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Extended11 studies in people cited

48-Hour Fast

2 days without food · Once or twice a month in most accounts, once a week at the outside

Written by Reviewed Sep 2026

Also called: Two day fast, Rolling 48

Two days with no calories. This is the best-measured hormone window in all fasting research. On day two, the body makes five times as much growth hormone, while IGF-1 does not move. And total energy burned across the day, measured in a sealed room, is 10.9% lower, not higher.

Overview

48 hours is where the hormone science gets specific. Nine healthy men had their growth hormone tested every 5 minutes for 24 hours, once on a normal eating day and once on the second day of a fast. It is the cleanest hormone measurement in this field. Growth hormone output over 24 hours went up five times. The body released it in more bursts, with more hormone per burst, and cleared it at the same speed as before.

The same paper tested IGF-1 at 56 hours and found it had not changed. That line almost always gets dropped. IGF-1 is the growth signal growth hormone normally works through. It does not follow growth hormone up on this timescale, and it does not fall until around day 5.

Two days is also where the metabolism claim can be properly tested. A study kept 12 lean men in a sealed room that measures energy burned for 60 hours straight, once fasting and once eating just enough to hold their weight. Total energy burned across the day fell 10.9% on day 2 and 5.5% on day 3 while fasting. Fat burning kept rising and peaked at 51 hours.

How it works

Two full days with no calories, most often dinner on day one to dinner on day three, drinking only things with no calories. In the studies, the fast was set by what was being measured, not by a schedule. There was 48 hours without food before a lab insulin test in 6 men. There was 48 hours before scans of the liver and muscle in men and women. There was 60 hours in a sealed room. There was a 60-hour fast from 36 hours before to 24 hours after chemotherapy, in a small early study where the same patients tried fasting and not fasting. And there was a 48-hour group inside a chemotherapy fasting study that tried longer and longer fasts.

People describe day two as the wall: the worst headache, the lowest energy, and the most people quitting. They say it breaks somewhere between hours 48 and 72. That is self-report, and the same people warn that day two can feel different from one fast to the next, even for the same person.

This is also when salt loss during a fast turns. In 9 women with obesity who kept eating their usual salt while fasting, salt in the urine started rising after about 48 hours and then went above what they took in. Across week one they lost 203 mEq more salt than they took in on average, ranging from 107 to 232. The peak comes later. In 8 people with obesity on a fixed salt intake of 51 mmol a day, salt in the urine rose from 35 to 109 mmol a day by day 4, then went back down.

What 2 days actually reaches

Growth hormone first, with the real numbers. In 9 healthy men on the second day of a fast, growth hormone made over 24 hours rose from 78 to 371 micrograms per litre, five times as much. Bursts went from 14 to 32 a day. The amount per burst went from 6.3 to 11 micrograms. The gap between bursts shrank from 143 to 88 minutes. How long growth hormone lasted in the blood did not change (18 against 20 minutes), so the body was really making more, not clearing it slower. IGF-1 in the blood was unchanged after 56 hours of fasting. Insulin handling drops at 48 hours, and the way it drops tells you something. In a lab insulin test in 6 men without diabetes, the whole body took up much less sugar: 24.1 against 39.8 micromol/kg/min. Sugar burning crashed from 21.8 to 3.9. But sugar being stored away did not change (18.0 against 20.2). Sugar uptake in the forearm fell from 59.4 to 15.4. Burning sugar collapsed, while storing it did not. That is the body switching fuels, not diabetes-type trouble handling sugar. Ketones (beta-hydroxybutyrate) measured directly in the brain by scanner reached 0.60 mmol/L after the second day, up from 0.05 when not fasting. From 36 hours on, the liver makes 96% of the body's sugar from scratch. There is also a difference between men and women in where fat goes. Across a 48-hour fast, fat built up in the livers of men and in the muscles of women. Sugar and energy handling in the whole body and the liver were the same in both. No study in people has measured autophagy (the cell's clean-up process) in any tissue between 36 and 72 hours. This page does not claim a 48-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

    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.

  7. 2d to 3d

    Ketones reach the brain

    past this window
    Burning
    Fat and ketones. The brain now gets a measurable share of its fuel from BHB.
    Blood sugar
    4.14 mmol/L (74.6 mg/dL) at 72 hours in 6 healthy men, about 25% below the 12 hour value
    Ketones
    74% of people whose test fast came back clear went above 2.7 mmol/L before the 72 hour mark. The middle rise from 54 to 72 hours was +167%.
    Insulin
    30.1 pmol/L (4.3 microunits/mL) at 72 hours, about half the 12 hour value
    • Brain BHB was measured directly with a strong MRI scanner aimed at the back of the brain. It rose from 0.05 mmol/L before fasting to 0.60 after the second day and 0.98 after the third. Brain lactate rose from 0.69 to 1.47 mmol/L.
    • Blood and brain BHB rose closely together. The brain level rose about 0.26 times as much as the blood level did.
    • Over a 48 hour fast, fat built up in the livers of men and in the muscles of women. Sugar handling and fuel burning, in the whole body and the liver, were the same in both sexes.
    • The two ways of measuring energy burned split apart here, and both are real. Resting energy burned, measured with a hood, rose 14.1% by day 3 in 11 lean people. Energy burned over the whole day, measured in a sealed room, was still 5.5% below a normal eating day on day 3 (10.3 against 10.9 MJ). The resting part can rise while the whole day falls.
    • The 72 hour supervised fast is a routine hospital test that doctors have used for decades to check hormone problems. That is the strongest evidence there is that healthy people handle this length of fast when they are being watched.

    Read the biologyKetogenesis and beta-hydroxybutyrateInsulin and insulin sensitivity during a fast

    Measured in people
    Klein 1993 sampled 6 healthy men at 12, 18, 24, 30, 42, 54 and 72 hours, using tracers and breath gas tests. Service 2005 measured BHB every 6 hours in 34 adults. Pan 2000 measured brain BHB and lactate with MRI scans in healthy adults. Browning 2012 measured liver and muscle fat with scans across 48 hours in healthy men and women. Zauner 2000 measured resting energy burned with a hood in 11 lean people. Andriessen 2023 measured energy burned over 24 hours in a sealed room in 12 lean men. The two are measuring different things. They do not contradict each other.

What studies in people found

There is one growth hormone study with samples every 5 minutes, one lab insulin test study, one sealed-room study where the same men fasted and ate, one scan study of where fat goes in men and women, one brain scan study, and two cancer studies that used fasts of 48 to 60 hours.

  • Bauersfeld 2018, 34 women with breast or ovarian cancer, in a small early study. Each patient fasted 60 hours around some chemotherapy rounds (from 36 hours before to 24 hours after) and ate normally around others. Quality of life dropped less than a meaningful amount on fasting rounds and more than that on normal rounds. Tiredness scores got worse by 10.4 points when fasting against 27.0 when not fasting in one group, and 11.0 against 14.1 in the other. There were no serious side effects. The authors say they have no conflicts of interest.
  • Dorff 2016, 20 patients getting platinum chemotherapy, in groups that fasted 24, 48 or 72 hours before treatment. Fasting proved doable, and side effects from fasting were all mild to moderate. DNA damage in white blood cells was lower at 48 hours or more, but not clearly so. A drop in severe low white cell counts in the longer groups was also not clear. IGF-1 fell 30%, 33% and 8% in the 24, 48 and 72-hour groups after the first fast. No group ate normally for comparison, and the authors say they cannot tell whether fasting reduces side effects.
  • Andriessen 2023, 12 healthy lean men in a sealed room for 60 hours straight, once fasting and once eating just enough to hold their weight. Total energy burned over 24 hours was 10.9% lower on day 2 and 5.5% lower on day 3 while fasting. Fat burning kept rising, peaking at 51 hours at 160 mg a minute, while sugar burning levelled off around 60 mg a minute. The daily rhythm of energy burned did not change. The daily rhythm of which fuel was burned did.
  • Hartman 1992, 9 healthy men with growth hormone tested every 5 minutes for 24 hours on a normal day and on the second day of a fast. Output over 24 hours rose from 78 to 371 micrograms per litre. Bursts went from 14 to 32, and the amount per burst from 6.3 to 11 micrograms. How long it lasted in the blood did not change. IGF-1 was unchanged at 56 hours.
  • Mansell 1990, 6 men without diabetes at a normal weight, given a lab insulin test after 48 hours without food. Whole-body sugar uptake fell from 39.8 to 24.1 micromol/kg/min. Sugar burning fell from 21.8 to 3.9. Sugar storage did not change. Forearm sugar uptake fell from 59.4 to 15.4. Heat made during the test was clear when fed, and not clear after 48 hours without food.
  • Browning 2012 scanned liver and muscle fat across a 48-hour fast in healthy men and women. Fat built up in the livers of men and in the muscles of women. Sugar and energy handling in the whole body and the liver were the same in both.
  • Pan 2000 measured ketones directly in the back of the brain in healthy adults, using a scanner. They were 0.05 mmol/L when not fasting, 0.60 after the second day, and 0.98 after the third. Brain and blood levels tracked closely, with the brain at about a quarter of the blood level.

What this does not tell you: The growth hormone study had 9 men and the insulin test study had 6, all healthy young men, and neither has been repeated with samples this frequent. The sealed-room study had 12 lean men. The cancer studies were small early studies: 34 patients in one and 20 in the other. The second had no comparison group, and we could not confirm its conflict-of-interest statement. Nothing here looked at real health outcomes in healthy people. No study has assigned people to repeated 48-hour fasts against a comparison group.

Reading the research record

Two claims usually get tied to this fast length, and both need correcting against the papers they come from.

The first is the growth hormone percentage. The figures of 1,300% in women and 2,000% in men come from a heart institute press release dated 3 April 2011. It described work shown at a heart meeting, covering over 200 people in two studies plus 30 more patients. The percentages are quoted correctly from that release. But three things get lost. They are figures for a 24-hour fast, not a multi-day one. They come from a meeting talk, not a paper you can check. And the same release says LDL cholesterol rose 14% and HDL 6% during the fast. The checked, published number for the second day of a fast is a five-fold rise in 24-hour output in 9 men, with IGF-1 unchanged at 56 hours.

The second claim is that metabolism speeds up. Zauner 2000 is a real study and is where this claim comes from. 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 for the same claim, but it measured something else entirely: heat made during a lab insulin test at 48 hours. And its result points the other way.

For the whole-day question, the stronger design is the sealed room, and there total energy burned fell 10.9% on day 2 and 5.5% on day 3. Both findings 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. Resting burn does fall eventually: by 20.3% across a 21-day fast.

What happens to muscle

No study has measured body make-up across a 48-hour fast. That is the honest answer, and it is also a sensible choice, because a scan at that point would mostly be measuring water.

The 10-day studies show why. Lean mass means the non-fat part of you, mostly muscle and water. In 16 men on a 10-day low-calorie fast, scan lean mass fell 3.53 kg, about 60% of total weight lost. But 44% of that was water outside the cells, 14% was stored sugar in muscle and liver with its water, and only 42% was working lean tissue. That came to 1.5 kg, or 25% of total weight lost.

In 13 men through 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 ended 17.2% down. Stored sugar holds about three times its own weight in water, so the lean part of a scan moves with water before it moves with protein.

What can be measured at 48 hours is protein flow, not mass. At 72 hours, the forearm muscles start leaking protein building blocks, and a key muscle-building switch (mTOR) drops by about 50%. Muscle loss then comes more from building slowing down than from breakdown speeding up. Across a 10-day fast, protein burned (measured as nitrogen in the urine) dropped 41% by day 5 and then held steady. So the protein-saving shift arrives after this window, not inside it.

The body fat warning applies strongly to anyone lean. Protein lost per kg of weight lost 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.

Doing it

This is where you start losing salt faster than you take it in
In the hospital ward study, salt in the urine started rising after about 48 hours and went above what people took in. They lost 203 mEq more salt than they took in across week one, on average. The peak came at day 4, at 109 mmol a day on a fixed intake of 51 mmol a day. The salt-holding hormone aldosterone cannot stop it. Given on day 4 of a fast, it had only about 40% of its normal salt-holding effect.
Blood tests that will look abnormal
Blood sugar falls about 25% between 12 and 72 hours. Insulin roughly halves. In a lab insulin test at 48 hours, sugar burning crashed while sugar storage did not change. Active thyroid hormone is already down, and the brain's thyroid signal is pushed down from 30 hours. None of these can be read as your normal levels until after you eat again.
Do not rely on symptoms
Glucose monitors on 12 healthy adults through a 7-day water fast showed a daily low of 50 mg/dL by day 5, with no symptoms of low blood sugar reported. In a healthy person the brain switches to ketones, so a low reading is mostly silent. That also means anyone on a blood-sugar-lowering drug has lost their warning system.
Uric acid is a real and predictable rise
Uric acid in the blood roughly doubles by day 7 of a long fast, and the rise tracks how deep into ketosis you are. In 1,610 people, blood uric acid rose 100 micromol/L in those with low urine ketones and 200 in those with high urine ketones. Ketones and uric acid compete for the same exit route in the kidneys, which is why drinking more water does not fix it.
Training
Community accounts describe lifting on day two as clearly weaker, with less stamina. No study has tested weight training during a 48-hour fast. What has been measured: in a 10-day low-calorie fast with up to 3 hours a day of light activity, daily steps rose 60%. Strength held steady in muscles that do not carry body weight, and rose 33% in muscles that do.
Gallbladder
The gallbladder squeezes when fat and protein reach the gut. On a water fast it does not squeeze, so bile sits and thickens. On a 500-calorie-a-day diet, the one-year risk of gallstones bad enough to need hospital care was about 1.5%, against 0.44% on 1,200 to 1,500 calories. That is 3.4 times the rate. For every 92 people on the very-low-calorie diet, one extra person needed hospital care for gallstones. No study that good exists for water fasting, and it is better to name that gap than to fill it with guesses.

Breaking the fast

Two days is short of every published refeeding risk line for a well-fed person. But it is long enough that you should understand why those lines exist before your first meal, not after it. During a fast, phosphate, potassium and magnesium leave the body while blood levels stay normal. The clearest proof is a 40-day hospital ward study of 6 people. 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. That pushes sugar, phosphate, potassium and magnesium into the cells all at once, and raises the body's need for thiamine (vitamin B1) at the same moment. That is why the deaths in older case reports happened mostly during refeeding, not during the fast. One death came during refeeding after a 3-week fast. Another came on day 7 or 8 of eating again after a 210-day fast. And in a hospital series, two of three crises involved heavy fluid build-up on refeeding. At 48 hours in a healthy person, the real problems are comfort, not danger, and people describe them the same way again and again. Bloating, cramps, diarrhoea, nausea, and heavy sleepiness within an hour of a carb-heavy first meal. Plus overeating over the next day. The community habit is a small, warm, low-carb first course, a 30 to 60 minute wait, then a meal at about half the normal size. Buy the refeed food before the fast ends, so you are not deciding while hungry. None of that has been tested against anything else. It is here as what people do, not as something shown to work.

Sources

  1. Study in people1992
    Augmented growth hormone (GH) secretory burst frequency and amplitude mediate enhanced GH secretion during a two-day fast in normal men

    Journal of Clinical Endocrinology and Metabolism

    9 healthy men had growth hormone checked every 5 minutes for 24 hours on a normal day and on day two of a fast. Growth hormone output went up five-fold, from more bursts that were bigger and closer together. How fast it cleared from the blood did not change. IGF-1, a growth signal, was unchanged after 56 hours of fasting.

    9 normal men, serum growth hormone sampled every 5 minutes for 24 hours on a fed control day and on the second day of a fast, analysed by multi-parameter deconvolution. 24 hour production rose from 78 ± 12 to 371 ± 57 micrograms per litre of distribution volume, a five-fold increase (p = 0.0001). Secretory bursts 14 ± 2.3 to 32 ± 2.4 per 24 hours (p = 0.0006); mass per burst 6.3 ± 1.2 to 11 ± 1.6 micrograms (p = 0.002); interval between volleys 143 ± 14 to 88 ± 4.2 minutes (p = 0.0001). Half-life unchanged at 18 ± 2.2 against 20 ± 1.5 minutes (p = 0.47). Serum IGF-1 after 56 hours of fasting was unchanged.

  2. Study in people1990
    The effect of starvation on insulin-induced glucose disposal and thermogenesis in humans

    Metabolism

    6 normal-weight men without diabetes had a lab insulin test after 48 hours without food. Their bodies took up much less sugar and burned far less of it, while the sugar they stored stayed about the same. The small rise in body heat the insulin test causes was no longer clear after starving. This paper did not measure resting calorie burn during fasting, though it is often cited as if it did.

    6 non-diabetic normal-weight men clamped at 100 mU/min/m2 insulin and 3.5 mmol/L glucose after 48 hours of starvation. Whole-body glucose disposal 39.8 ± 4.6 to 24.1 ± 2.1 micromol/kg/min (p < 0.01); glucose oxidation 21.8 ± 1.3 to 3.9 ± 1.4 (p < 0.001) while non-oxidative disposal was unchanged at 18.0 ± 3.9 against 20.2 ± 1.2; forearm glucose uptake 59.4 to 15.4 micromol/min/L of forearm (p < 0.01). Net thermogenesis during the clamp was significant fed (0.27 ± 0.08 kJ/min, p < 0.01) and not significant after starvation (0.11 ± 0.09). This paper measured clamp-induced thermogenesis, not resting metabolic rate during fasting, and is frequently miscited for the latter.

  3. Study in people2023
    The impact of prolonged fasting on 24h energy metabolism and its 24h rhythmicity in healthy, lean males: A randomized cross-over trial

    Clinical Nutrition

    12 healthy lean men spent 60 hours in a sealed room that measures calories burned, once fasting and once eating enough to hold their weight. Fasting cut calories burned by 10.9% on day 2 and 5.5% on day 3. Fat burning rose the whole time and peaked at 51 hours. The daily rhythm of calories burned did not change, but the rhythm of which fuels were burned did.

    12 healthy lean males in a respiration chamber for 60 continuous hours, randomised crossover against a 60 hour fed condition at energy balance. 24 hour energy expenditure was significantly lower fasting: 9.8 ± 0.2 against 11.0 ± 0.4 MJ on day 2 and 10.3 ± 0.3 against 10.9 ± 0.3 MJ on day 3, both p < 0.01, falls of 10.9% and 5.5%. Fat oxidation rose throughout and peaked at 51 hours at 160 mg/min while carbohydrate oxidation stabilised around 60 mg/min. The 24 hour rhythm of energy expenditure was unchanged by fasting; the rhythm of substrate oxidation was.

  4. Study in people2000
    Resting energy expenditure in short-term starvation is increased as a result of an increase in serum norepinephrine

    American Journal of Clinical Nutrition

    11 healthy lean people went 84 hours without food, with resting calorie burn measured each day. Resting burn rose 14.1% by day 3, and noradrenaline, a stress hormone, rose from 1716 to 3728 pmol/L by day 4. Blood sugar fell from 4.9 to 3.5 mmol/L, and insulin did not clearly change. This measured resting burn under a hood, not calories burned across a whole day.

    11 healthy lean subjects, resting energy expenditure by indirect calorimetry on days 1 to 4 of an 84 hour starvation period. Resting expenditure rose from 3.97 ± 0.9 to 4.53 ± 0.9 kJ/min by day 3 (p < 0.05), a 14.1% rise. Noradrenaline rose from 1716 ± 574 to 3728 ± 1636 pmol/L by day 4 (p < 0.05) and serum glucose fell from 4.9 ± 0.5 to 3.5 ± 0.5 mmol/L (p < 0.05). Insulin did not change significantly. This is hood-measured resting expenditure in lean subjects, not whole-day expenditure.

  5. Study in people2012
    The effect of short-term fasting on liver and skeletal muscle lipid, glucose, and energy metabolism in healthy women and men

    Journal of Lipid Research

    Healthy men and women fasted 48 hours while scans measured fat in the liver and muscle. Fat built up in the livers of men and in the muscles of women. How the body and the liver handled sugar and energy was the same in both sexes.

    Liver and muscle triglyceride measured by magnetic resonance spectroscopy across a 48 hour fast in healthy men and women. Triglyceride accumulated in the livers of men and in the muscles of women. Whole-body and hepatic glucose and oxidative metabolism responses were identical between the sexes.

  6. Study in people2000
    Human brain beta-hydroxybutyrate and lactate increase in fasting-induced ketosis

    Journal of Cerebral Blood Flow and Metabolism

    Scans measured ketones directly in the brains of healthy adults. Brain ketones went from 0.05 mmol/L when fed to 0.60 after the second day of fasting and 0.98 after the third. Brain lactate also rose, from 0.69 to 1.47 mmol/L. Brain levels tracked blood levels closely.

    Beta-hydroxybutyrate measured directly in the occipital lobe of healthy adults by 4-tesla magnetic resonance spectroscopy: 0.05 ± 0.05 mmol/L non-fasted, 0.60 ± 0.26 after the second day of fasting and 0.98 ± 0.16 after the third. Brain lactate rose from 0.69 ± 0.17 to 1.47 ± 0.22 mmol/L. Plasma and brain beta-hydroxybutyrate correlated at r = 0.86 with a brain-to-plasma slope of 0.26.

  7. Study in people2018
    The effects of short-term fasting on quality of life and tolerance to chemotherapy in patients with breast and ovarian cancer: a randomized cross-over pilot study

    BMC Cancer

    34 patients with breast or ovarian cancer fasted for 60 hours around some chemo cycles, from 36 hours before to 24 hours after, and ate normally around others. Quality of life and tiredness got less worse in the fasting cycles. In one group, tiredness scores worsened by 10.4 fasting against 27.0 not fasting, and in the other by 11.0 against 14.1. There were no serious side effects, and the authors declare no conflicts.

    34 patients with breast and ovarian cancer, individually randomised crossover, a 60 hour fast from 36 hours before to 24 hours after chemotherapy. Quality-of-life deterioration was below the minimally important difference during fasted cycles and above it during non-fasted cycles: mean fatigue-scale deterioration 10.4 ± 5.3 fasted against 27.0 ± 6.3 non-fasted in group A and 11.0 ± 5.6 against 14.1 ± 5.6 in group B. No serious adverse effects. The authors declare no competing interests.

  8. Study in people2016
    Safety and feasibility of fasting in combination with platinum-based chemotherapy

    BMC Cancer

    20 people, median age 61 and 85% women, fasted 24, 48 or 72 hours before platinum chemo, to test safety. Fasting was doable, and its side effects were all mild to moderate, with none of the serious kind. Less DNA damage in white blood cells and fewer severe drops in white cells in the longer fasts were not clear results. Nobody ate normally for comparison, so the authors say it cannot show fasting reduces side effects.

    Dose escalation of fasting before platinum-based chemotherapy in 20 patients, median age 61, 85% women, in three cohorts fasting 24, 48 or 72 hours. Feasibility was met and fasting-related toxicities were all grade 2 or below, with no grade 3 event attributed to fasting. Leukocyte DNA damage by COMET assay was reduced at 48 hours or more but at p = 0.08, not significant, and a trend to less grade 3/4 neutropenia in the longer cohorts was p = 0.17. IGF-1 fell 30%, 33% and 8% in the 24, 48 and 72 hour cohorts after the first fast. No control arm ate normally, and the authors state they cannot address whether fasting reduces side effects.

  9. Study in people1990
    Relationship between natriuresis and changes in plasma atrial natriuretic factor, renin activity and aldosterone levels in fasting obese subjects

    Diabete et Metabolisme

    8 people with obesity fasted 7 days on a fixed salt intake. Sodium in the urine rose from 35 to 109 mmol a day by day 4, then fell back. A heart hormone that makes the kidneys shed salt (atrial natriuretic factor) fell 36%, and aldosterone rose. So that hormone does not explain the salt loss.

    8 obese subjects fasted 7 days on a constant 51 mmol/day sodium intake. Urinary sodium rose from 35 ± 7 to 109 ± 8 mmol/day after 4 days (p < 0.001), then declined. Plasma atrial natriuretic factor fell 36% and aldosterone rose, so the natriuresis is not explained by atrial natriuretic factor.

  10. Study in people2024
    Long-Term Fasting-Induced Ketosis in 1610 Subjects: Metabolic Regulation and Safety

    Nutrients

    1,610 people fasted 4 to 21 days on about 75 to 250 calories a day. More than 95% had ketones in their urine from day 4 on. Blood uric acid rose by 200 micromol/L in people with the most ketones, against 100 in those with the fewest. People who had only soup, with no fruit juice or honey, ate less and made more ketones.

    1,610 subjects fasting 4 to 21 days on roughly 75 to 250 kcal/day. Ketonuria was detectable in more than 95% of subjects from day 4 onward. Blood uric acid rose 100 ± 4.52 micromol/L in the low-ketonuria group against 200 ± 4.88 micromol/L in the high-ketonuria group (p = 1.1 x 10^-44), with change in ketonuria correlating with change in uric acid at r = 0.48. Subjects taking only soup, without fruit juice or honey, had lower intake and more intense ketonuria.

  11. Study in people2014
    Risk of symptomatic gallstones and cholecystectomy after a very-low-calorie diet or low-calorie diet in a commercial weight loss program: 1-year matched cohort study

    International Journal of Obesity

    3,320 people on a 500-calorie diet for 6 to 10 weeks were matched with 3,320 people eating 1,200 to 1,500 calories, and followed for about a year. The very low-calorie group had more gallstones needing hospital care, 48 cases against 14, and more gallbladder removals, 29 against 9. Most of the extra risk came from how fast people lost weight, not what the diet was made of.

    3,320 people on 500 kcal/day for 6 to 10 weeks matched to 3,320 on 1,200 to 1,500 kcal/day, 6,361 person-years of follow-up. Symptomatic gallstones requiring hospital care: 48 against 14 cases, 152 against 44 per 10,000 person-years, hazard ratio 3.4 (95% CI 1.8 to 6.3, P < 0.001), number needed to harm 92. Cholecystectomy 29 against 9, hazard ratio 3.2 (95% CI 1.5 to 6.8, P = 0.003). Adjusting for 3-month weight loss attenuated the gallstone hazard ratio to 2.5, so most of the effect runs through the rate of weight loss rather than diet composition.

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.

  • Day two described as the wall: the worst headache, the lowest energy and the most quitting. People describe nearly stopping several times.
  • The wall breaking somewhere between hours 48 and 72, felt as a clear switch rather than a slow change.
  • Nail-polish breath (acetone) and a metal taste, strong by the end of day two. People say brushing does not fix it.
  • Dizziness when standing up fast. Almost everyone advises standing up in stages.
  • Shorter, lighter sleep on the second night, often waking wide awake at 3am or 4am, with vivid dreams that are often about food.
  • Lifting feeling clearly weaker, with less stamina, while light walking feels fine and often pleasant.
  • A real split on day two energy. Some call it the flattest day of any fast. Others describe a clear lift in the afternoon.

Sources: These are self-reports from public fasting communities, with no comparison group and no count of how many people tried. People who have a bad time often leave instead of posting, so these reports make it sound easier than it is. Two items have supervised matches. In a review of water-only fasting, feeling faint or light-headed was recorded at 217 of 768 visits, and trouble sleeping at 257 of 768, with 37 of those rated severe.

Who this is wrong for

  • Anyone taking an SGLT2 inhibitor, unless a prescriber has stopped it at least 3 days ahead, or 4 for ertugliflozin. That is the same lead time the FDA advises before planned surgery. The ketoacidosis (dangerous acid build-up) it causes happens while blood sugar looks normal. A urine strip will not catch it, because it tests for a different ketone. One published case shows the risk lasting after the drug was stopped. If nausea, vomiting, belly pain, gasping for air or confusion show up, the tests that matter are a blood gas and a blood ketone (beta-hydroxybutyrate) level.
  • Anyone on insulin, a sulfonylurea or a meglitinide, unless a prescriber changes the dose first. Do not assume a dose cut keeps you safe: a 50% cut in long-acting insulin and a 70% cut 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, because the risk runs both ways at once.
  • Anyone with primary adrenal insufficiency. In a study of 30 well-treated, well-taught patients who tried both fasting and not fasting, 67% had problems while fasting against 23% when not fasting. 37% broke the fast because of a problem, even though glucose monitors showed no difference in low blood sugar.
  • Pregnancy and breastfeeding. The published Ramadan research is about fasting from dawn to sunset with eating at night. It says nothing about fasting for days, when the baby and milk-making need sugar all the time.
  • Anyone with a past eating disorder, or who now has episodes of eating they cannot control.
  • Children and teens.
  • Anyone with a BMI under 18.5, or who already meets the UK NICE risk list for refeeding problems. One large supervised fasting clinic turns away anyone with severe wasting or anorexia outright. It also turns away people with advanced kidney, liver or brain blood-flow problems, and people with dementia.
  • Anyone with chronic kidney disease on an ACE inhibitor, ARB or water pill. A blood marker of kidney strain (creatinine) rose in 60.4% of 131 fasting kidney patients by day 7. People on this family of blood pressure drugs were twice as likely to have the rise. Serious heart problems happened in six fasting patients against one who was not fasting, and the early creatinine rise predicted them.
  • Anyone with gout. Uric acid rises during fasting are big and happen to almost everyone. The one gout attack recorded in a supervised group of 1,422 people happened to a 72-year-old man who was already on allopurinol and had frequent attacks before.
  • Anyone with known heart disease who is not under supervision. In the same group of 1,422 people, a 75-year-old man with known heart artery disease had a heart attack on fasting day 9. In older case reports, two of the deaths were in people with severe heart failure before the fast, which at first seemed to get better with fasting.
  • Anyone taking lithium who will not be drinking freely, and anyone who takes an anti-inflammatory painkiller (NSAID) long term.

Questions

How much does growth hormone actually rise on a 48 hour fast?
Five times as much made over 24 hours, from 78 to 371 micrograms per litre, in 9 healthy men on the second day of a fast. That came from about twice as many bursts and about twice as much hormone per burst. How long it lasted in the blood did not change, so the body was really making more, not clearing it slower. The 1,300% and 2,000% figures come from a 2011 press release about a 24-hour fast. The same release says LDL cholesterol rose 14%.
Does IGF-1 fall at 48 hours?
No. IGF-1 in the blood had not changed at 56 hours in the same men whose growth hormone output rose five times. It falls later. By day 5 of a 5-day fast it went from 1.31 to 0.77 U/mL in 6 men, about a 41% drop. Lower IGF-1 is a multi-day effect, not a 48-hour one.
Does my metabolism speed up or slow down over two days?
Total energy burned across the day, measured in a sealed room, fell 10.9% on day 2 and 5.5% on day 3 in 12 lean men. Resting energy burn measured under a hood in a separate study rose 14.1% by day 3 in 11 lean people. Both are real. Resting burn can rise while total daily burn falls, because you are no longer burning energy digesting food, and you move around less. The blanket claim that fasting does not slow metabolism is not backed up when the whole day is measured.
Why did my glucose tolerance get worse?
In a lab insulin test at 48 hours, the whole body took up about 40% less sugar, and sugar burning fell more than 80%. But storing sugar did not change at all. That is the body choosing not to burn sugar because it is burning fat. It is not your tissues failing to handle sugar. It goes back to normal when you eat again. It is also why a sugar tolerance test taken during or right after a fast this long cannot be read normally.
Is autophagy happening at 48 hours?
Nobody has measured it in a person at that hour. The measurements in human tissue are at 24 and 36 hours, where the muscle markers fell, and at 72 hours, where one marker (LC3B-II) rose about 30% but another (p62) rose too. The authors say that makes it impossible to tell from those markers how fast clean-up was running. Between 36 and 72 hours there is no measurement in any human tissue, so this page does not claim a 48-hour fast reaches autophagy.