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Extended11 human studies 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 without calories, and the best-characterised hormonal window in the whole fasting literature. Growth hormone output rises five-fold on day two while IGF-1 does not move, and whole-day energy expenditure measured in a chamber is 10.9% lower rather than higher.

Overview

48 hours is where the endocrinology gets specific. Nine normal men had serum growth hormone sampled every 5 minutes for 24 hours on a fed day and again on the second day of a fast, with deconvolution analysis, and the result is the cleanest hormonal measurement in this field: a five-fold rise in 24 hour production, driven by more secretory bursts and more hormone per burst, with unchanged clearance.

The same paper measured serum IGF-1 at 56 hours and found it unchanged. That line is almost always dropped. The growth factor growth hormone normally works through does not follow it up on this timescale, and does not fall until around day 5.

Two days is also where the metabolism claim can be tested properly, because a randomised crossover put 12 lean men in a respiration chamber for 60 continuous hours. Whole-day energy expenditure fell 10.9% on day 2 and 5.5% on day 3 against a fed condition at energy balance, while fat oxidation kept rising and peaked at 51 hours.

How it works

Two full days without calories, most often dinner on day one to dinner on day three, with non-caloric fluids. In the trials the protocol was defined by what was measured rather than by a schedule: 48 hours of starvation before a hyperinsulinaemic clamp in 6 men, 48 hours before magnetic resonance spectroscopy of liver and muscle in men and women, 60 hours in a respiration chamber, a 60 hour fast from 36 hours before to 24 hours after chemotherapy in a randomised crossover pilot, and a 48 hour cohort inside a chemotherapy fasting dose-escalation.

The practical shape people describe is that day two contains the wall: the worst headache, the lowest energy and the highest quit rate, with the report that it breaks somewhere between hours 48 and 72. That is self-report, and the same people caution that the day-two experience varies fast to fast for the same person.

This is also the duration at which the natriuresis of fasting turns. In 9 obese women fasting on maintained salt intake, sodium excretion began to rise after roughly 48 hours and then exceeded intake, with a mean cumulative negative sodium balance of 203 mEq across week one, range 107 to 232. The peak comes later: in 8 obese subjects on a constant 51 mmol/day sodium intake, urinary sodium rose from 35 ± 7 to 109 ± 8 mmol/day by day 4 (p < 0.001) and then declined.

What 2 days actually reaches

Growth hormone first, with the real numbers. In 9 normal men on the second day of a fast, 24 hour endogenous growth hormone production rose from 78 ± 12 to 371 ± 57 micrograms per litre of distribution volume (p = 0.0001), a five-fold increase. Secretory bursts went from 14 ± 2.3 to 32 ± 2.4 per 24 hours (p = 0.0006), the mass per burst from 6.3 ± 1.2 to 11 ± 1.6 micrograms (p = 0.002), and the interval between secretory volleys from 143 ± 14 to 88 ± 4.2 minutes (p = 0.0001). Growth hormone half-life was 18 ± 2.2 fasted against 20 ± 1.5 minutes fed (p = 0.47), so clearance did not change and the rise is genuine secretion. Serum IGF-1 after 56 hours of fasting was unchanged. Insulin sensitivity falls at 48 hours, and the shape of the fall is informative. Under a clamp at 100 mU/min/m2 insulin and 3.5 mmol/L glucose in 6 non-diabetic men, whole-body glucose disposal fell from 39.8 ± 4.6 to 24.1 ± 2.1 micromol/kg/min (p < 0.01) and glucose oxidation collapsed from 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 fell from 59.4 to 15.4 micromol/min/L of forearm (p < 0.01). Oxidative disposal collapsed and storage disposal did not, which is a fuel-selection change rather than glucose intolerance in the diabetic sense. Brain beta-hydroxybutyrate measured directly in the occipital lobe by 4-tesla magnetic resonance spectroscopy had reached 0.60 ± 0.26 mmol/L after the second day, from 0.05 ± 0.05 non-fasted. Gluconeogenesis is supplying 96 ± 1% of glucose production from 36 hours onward. And there is a sex difference in where fat goes: across a 48 hour fast, triglyceride accumulated in the livers of men and in the muscles of women, while whole-body and hepatic glucose and oxidative metabolism responses were identical between the sexes. No human study has measured autophagy in any tissue between 36 and 72 hours. This page does not claim that a 48 hour fast reaches autophagy, and claims mitophagy at no duration, because no human trial has measured mitophagy in response to any fast.

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

    Still absorbing

    Burning
    Glucose from the meal just eaten, with insulin high enough to hold fat release down
    • Insulin is at its post-meal peak, and while it is there, adipose tissue lipase stays suppressed and fat is being stored rather than released.
    • The hour count on most fasting charts starts at the last bite. Absorption of a mixed meal is still running here, so the clock and the physiology are not yet the same thing.

    Read the biologyInsulin and insulin sensitivity during a fast

    Measured in people
    The human hour-by-hour fasting series begin later than this. Klein 1993 took its first sample at 12 hours in 6 healthy men, and Rothman 1991 reports its first gluconeogenesis figure as an average across the first 22 hours. This row is the fed state those measurements are compared against, and no extended-fasting measurement exists inside it.

  2. 4h to 12h

    Post-absorptive

    Burning
    A mix of remaining glucose and fatty acids, with the liver covering the brain's glucose demand
    Blood glucose
    5.58 ± 0.08 mmol/L (100.5 mg/dL) at the 12 hour mark, in 6 healthy men
    Ketones (BHB)
    Total ketone bodies around 0.20 mmol/L in overnight-fasted subjects. That figure is acetoacetate plus beta-hydroxybutyrate, not BHB on its own.
    Insulin
    64.6 ± 12.9 pmol/L (9.3 microunits/mL) at 12 hours, in the same 6 men
    • Glycerol release runs at 2.08 ± 0.22 and palmitic acid release at 1.63 ± 0.20 micromol/kg/min at 12 hours. Those are the numbers everything later in the fast is measured against.
    • Gluconeogenesis is already the majority contributor to glucose production rather than a reserve waiting to be called on, which is the first place the popular switch model breaks.

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

    Measured in people
    Klein 1993 measured glucose, insulin and stable-isotope lipid kinetics at 12 hours in 6 healthy men. Féry 1983 measured total ketone body turnover in overnight-fasted subjects and put the baseline at 0.20 mmol/L. Rothman 1991 measured hepatic glycogen serially by 13C nuclear magnetic resonance through a 68 hour fast in healthy adults and calculated that gluconeogenesis supplied 64 ± 5% of total glucose production across the first 22 hours.

  3. 12h to 18h

    Ketones have not moved yet

    Burning
    Fatty acids rising, with glucose production increasingly made rather than released from store
    Ketones (BHB)
    Median change of 0% between 12 and 18 hours, sampled every 6 hours in 34 adults
    • Beta-hydroxybutyrate does not move in this window. In 34 adults sampled every 6 hours through the standard diagnostic 72 hour fast, the median change from 12 to 18 hours was zero.
    • Gluconeogenesis is supplying 64 ± 5% of glucose production across the first 22 hours, confirmed by an orthogonal method at about 47% by 14 hours. Glucose production does not switch from glycogen to fat at a threshold hour.
    • Whole blood LC3A messenger RNA was 22 ± 5% higher at 18 hours than in a 12 hour control arm in 11 overweight adults (p = 0.001). In the same subjects MTOR expression also rose, by 9 ± 3%.

    Read the biologyGlycogen depletion and gluconeogenesisKetogenesis and beta-hydroxybutyrateAutophagy

    Measured in people
    Service 2005 measured beta-hydroxybutyrate every 6 hours in 34 adults whose 72 hour fasts were negative for insulinoma. Rothman 1991 measured the gluconeogenic share by 13C NMR. Jamshed 2019 measured whole blood messenger RNA in an 11-person 4 day randomised crossover of an 08:00 to 14:00 eating window against 08:00 to 20:00. That is one gene's expression in blood cells, not autophagic flux and not muscle. No study in any species has tested a 16 hour timepoint for autophagy.

  4. 18h to 1d

    The steepest hours for fat release

    Burning
    Fatty acids and glycerol, with ketone production starting its climb
    Ketones (BHB)
    Climbing. The median rise from 18 to 36 hours was +333%, the steepest proportional stretch of a 72 hour fast.
    Insulin
    70% of the entire decline seen across a 72 hour fast has already happened by 24 hours
    • The largest single interval increase in lipolysis across a whole 72 hour fast falls between 18 and 24 hours, and 60% of the total rise in lipid kinetics happens between 12 and 24 hours. The authors attribute that to the falling insulin rather than to any change in glucose.
    • Growth hormone was significantly higher after a 24 hour water-only fast than after a fed day in a randomised crossover of 30 healthy adults (p = 1.1 x 10^-4). The size of that rise is not in the accessible record and is not published here.
    • Haemoglobin, red cell count and haematocrit all rose together at 24 hours in the same trial. That combination is the signature of plasma volume contraction, not of new red cells.
    • 24 hour energy expenditure measured in a whole-room calorimeter fell during 24 hour fasting in 20 volunteers, with the size of the fall varying between individuals.

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

    Measured in people
    Horne 2013 randomised 30 apparently healthy adults to a 24 hour water-only fast or a day of usual eating in crossover and measured growth hormone, blood counts, cholesterol, triglycerides, bicarbonate and weight. Klein 1993 measured the lipid kinetics by stable isotope tracer. Hollstein 2020 measured 24 hour energy expenditure by whole-room calorimetry in 20 volunteers; the exact percentage decrease is not in the accessible record and is not published here. On autophagy, a 24 hour fast raised p62/SQSTM1 in the vastus lateralis of 50 women, which is movement in the direction of less autophagic degradation rather than more.

  5. 1d to 36h

    Thyroid drops before metabolic rate does

    Burning
    Fat, with the remaining glucose built from glycerol, lactate and amino acids
    Ketones (BHB)
    1.308 ± 1.053 mmol/L after day 1 in 13 men beginning a 10 day water fast, from a baseline of 0.177 ± 0.044. The spread on that mean is as wide as the mean.
    • Gluconeogenesis supplies 82 ± 5% of glucose production between 22 and 36 hours.
    • Serum T3 fell from 2.30 ± 0.06 to 1.84 ± 0.03 nmol/L by 30 hours in 8 healthy men (p < 0.01), TSH fell below 1 mU/L and the nocturnal TSH peak was abolished. Serum cortisol did not differ across conditions.
    • Thyroid suppression therefore begins inside the first 30 hours, well before any measurable fall in resting metabolic rate.
    • Sodium excretion has not turned yet. In 9 obese women fasting with pre-fast salt intake maintained, urinary sodium began to rise after roughly 48 hours and then exceeded intake.

    Read the biologyGlycogen depletion and gluconeogenesisKetogenesis and beta-hydroxybutyrateAutophagy

    Measured in people
    Rothman 1991 measured the gluconeogenic share by 13C NMR in healthy adults. Hugues 1984 fasted 8 healthy men for 30 hours with serial thyroid sampling. Dai 2022 measured blood beta-hydroxybutyrate daily in 13 men fasting in a controlled facility. Sigler 1975 collected urine in 3 hour blocks from 9 obese women on a metabolic ward. On autophagy, the only human study with several within-fast timepoints took vastus lateralis biopsies at 2, 12, 24 and 36 hours: LC3I, LC3II and p62 all fell, in untrained subjects only, which the authors summarise as skeletal muscle autophagy being only modestly affected by 36 hours of fasting.

  6. 36h to 2d

    Growth hormone multiplies, IGF-1 does not follow

    Burning
    Fat, with ketones now carrying a real share of the load and glucose production almost entirely gluconeogenic
    Ketones (BHB)
    Median +210% between 36 and 54 hours in 34 adults
    • 24 hour growth hormone production rose from 78 ± 12 to 371 ± 57 micrograms per litre of distribution volume on the second day of a fast in 9 normal men (p = 0.0001), a five-fold rise. Secretory bursts went from 14 ± 2.3 to 32 ± 2.4 per 24 hours and the mass per burst from 6.3 ± 1.2 to 11 ± 1.6 micrograms.
    • Growth hormone half-life was unchanged at 18 ± 2.2 against 20 ± 1.5 minutes (p = 0.47), so this is genuine secretion rather than slower clearance.
    • Serum IGF-1 was unchanged at 56 hours. It does not fall until around day 5, when it dropped from 1.31 ± 0.22 to 0.77 ± 0.18 U/mL in 6 men.
    • Gluconeogenesis supplies 96 ± 1% of glucose production from 36 hours onward.
    • Under an insulin clamp at 48 hours, whole-body glucose disposal fell from 39.8 ± 4.6 to 24.1 ± 2.1 micromol/kg/min (p < 0.01) and glucose oxidation collapsed from 21.8 ± 1.3 to 3.9 ± 1.4 (p < 0.001), while non-oxidative disposal was unchanged. That is a change in fuel selection, not glucose intolerance in the diabetic sense.
    • 24 hour energy expenditure in a respiration chamber was 10.9% lower on day 2 of a fast than in a fed condition at energy balance (9.8 ± 0.2 against 11.0 ± 0.4 MJ, p < 0.01).

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

    Measured in people
    Hartman 1992 sampled serum growth hormone every 5 minutes for 24 hours in 9 normal men, on a fed day and on the second day of a fast, and applied multi-parameter deconvolution; IGF-1 was measured at 56 hours. Ho 1988 measured somatomedin C through a 5 day fast in 6 men. Mansell 1990 clamped 6 non-diabetic men at 48 hours of starvation. Andriessen 2023 put 12 healthy lean males in a respiration chamber for 60 continuous hours in a randomised crossover against a fed condition. No human study has measured autophagy in any tissue between 36 and 72 hours, which is why autophagy is not among the mechanisms linked from this row.

  7. 2d to 3d

    Ketones reach the brain

    past this window
    Burning
    Fat and ketone bodies, with the brain now taking a measurable share of its fuel as beta-hydroxybutyrate
    Blood glucose
    4.14 ± 0.10 mmol/L (74.6 mg/dL) at 72 hours in 6 healthy men, about 25% below the 12 hour value
    Ketones (BHB)
    74% of people whose diagnostic fast was negative exceeded 2.7 mmol/L before the 72 hour mark. The median rise from 54 to 72 hours was +167%.
    Insulin
    30.1 ± 7.9 pmol/L (4.3 microunits/mL) at 72 hours, roughly half the 12 hour value (p < 0.001)
    • Brain beta-hydroxybutyrate, measured directly in the occipital lobe by 4-tesla magnetic resonance spectroscopy, rose from 0.05 ± 0.05 mmol/L non-fasted to 0.60 ± 0.26 after the second day 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.
    • Across a 48 hour fast, triglyceride accumulated in the livers of men and in the muscles of women, while whole-body and hepatic glucose and oxidative responses were identical between the sexes.
    • The two energy expenditure measurements diverge here and both are real. Hood-measured resting expenditure rose 14.1% by day 3 in 11 lean subjects, while chamber-measured 24 hour expenditure was still 5.5% below the fed condition on day 3 (10.3 ± 0.3 against 10.9 ± 0.3 MJ, p < 0.01). The resting component can rise while the whole day falls.
    • The 72 hour supervised fast is a decades-old routine diagnostic procedure in endocrinology, which is the strongest available evidence that healthy people tolerate this duration under observation.

    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 with stable isotope tracers and indirect calorimetry. Service 2005 measured beta-hydroxybutyrate every 6 hours in 34 adults. Pan 2000 measured brain beta-hydroxybutyrate and lactate by magnetic resonance spectroscopy in healthy adults. Browning 2012 measured liver and muscle triglyceride by spectroscopy across 48 hours in healthy men and women. Zauner 2000 measured resting expenditure by hood in 11 lean subjects and Andriessen 2023 measured 24 hour expenditure in a chamber in 12 lean men, and the two are measuring different quantities rather than contradicting each other.

Human evidence

One 5-minute-sampling growth hormone study with deconvolution, one hyperinsulinaemic clamp, one respiration chamber crossover, one magnetic resonance spectroscopy study of tissue fat by sex, one brain spectroscopy series, and two oncology trials that used fasts of 48 to 60 hours.

  • Hartman 1992, 9 normal men, serum growth hormone every 5 minutes for 24 hours on a fed day and on the second day of a fast, multi-parameter deconvolution. 24 hour production 78 ± 12 to 371 ± 57 micrograms per litre of distribution volume (p = 0.0001); bursts 14 ± 2.3 to 32 ± 2.4 (p = 0.0006); mass per burst 6.3 ± 1.2 to 11 ± 1.6 micrograms (p = 0.002); half-life unchanged, 18 ± 2.2 against 20 ± 1.5 minutes (p = 0.47). Serum IGF-1 unchanged at 56 hours (Journal of Clinical Endocrinology and Metabolism, 1992).
  • Mansell 1990, 6 non-diabetic normal-weight men clamped 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); non-oxidative disposal unchanged; forearm glucose uptake 59.4 to 15.4 micromol/min/L (p < 0.01). Net thermogenesis during the clamp was significant fed at 0.27 ± 0.08 kJ/min (p < 0.01) and not significant after starvation at 0.11 ± 0.09 (Metabolism, 1990).
  • Andriessen 2023, 12 healthy lean males in a respiration chamber for 60 continuous hours in a randomised crossover against a 60 hour fed condition at energy balance. 24 hour energy expenditure 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 kept rising throughout, peaking at 51 hours at 160 mg/min, while carbohydrate oxidation stabilised around 60 mg/min. The 24 hour rhythm of energy expenditure was unchanged; the rhythm of substrate oxidation was not (Clinical Nutrition, 2023).
  • Browning 2012 measured liver and muscle triglyceride 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, while whole-body and hepatic glucose and oxidative metabolism responses were identical between sexes (Journal of Lipid Research, 2012).
  • Pan 2000 measured beta-hydroxybutyrate 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, 0.98 ± 0.16 after the third. Plasma and brain correlated at r = 0.86 with a brain-to-plasma slope of 0.26 (Journal of Cerebral Blood Flow and Metabolism, 2000).
  • Bauersfeld 2018, an individually randomised crossover pilot in 34 patients with breast and ovarian cancer, using 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, with mean fatigue-scale deterioration of 10.4 ± 5.3 fasted against 27.0 ± 6.3 non-fasted in one group and 11.0 ± 5.6 against 14.1 ± 5.6 in the other. No serious adverse effects. The authors declare no competing interests (BMC Cancer, 2018).
  • Dorff 2016, a dose escalation of fasting before platinum-based chemotherapy in 20 patients across cohorts fasting 24, 48 or 72 hours. Feasibility was met and fasting-related toxicities were all grade 2 or below. 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. There was no normally eating control arm and the authors state they cannot address whether fasting reduces side effects (BMC Cancer, 2016).

What this does not tell you: The growth hormone work is 9 men and the clamp work is 6, both in healthy young males, and neither has been repeated at this sampling density. The chamber study is 12 lean men. The oncology trials are pilots: 34 patients in the crossover and 20 in the dose escalation, the latter with no control arm and a conflict-of-interest statement that could not be verified. Nothing here has a clinical endpoint in healthy people, and no trial has randomised repeated 48 hour fasts against a control.

Reading the research record

Two claims are usually attached to this duration 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 trace to a press release from a heart institute dated 3 April 2011, describing work presented at a cardiology conference across over 200 individuals in two studies plus a further 30 patients. The percentages are quoted accurately from that release. Three things travel badly: they are figures for a 24 hour fast rather than for multi-day fasting, they come from a conference presentation rather than a paper a reader can check, and the same release reports that LDL cholesterol rose 14% and HDL 6% during the fast. The peer-reviewed number for the second day of a fast is a five-fold rise in 24 hour production in 9 men, with IGF-1 unchanged at 56 hours.

The second is that metabolism speeds up. Zauner 2000 is a real study and is the origin of the claim: 11 healthy lean subjects, resting energy expenditure by hood, rising from 3.97 ± 0.9 to 4.53 ± 0.9 kJ/min by day 3 of an 84 hour fast (p < 0.05), a 14.1% rise, with noradrenaline going from 1716 ± 574 to 3728 ± 1636 pmol/L by day 4. Mansell 1990 is routinely cited alongside it for the same claim and measured something else entirely, clamp-induced thermogenesis at 48 hours, whose result points the other way. The stronger design for the whole-day question is the respiration chamber, and there 24 hour energy expenditure fell 10.9% on day 2 and 5.5% on day 3. Both findings are true and they are different quantities: the resting component can rise while whole-day expenditure falls, because the thermic effect of food is removed and spontaneous activity drops. Resting rate itself does eventually fall, by 20.3 ± 11.13% across a 21 day fast.

What happens to muscle

No trial has measured body composition across a 48 hour fast, which is the honest answer and also a reasonable design decision, because a scan taken at that point would be measuring water.

The reason is quantified in the 10 day studies. In 16 men on a 10 day modified fast, lean soft tissue fell 3.53 ± 0.13 kg, about 60% of total weight lost, but the decomposition was 44% extracellular water, 14% muscle and liver glycogen with its bound water, and only 42% metabolically active lean tissue, which came to 1.5 ± 0.1 kg or 25% of total weight loss. In 13 men through a 10 day complete water fast with DXA at baseline, fasting day 6 and recovery day 5, lean mass was down 9.2% at day 6 and back to baseline by recovery day 5 while fat mass ended 17.2% down. Glycogen binds roughly three times its own weight in water, so the lean compartment on a DXA moves with hydration before it moves with protein.

What is measurable at 48 hours is the protein flux rather than the mass. At 72 hours, forearm net phenylalanine release rises and skeletal muscle mTOR phosphorylation falls about 50%, with net muscle amino acid loss driven more by suppressed synthesis than by accelerated breakdown. Across a 10 day fast, protein oxidation measured as total nitrogen excretion dropped 41 ± 7% by day 5 and then held, which means the sparing adaptation arrives after this window rather than inside it.

The scaling caveat applies with force to anyone lean: nitrogen loss per kilogram of weight lost is roughly 20 g of nitrogen per kg in non-obese people against roughly 10 g per kg in people carrying 50 kg or more of fat.

Doing it

This is where sodium starts leaving faster than it arrives
In the metabolic ward study, sodium excretion began to rise after approximately 48 hours and ultimately exceeded intake, with a mean cumulative negative sodium balance of 203 mEq in week one. The peak came at day 4, at 109 ± 8 mmol/day on a constant 51 mmol/day intake. Aldosterone cannot hold it: infused on day 4 of a fast it produced only about 40% of its pre-fast antinatriuretic effect.
The chemistry that will look abnormal
Glucose falls about 25% between 12 and 72 hours. Insulin roughly halves. Under a clamp at 48 hours, glucose oxidation collapsed while non-oxidative disposal was unchanged. T3 is already down and TSH suppressed from 30 hours. None of these are interpretable as a baseline until after refeeding.
Symptoms are not a monitor
Continuous glucose monitors on 12 healthy adults through a 7 day water-only fast recorded a minimum daily glucose of 50 ± 7 mg/dL by day 5 with no symptoms of hypoglycaemia reported. In a healthy person the brain adapts to ketone fuel and the low reading is largely silent, which also means anyone on a glucose-lowering drug has lost their warning system.
Uric acid is a real and predictable rise
Serum urate roughly doubles by day 7 of a prolonged fast, and the rise scales with the depth of ketosis: in 1,610 subjects, blood uric acid rose 100 ± 4.52 micromol/L in the low-ketonuria group against 200 ± 4.88 in the high-ketonuria group (p = 1.1 x 10^-44). Ketoacids and urate compete for the same proximal tubular transport, which is why drinking more water does not fix it.
Training
Lifting at day two is described in community accounts as noticeably weaker with reduced work capacity. There is no trial of resistance training during a 48 hour fast. What is measured is that in a 10 day modified fast with up to 3 hours a day of low-intensity activity, daily step count rose 60% and strength was maintained in non-weight-bearing muscles and rose 33% in weight-bearing ones.
Gallbladder
Gallbladder contraction is driven by cholecystokinin, which is released by fat and protein reaching the duodenum, so a water fast means no contraction and bile sits concentrating. On a 500 kcal/day diet the one-year risk of a symptomatic gallstone needing hospital care was about 1.5% against 0.44% on 1,200 to 1,500 kcal, hazard ratio 3.4 (95% CI 1.8 to 6.3), number needed to harm 92. No cohort of that quality exists for water fasting, and that gap is worth naming rather than filling by inference.

Breaking the fast

Two days is short of every published refeeding risk threshold in a well-nourished person, and it is long enough that the mechanism behind those thresholds is worth understanding before the first meal rather than after it. During a fast, phosphate, potassium and magnesium leave the body while serum values stay normal. The cleanest demonstration is a 40 day metabolic ward study in 6 subjects where mean daily balances were -104 mg calcium, -48 mg magnesium and -363 mg phosphorus while serum phosphorus and magnesium did not change at all. Serum testing does not see the deficit. Reintroducing carbohydrate raises insulin, which drives glucose, phosphate, potassium and magnesium into cells simultaneously and raises thiamine demand at the same moment. That is why the deaths in the historical case literature cluster in the refeeding period rather than in the fast: a death during refeeding after a 3 week fast, a death on the 7th or 8th day of realimentation after a 210 day fast, and gross fluid retention on refeeding in two of three crises in an inpatient series. At 48 hours in a healthy person the realistic problems are comfort rather than danger, and they are consistently described: bloating, cramping, diarrhoea, nausea and a profound sleepiness within an hour of a carbohydrate-heavy first meal, plus reactive over-eating over the following day. The community practice is a small warm low-carbohydrate first course, a 30 to 60 minute wait, then a meal at roughly half normal size, with the refeed food bought before the fast ends so the decision is not made while hungry. None of that has been tested against an alternative, and it is offered here as what people do rather than as what has been shown to work.

Citations

  1. Human1992
    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 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. Human1990
    The effect of starvation on insulin-induced glucose disposal and thermogenesis in humans

    Metabolism

    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. Human2023
    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 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. Human2000
    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 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. Human2012
    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

    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. Human2000
    Human brain beta-hydroxybutyrate and lactate increase in fasting-induced ketosis

    Journal of Cerebral Blood Flow and Metabolism

    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. Human2018
    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 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. Human2016
    Safety and feasibility of fasting in combination with platinum-based chemotherapy

    BMC Cancer

    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. Human1990
    Relationship between natriuresis and changes in plasma atrial natriuretic factor, renin activity and aldosterone levels in fasting obese subjects

    Diabete et Metabolisme

    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. Human2024
    Long-Term Fasting-Induced Ketosis in 1610 Subjects: Metabolic Regulation and Safety

    Nutrients

    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. Human2014
    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 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 uncontrolled self-reports, not evidence. They are here because they tell you what to expect and what to watch for, which the trial literature does not. They cannot tell you whether anything works.

  • Day two described as the wall: the worst headache, the lowest energy and the highest quit rate, with reports of nearly stopping several times
  • The wall breaking somewhere between hours 48 and 72, described as a noticeable transition rather than a gradual one
  • Acetone breath and a metallic taste, described as strong by the end of day two, with brushing reported not to fix it
  • Orthostatic dizziness on standing quickly, with standing in stages as the near-universal advice
  • Shortened, lighter sleep on the second night, often waking at 3am or 4am wide awake, alongside vivid and frequently food-focused dreams
  • Lifting described as noticeably weaker with reduced work capacity, while light walking is reported as fine and often pleasant
  • A genuine split on day two energy, with one group describing it as the flattest day of any fast and another describing a distinct afternoon lift

Sources: Uncontrolled self-report gathered from public fasting communities, with no control group and no denominator. The people who have a bad time often leave rather than post, so tolerability is overstated. Two items have supervised counterparts: presyncope was recorded in 217 of 768 visits in a water-only fasting chart review, and insomnia in 257 of 768, with 37 of those graded 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, on the same lead time the FDA advises before scheduled surgery. The ketoacidosis this produces happens at a normal-looking glucose, a urine strip will not catch it because it measures acetoacetate rather than beta-hydroxybutyrate, and one published case documents the risk persisting after the drug was stopped. If nausea, vomiting, abdominal pain, air hunger or confusion appear, the relevant test is a venous blood gas and a beta-hydroxybutyrate level.
  • Anyone on insulin, a sulfonylurea or a meglitinide without a prescriber adjusting the dose in advance, and not on the assumption that a dose cut is protective: a 50% basal reduction and a 70% prandial reduction each still produced significant hypoglycaemia rates.
  • Anyone with type 1 diabetes outside a structured programme with continuous glucose monitoring and an insulin plan, because the risk runs in both directions at once.
  • Anyone with primary adrenal insufficiency. In a randomised crossover in 30 well-treated, well-educated patients, complications occurred in 67% during fasting against 23% outside it (P = 0.001), and 37% broke the fast because of one, while monitor-detected hypoglycaemia did not differ.
  • Pregnancy and breastfeeding. The published Ramadan literature is about a dawn-to-sunset pattern with a night eating window and says nothing about multi-day fasting, where the fetal and lactational glucose demand is continuous.
  • Anyone with a history of an eating disorder or current loss-of-control eating.
  • Children and adolescents.
  • Anyone with a BMI under 18.5 or meeting the NICE refeeding risk criteria on the way in. A large supervised fasting clinic excludes cachexia and anorexia nervosa outright, alongside advanced kidney, liver or cerebrovascular insufficiency and dementia.
  • Anyone with chronic kidney disease on an ACE inhibitor, ARB or diuretic. Creatinine rose in 60.4% of 131 fasting CKD patients by day 7, associated with RAAS antagonist use (relative risk 2, P = 0.002), and major adverse cardiovascular events occurred in six fasting patients against one non-fasting (P = 0.036), predicted by that early creatinine rise.
  • Anyone with established gout. Fasting urate rises are large and near-universal, and the one gout attack recorded in a supervised cohort of 1,422 people happened to a 72-year-old man already on allopurinol with frequent prior attacks.
  • Anyone with known cardiac disease who is not under supervision. In the same 1,422-subject cohort, a 75-year-old man with known coronary artery disease had a non-ST-elevation myocardial infarction on the 9th fasting day, and in the historical case series two of the fatal cases had severe pre-fast congestive heart failure that had initially appeared to improve with fasting.
  • Anyone taking lithium who will not be drinking freely, and anyone on a long-term NSAID.

Questions

How much does growth hormone actually rise on a 48 hour fast?
Five-fold in 24 hour production, from 78 ± 12 to 371 ± 57 micrograms per litre of distribution volume in 9 normal men on the second day of a fast (p = 0.0001), through roughly a doubling of both the number of secretory bursts and the mass per burst. Half-life was unchanged, so this is real secretion rather than slower clearance. The figures of 1,300% and 2,000% come from a 2011 press release about a 24 hour fast, and the same release reports LDL cholesterol rising 14%.
Does IGF-1 fall at 48 hours?
No. Serum IGF-1 was unchanged at 56 hours in the same men whose growth hormone output had risen five-fold. It falls later: by day 5 of a 5 day fast it went from 1.31 ± 0.22 to 0.77 ± 0.18 U/mL in 6 men, roughly a 41% drop. IGF-1 suppression is a multi-day effect, not a 48 hour one.
Does my metabolism speed up or slow down over two days?
Whole-day expenditure measured in a respiration chamber fell 10.9% on day 2 and 5.5% on day 3 in 12 lean men. Hood-measured resting expenditure in a separate study rose 14.1% by day 3 in 11 lean subjects. Both are real: the resting component can rise while total daily expenditure falls, because the thermic effect of food is gone and spontaneous activity drops. The blanket claim that fasting does not slow metabolism is not supported by whole-day measurement.
Why did my glucose tolerance get worse?
Under a clamp at 48 hours, whole-body glucose disposal fell about 40% and glucose oxidation fell more than 80%, while non-oxidative, storage disposal did not change at all. That is the body declining to burn glucose because it is burning fat, not the tissue failing to handle it. It reverses on refeeding, and it is the reason a glucose tolerance test taken during or immediately after a fast of this length is not interpretable.
Is autophagy happening at 48 hours?
Nobody has measured it in a person at that hour. The human tissue measurements sit at 24 and 36 hours, where the muscle markers fell, and at 72 hours, where LC3B-II rose about 30% but p62 rose too, which the authors say makes flux impossible to read from those markers. Between 36 and 72 hours there is no human measurement in any tissue, so this page does not claim a 48 hour fast reaches autophagy.