fresh.
    Ketones
    A middle rise of +210% between 36 and 54 hours, in 34 adults
    • In 9 healthy men, daily growth hormone output rose from 78 to 371 micrograms per litre on the second day of a fast. That is a five-fold rise. The number of bursts went from 14 to 32 a day, and the amount per burst went from 6.3 to 11 micrograms.
    • How long growth hormone lasted in the blood did not change, at 18 against 20 minutes. So the body was truly making more, not just clearing it more slowly.
    • Blood IGF-1 had not changed at 56 hours. It does not fall until around day 5, when it dropped from 1.31 to 0.77 U/mL in 6 men.
    • From 36 hours on, making new sugar supplies 96% of the body's sugar.
    • At 48 hours, in a clamp test where insulin is dripped in, the body's sugar use fell from 39.8 to 24.1 micromol/kg/min. Sugar burned for energy crashed from 21.8 to 3.9. Sugar stored did not change. That is a change in which fuel the body burns. It is not the kind of sugar problem seen in diabetes.
    • Energy burned over 24 hours, measured in a sealed room, was 10.9% lower on day 2 of a fast than on a day of normal eating (9.8 against 11.0 MJ).

    Read the biologyThe growth hormone responseGlycogen depletion and gluconeogenesisInsulin and insulin sensitivity during a fast

    Measured in people
    Hartman 1992 took blood every 5 minutes for 24 hours in 9 healthy men, on a normal day and on the second day of a fast, and worked out the true output with maths. IGF-1 was measured at 56 hours. Ho 1988 measured IGF-1 (as somatomedin C) through a 5 day fast in 6 men. Mansell 1990 did clamp tests on 6 men without diabetes at 48 hours without food. Andriessen 2023 put 12 healthy lean men in a sealed room for 60 hours straight, once fasting and once eating. No study in people has measured autophagy in any tissue between 36 and 72 hours. That is why autophagy is not linked from this row.

What studies in people found

There is one study comparing 36 hours with 12 hours, looking at insulin release and how the liver responds to insulin. There is the only muscle clean-up study in people with several time points, a 30-hour thyroid study, and the ketone and liver sugar studies on either side of this hour.

  • Service 2005 tested ketones every 6 hours in 34 adults through the standard 72-hour hospital test fast. The typical rise between 18 and 36 hours was 333%, the steepest stretch of the whole fast.
  • Jorgensen 2021, 13 healthy young men after 36 hours of fasting compared with 12 hours. The first burst of insulin was a little smaller. The liver responded to insulin better. And the body as a whole resisted insulin more, all at the same time. The authors read the smaller insulin burst as a healthy response to the liver working better.
  • Hugues 1984, 8 healthy men who fasted 30 hours. Active thyroid hormone (T3) fell from 2.30 to 1.84 nmol/L. The brain's thyroid signal (TSH) fell below 1 mU/L, its night-time peak disappeared, and inactive reverse T3 rose. Eating 800 calories did not measurably change T3. Cortisol did not change.
  • Norrelund 2001, 8 healthy people across a 40-hour fast. Growth hormone was blocked with a drug and then given back, while insulin and glucagon were kept the same, so only growth hormone changed. Protein waste in the urine (urea) was 440 mmol per 24 hours when fasting, 609 with growth hormone blocked, and 408 with it given back. Blocking growth hormone during a 40-hour fast raised protein waste by about 50%.
  • Norrelund 2003, 7 healthy people through a 37-hour fast, under four different conditions. Blocking fat release with a drug (acipimox) raised protein waste in the urine and blood, and muscle breakdown, by almost 50%. Adding growth hormone while fat release was blocked did nothing to protein breakdown. Putting fat back into the blood with a drip (intralipid) lowered protein waste and cut whole-body protein breakdown by 10% to 15%.
  • Rothman 1991 scanned the liver's stored sugar through a 68-hour fast in healthy adults. The liver made 82% of the body's sugar from scratch between 22 and 36 hours.
  • Dethlefsen 2018 is the only study in people with several time points for muscle clean-up during a fast. Muscle samples at 2, 12, 24 and 36 hours showed LC3I, LC3II and p62 all going down, only in people who did not exercise. Its own summary is that muscle clean-up was only a little affected by 36 hours of fasting.

What this does not tell you: The insulin study had 13 young men. The muscle clean-up study did not report a size we could confirm, because the full paper is behind a paywall, and it used snapshot markers, not a measure of how fast clean-up ran. The thyroid study had 8 men at 30 hours, not 36. The best hour-by-hour study did not take samples at 36 hours at all. So the blood sugar, insulin and fat release numbers for this hour are guesses from either side, not measurements. No study of a repeated 36-hour water fast has looked at real health outcomes.

Reading the research record

The claim that belongs here is that the growth hormone rise during fasting protects muscle. It belongs here because the key experiment ran for 37 hours.

The growth hormone rise is real, and it does help save protein. Blocking growth hormone during a 40-hour fast raised protein waste in the urine by about 50%, from 440 to 609 mmol per 24 hours. Giving it back brought that down to 408. Muscle breakdown in the forearm moved the same way.

But it does not work the way the popular claim assumes. In 7 healthy people fasted for 37 hours, blocking fat release with acipimox raised protein waste in the urine and blood, and muscle breakdown, by almost 50%. Adding growth hormone while fat release was blocked did nothing to protein breakdown. Putting fat back into the blood with intralipid cut whole-body protein breakdown by 10% to 15%. The authors concluded that once fat release is blocked, growth hormone can no longer limit protein loss during a fast. Its main way of saving protein is releasing fat.

So growth hormone saves protein by freeing up fat, so the body burns fat instead of protein. It does not build muscle during a fast. In people fully adapted to long starvation, it failed outright. In people with obesity fasted for 5 to 6 weeks, growth hormone led to a 2 to 3 fold rise in insulin, a 50% rise in blood sugar, ketone levels of 10 to 12 mmol/L, and nausea, vomiting, weakness and muscle aches.

One more thing to carry from the 48-hour studies into this one. A five-fold rise in growth hormone over two days left IGF-1 in the blood unchanged at 56 hours. IGF-1 is the growth signal growth hormone normally works through, and it does not follow along on this timescale.

What happens to muscle

No study has measured body make-up across a single 36-hour fast. The alternate-day studies that use this length report body make-up for the whole plan, not for the fast day.

What can be measured at this length is how the body handles protein. Studies in people show the body's protein-saving system is already starting up. Across a 10-day low-calorie fast in 16 men, total protein burned (measured as nitrogen in the urine) dropped 41% by day 5, then held steady through day 10. Urea in the blood fell 41% alongside it. A marker of muscle breakdown (3-methylhistidine) rose until day 5, then fell. That is early protein breakdown followed by protein saving. 36 hours sits at the front of that curve, before the saving kicks in.

How much of any scan's lean loss is real tissue is a separate question. Lean mass means the non-fat part of you, mostly muscle and water. In that same 10-day study, 3.53 kg of scan lean mass was lost. Of that, 44% was water outside the cells, 14% was stored sugar with its water, and 42% was working lean tissue.

For anyone lean, body fat matters more than fast length. Total protein lost per kg of weight lost is lower the more body fat you carry. It is roughly 20 g of nitrogen per kg in people without obesity, and roughly 10 g per kg in people carrying 50 kg or more of fat. So a lean person loses about twice as much protein per kg lost as a person with obesity doing the same fast. That is the single biggest warning about applying the classic starvation research to everyone, because all of it was measured in people with obesity.

Doing it

Salt loss has not peaked yet
In the hospital ward study, salt loss in the urine started rising after about 48 hours, and only then went above what people took in. In week one, people lost 203 mEq of salt more than they took in, on average. At 36 hours the loss has started but has not peaked. Fasting communities say 24 to 36 hours is when salt goes from optional to routine. Nobody has ever tested that line.
Thyroid drops before energy burn does
Active thyroid hormone (T3) fell 20% by 30 hours in 8 healthy men, with the brain's thyroid signal (TSH) pushed below 1 mU/L. In a separate study, resting energy burn measured under a hood was still rising at day 3. A thyroid blood test taken during or just after a fast this long cannot be read as your normal level.
Skip sugar tolerance tests here
At 36 hours, the body as a whole resists insulin more, even as the liver responds to it better. At 48 hours in a lab insulin test, burning of sugar crashed, while sugar stored away did not change. That is the body switching fuels, not diabetes-type trouble handling sugar. But it will make a sugar tolerance test look abnormal.
Medication review, same as at 24 hours
The published guide says to skip a sulfonylurea for a 24-hour fast, and a longer fast is a bigger risk, not a smaller one. In one study, people cut long-acting insulin by 50% on fasting days and still often had low blood sugar. An SGLT2 inhibitor has to be stopped days ahead, the same lead time used before surgery.
Protein after the fast
The short-window studies that kept lean mass fed people 1.6 to 2.2 g of protein per kg of body weight a day, with weight training. A fast day inside a weekly routine makes it harder to hit that weekly average. That is the real lever, not anything about the fast itself.

Breaking the fast

A day and a half is short of every published refeeding risk line for a well-fed person. The UK NICE list counts more than 5 days of little or no food as one of two warning signs, and more than 10 days as enough on its own. The US nutrition society's (ASPEN) bands are 5 to 6 days for medium risk and more than 7 for high risk. You are nowhere near either here. The reason those lines exist is worth knowing, because a blood test cannot see it. During a fast, the body runs low on phosphate, potassium and magnesium even while blood levels look normal. In 6 people studied for 40 days on a hospital ward, they lost 104 mg of calcium, 48 mg of magnesium and 363 mg of phosphorus a day on average, while blood phosphorus and magnesium did not change at all. Blood tests miss the shortfall. When you eat carbs again, insulin rises and pushes sugar, phosphate, potassium and magnesium into the cells all at once. The real-world reports at this length are about comfort, not danger. People often describe bloating, cramps and diarrhoea after breaking a fast of 48 hours or more with a normal-sized meal. The community's fix is a small, warm, low-carb first course, a wait, then a real meal at about half the normal size. None of that has been tested against anything else.

Sources

  1. Study in people2021
    Impact of prolonged fasting on insulin secretion, insulin action, and hepatic versus whole body insulin secretion disposition indices in healthy young males

    American Journal of Physiology, Endocrinology and Metabolism

    13 healthy young men were tested after 36 hours of fasting and after 12 hours. After the long fast, the first burst of insulin was a little smaller, the liver responded to insulin better, and the body as a whole responded to insulin worse. The authors see the smaller insulin burst as a likely healthy response, not a fault.

    13 healthy young males compared after 36 hours of fasting against 12 hours. A minor reduction in first-phase insulin secretion alongside improved hepatic insulin action and increased whole-body insulin resistance. The authors frame the reduced secretion as a possible healthy response to improved hepatic insulin action rather than as a defect.

  2. 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, and the authors said the effect was mild. These were snapshot markers, not a measure of the clean-up process actually running. 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, which the authors summarise as skeletal muscle autophagy being only modestly affected by 36 hours of fasting. Static markers, not a flux assay. The exact N could not be verified from the paywalled full text.

  3. Study in people1984
    Rapid adaptations of serum thyrotrophin, triiodothyronine and reverse triiodothyronine levels to short-term starvation and refeeding

    Acta Endocrinologica

    8 healthy men fasted 30 hours. Active thyroid hormone (T3) fell from 2.30 to 1.84 nmol/L, the thyroid signal TSH fell and lost its usual night-time peak, and reverse T3 rose. An 800-calorie meal afterwards did not measurably change T3. Cortisol did not change.

    8 healthy men fasted 30 hours. Serum T3 fell from 2.30 ± 0.06 to 1.84 ± 0.03 nmol/L (p < 0.01), TSH fell below 1 mU/L with the nocturnal TSH peak abolished, and reverse T3 rose. Refeeding 800 kcal did not measurably change T3, and serum cortisol did not differ across conditions.

  4. Study in people2003
    The decisive role of free fatty acids for protein conservation during fasting in humans with and without growth hormone

    Journal of Clinical Endocrinology and Metabolism

    7 healthy people fasted 37 hours under four conditions, including growth hormone replacement and a drug, acipimox, that blocks fat release. Blocking fat release raised muscle protein breakdown by almost 50%. Adding growth hormone did not change that, but putting fat back in through a drip cut protein breakdown by 10% to 15%. The authors conclude growth hormone protects protein mainly by releasing fat.

    7 normal subjects, 37 hour fast, four conditions including growth hormone replacement and antilipolysis with acipimox. Blocking lipolysis raised urea excretion, serum urea and muscle protein breakdown by almost 50%. Adding growth hormone during fasting with antilipolysis did not influence indexes of protein degradation. Restoring free fatty acids with intralipid cut whole-body protein degradation by 10% to 15%. The authors conclude that stimulation of lipolysis is the principal protein-conserving mechanism of growth hormone.

  5. Study in people2001
    The protein-retaining effects of growth hormone during fasting involve inhibition of muscle-protein breakdown

    Diabetes

    8 healthy people were tested four ways: after an overnight fast, after a 40-hour fast, after a 40-hour fast with growth hormone blocked, and the same with growth hormone put back. Other hormones were kept the same, so only growth hormone differed. Protein loss, measured in urine, was highest with growth hormone blocked and fell back when it was replaced. Muscle breakdown moved the same way, so growth hormone really does help save protein during a fast.

    8 normal subjects in four conditions: basal postabsorptive, 40 hour fast, 40 hour fast with growth hormone suppressed by somatostatin, and the same with growth hormone replaced, with insulin and glucagon replaced identically so only growth hormone differed. Urinary urea excretion 392 ± 44 basal, 440 ± 32 fasted, 609 ± 76 without growth hormone and 408 ± 36 with it replaced (p < 0.05). Muscle protein breakdown moved the same way. Growth hormone is a genuine component of physiological protein conservation.

  6. 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, with ketones checked every 6 hours. Ketones did not rise from 12 to 18 hours. They then rose 333% from 18 to 36 hours, the steepest stretch of the whole fast.

    Beta-hydroxybutyrate every 6 hours in 34 adults through the standard 72 hour diagnostic fast. The median rise from 18 to 36 hours was +333%, the steepest proportional stretch of the whole fast, following a median change of 0% from 12 to 18 hours.

  7. 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. Sugar made fresh by the body supplied 82% of blood sugar between 22 and 36 hours, up from 64% in the first 22 hours. After that it supplied 96%.

    Gluconeogenesis supplied 82 ± 5% of total glucose production between 22 and 36 hours, having already supplied 64 ± 5% across the first 22, and 96 ± 1% thereafter. Measured by serial 13C nuclear magnetic resonance of hepatic glycogen through a 68 hour fast in healthy adults.

  8. Study in people1975
    The mechanism of the natriuresis of fasting

    Journal of Clinical Investigation

    9 women with obesity on a hospital research ward fasted 5 to 7 days, still taking their usual salt as tablets. They started losing more sodium in urine after about 48 hours, and in the end lost more than they took in. Over week one they lost 203 mEq of sodium on average. Chloride loss was much smaller, only 6 mEq.

    9 obese women on a metabolic ward, stabilised 7 to 14 days then fasted 5 to 7 days with pre-fast sodium chloride intake maintained as salt tablets, urine collected in 3 hour blocks. Sodium excretion began to rise after approximately 48 hours and ultimately exceeded intake. Mean cumulative negative sodium balance in week one was 203 mEq, range 107 to 232. Chloride dissociated from sodium, with a mean cumulative negative balance of only 6 mEq.

  9. Study in people1981
    Adverse effects of liquid protein fast on the handling of magnesium, calcium and phosphorus

    American Journal of Medicine

    6 people with obesity spent 40 days on a liquid protein fast on a hospital research ward. They lost more minerals in urine than they took in: 58% more calcium, 75% more phosphorus and 500% more magnesium. Yet blood phosphorus and magnesium did not change, and blood calcium fell only 0.5 mg/dL. A normal blood test during a fast does not mean your body's stores are fine.

    6 obese subjects, 40 days on a metabolic ward. Cumulative urinary losses exceeded intake by 58% for calcium, 75% for phosphorus and 500% for magnesium, with mean daily balances of -104 mg calcium, -48 mg magnesium and -363 mg phosphorus, while serum phosphorus and magnesium did not change. Serum calcium fell 0.5 mg/dL (p < 0.05). The clearest demonstration that a normal serum value during a fast does not mean the store is intact.

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.

  • Hours 30 to 40 described as the hardest stretch mentally, and worse than hours 44 to 48.
  • The second missed dinner, not the time itself, being the hard part. People describe it as social and emotional more than physical.
  • A dull headache from about hour 24 to 36, blamed on low salt and said to get better with salt.
  • Dizziness or greying out when standing up fast, seen as normal rather than scary unless it gets worse.
  • Feeling cold, especially hands and feet.
  • The first signs of nail-polish breath (acetone) and a metal taste, usually from about hour 30.
  • Grumpiness and a short fuse on the second day. It is common enough that people advise planning around hard meetings.

Sources: These are self-reports from public fasting communities. There is no comparison group, no count of how many people tried, no follow-up on people who stopped posting, and bad experiences show up more than they should. Only one item has a supervised match: feeling faint or light-headed. It was recorded at 217 of 768 visits in a review of water-only fasting, and rated medium every time.

Who this is wrong for

  • Anyone taking an SGLT2 inhibitor, unless a prescriber has stopped it first, with the same lead time used before surgery: at least 3 days for canagliflozin, dapagliflozin and empagliflozin, and at least 4 for ertugliflozin. The ketoacidosis (dangerous acid build-up) this causes happens while blood sugar looks normal. A blood sugar meter will not warn you. A blood ketone test is the one that matters.
  • Anyone on insulin, a sulfonylurea or a meglitinide, unless a prescriber changes the dose first. Cutting the dose alone does not keep you safe: 50% cuts in long-acting insulin and 70% cuts in mealtime insulin each still led to a lot of low blood sugar.
  • Anyone with type 1 diabetes outside a set program with a continuous glucose monitor and an insulin plan.
  • Anyone with primary adrenal insufficiency. A fixed hydrocortisone dose cannot rise the way the body's own cortisol would. In 30 treated patients, 67% had problems while fasting against 23% when not fasting, and 37% broke the fast because of a problem.
  • Pregnancy and breastfeeding. The baby, and milk-making, need sugar all the time, not now and then. 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.
  • Children and teens. There are no long-term studies on growth or development.
  • Anyone with a BMI under 18.5, or who already meets the UK NICE risk list for refeeding problems.
  • Anyone with chronic kidney disease on an ACE inhibitor or ARB. Losing fluid while on these drugs takes away both of the ways the kidneys protect their own blood flow. 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 taking lithium who will also cut back on fluids. Losing fluid makes the kidneys take lithium back in along with salt, and the gap between a safe and a toxic level is small.
  • Anyone who takes an anti-inflammatory painkiller (NSAID) long term. Up to 70% of long-term users have damage visible on a camera test of the gut, but only about 10% notice stomach symptoms. So serious bleeding can happen without warning. Without food, nothing buffers the stomach, but it keeps making acid.

Questions

What actually happens at 36 hours that has not happened at 24?
Ketones finish the steepest part of their climb, a typical 333% rise between 18 and 36 hours. The liver makes 82% of the body's sugar from scratch. Active thyroid hormone has dropped about 20%, and the brain's thyroid signal is pushed below 1 mU/L by 30 hours. And compared with a 12-hour fast, the first burst of insulin is a little smaller while the liver responds to insulin better and the body as a whole resists insulin more. The study that tracked the rest of the fast hour by hour did not take samples at 36 hours at all.
Does the growth hormone rise stop me losing muscle?
It helps, but not the way the claim suggests. Blocking growth hormone during a 40-hour fast raised protein waste in the urine by about 50%, so it really is part of how the body saves protein. But when fat release was blocked with acipimox during a 37-hour fast, adding growth hormone did nothing to protein breakdown, and putting fat back into the blood brought breakdown back down. Growth hormone saves protein by freeing up fat, not by acting on muscle.
Is autophagy running at 36 hours?
The only study in people that sampled muscle at 2, 12, 24 and 36 hours found three clean-up markers (LC3I, LC3II and p62) all falling at the later times, only in people who did not exercise. Its own summary is that clean-up was only a little affected. Those are snapshot markers, not a measure of how fast clean-up runs, so a falling marker does not prove clean-up fell. But nothing measured in a person at this hour backs the claim that it is surging. So this page does not list autophagy as something a 36-hour fast reaches.
Why is hour 36 harder than hour 48?
Fasting communities report this often enough to note. They say hours 30 to 40 are the worst stretch, worse than hours 44 to 48, and that the hard part is social and emotional more than physical. Nobody has measured how hard fasting feels hour by hour in a controlled study. So this is what people describe, not what has been measured.