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Extended10 human studies cited

24-Hour Fast

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

Written by Reviewed Sep 2026

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

A single day without calories, and the shortest fast with a randomised crossover behind it. That trial measured a growth hormone rise, a blood count rise that is plasma volume rather than new red cells, and a triglyceride fall that had not returned to baseline a day later.

Overview

One day is the shortest window on this page that produces a distinct state rather than a longer version of overnight. What exists for it is one randomised crossover in 30 healthy adults, one whole-room calorimetry study in 20, two muscle biopsy studies that sampled at exactly this hour, and the hour-by-hour tracer work that brackets it.

The useful thing about 24 hours is how much has not happened yet. Beta-hydroxybutyrate barely moved through the first 18 hours and is only partway up its steepest climb. Gluconeogenesis has been the majority contributor to glucose production since the first day rather than switching on at a threshold. Against that, 70% of the entire insulin decline seen across a 72 hour fast is already complete, and the single largest interval increase in fat release across three days falls between hours 18 and 24.

One piece of context before starting: mild ketosis after a day without food is the base rate rather than an achievement. Among 1,535 non-diabetic patients fasted only for surgical preparation, 59.4% had postoperative ketonuria and 29.0% of the 929 with intraoperative blood gases showed metabolic acidosis.

How it works

Dinner to dinner, or waking to waking, with non-caloric fluids throughout. The randomised crossover used water only for 24 hours against a day of usual eating. The 8-week biopsy trial ran 24 hour fasts on three non-consecutive days a week in 50 women. The shortest cohort of the chemotherapy dose-escalation fasted 24 hours around treatment.

The medication review is the part of the setup that actually matters, and there is a drug-by-drug table written for exactly this duration. It advises skipping a sulfonylurea on a 24 hour fast day, one third of the usual basal insulin dose if control is good and half if it is not, skipping prandial insulin when no carbohydrate meal follows, and skipping an SGLT2 inhibitor on a fast day and certainly if dehydration is a concern. The same paper carries two lines worth reading twice: one study cut basal insulin by 50% on fasting days and still had significant hypoglycaemia rates, and another cut prandial insulin by 70% and still had significant hypoglycaemia rates.

Community practice at this duration is salt if a headache appears and nothing else, with potassium and magnesium widely described as unnecessary. That is practice, not evidence, and no trial has tested an electrolyte protocol at any fasting duration.

What 1 day actually reaches

In the randomised crossover, growth hormone was significantly higher after the fasting day than after the fed day (p = 1.1 x 10^-4). Haemoglobin (p = 4.8 x 10^-7), red cell count (p = 2.5 x 10^-6), haematocrit (p = 3.0 x 10^-6), total cholesterol (p = 5.8 x 10^-5) and HDL (p = 0.0015) all rose, while triglycerides (p = 1.3 x 10^-4), bicarbonate (p = 3.9 x 10^-4) and weight (p = 1.0 x 10^-7) fell. In the 16 subjects randomised to fast first, most markers including growth hormone and cholesterol had returned to baseline by 48 hours; weight and triglycerides had not. The three blood count measures rising together is the signature of plasma volume contraction rather than of erythropoiesis, and the paper's growth hormone effect size is not printed in the accessible record, so no percentage for it is published here. The insulin and lipid picture comes from serial tracer sampling in 6 healthy men. By 24 hours, 70% of the entire insulin decline measured across a 72 hour fast has already occurred, glycerol and palmitic acid release are climbing toward roughly double their 12 hour values, and 60% of that total rise happens between 12 and 24 hours with the largest single interval change between 18 and 24. Autophagy at 24 hours has been measured twice in human skeletal muscle, and this is the duration where the popular claim is most directly contradicted by the human record. In the vastus lateralis of 50 women, a 24 hour fast raised p62/SQSTM1, which moves in the direction of less autophagic degradation rather than more. In the only human study with several within-fast timepoints, LC3I, LC3II and p62 all fell at 24 and 36 hours, in untrained subjects only. Neither used a flux assay, and static markers count autophagosomes rather than measuring how fast material moves through them. This page therefore does not list autophagy as something a 24 hour fast reaches, and no protocol on this site claims mitophagy, 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

    past this window
    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.

Human evidence

One randomised crossover with a full biochemistry panel, one whole-room calorimetry study, two muscle biopsy studies at exactly 24 hours, the serial tracer work that brackets the hour, and a large surgical cohort that establishes the base rate of fasting ketosis.

  • Horne 2013, a randomised crossover in 30 apparently healthy volunteers comparing a one-day water-only fast against a day of usual eating. Growth hormone rose (p = 1.1 x 10^-4). Haemoglobin, red cell count, haematocrit, total cholesterol and HDL rose; triglycerides, bicarbonate and weight fell. In the 16 who fasted first, most markers including growth hormone and cholesterol were back to baseline at 48 hours while weight and triglycerides were not (Nutrition, Metabolism and Cardiovascular Diseases, 2013).
  • Hollstein 2020 measured 24 hour energy expenditure in a whole-room calorimeter during 24 hour fasting in 20 volunteers and reports a decrease, with the size varying between individuals and correlating with the thermogenic response to mild cold and with changes in FGF21. The exact percentage is not in the accessible record and is not published here (Diabetes, 2020).
  • Chaudhary 2022, 50 women, mean age 51, randomised to 24 hour fasts on three non-consecutive days a week at 70% or 100% of energy requirements for 8 weeks, with vastus lateralis biopsies after both 12 and 24 hour fasts. The 24 hour fast raised SQSTM1. The paper's title states its own conclusion: markers were activated in mouse liver but not in muscle from mouse or humans (Nutrition, 2022).
  • Dethlefsen 2018 took vastus lateralis biopsies at 2, 12, 24 and 36 hours, the only human study with several within-fast timepoints for muscle autophagy markers. LC3I, LC3II and p62 all decreased (p < 0.05), in untrained subjects only. The authors summarise it as skeletal muscle autophagy being only modestly affected by 36 hours of fasting. The exact N could not be verified from the paywalled full text (Journal of Applied Physiology, 2018).
  • Rothman 1991 measured hepatic glycogen serially by 13C nuclear magnetic resonance through a 68 hour fast in healthy adults and calculated gluconeogenesis by subtraction: 64 ± 5% of total glucose production across the first 22 hours, 82 ± 5% from 22 to 36 hours, 96 ± 1% thereafter. Landau 1996 reached the same place by deuterated water, about 47% at 14 hours and 67% at 22 hours. There is no hour at which the body switches from glycogen to fat (Science, 1991).
  • Lim 2024, 1,535 non-diabetic patients fasted only for preoperative preparation: 59.4% had postoperative ketonuria and 29.0% of the 929 with intraoperative blood gases showed metabolic acidosis, with higher risk in younger patients with lower BMI (Acta Anaesthesiologica Scandinavica, 2024).
  • Service 2005 measured beta-hydroxybutyrate every 6 hours through the standard 72 hour diagnostic fast in 34 adults. The median change from 12 to 18 hours was 0%, and from 18 to 36 hours was +333%. A 24 hour fast sits partway up that second stretch (Journal of Clinical Endocrinology and Metabolism, 2005).

What this does not tell you: The randomised crossover is 30 people over a single day with biochemistry as its endpoint and no clinical outcome. The calorimetry study's exact effect size is not in the accessible record. Both biopsy studies measured static markers rather than flux, and one has an N that could not be verified. No randomised trial has tested repeated 24 hour water fasts against a control with a clinical endpoint, and no trial of any fasting protocol has used cardiovascular events or mortality as its endpoint.

Reading the research record

The argument that starts at this duration and never stops is whether fasting slows metabolism, and it is worth setting out here because the shortest whole-day measurement is at 24 hours.

The source of the popular claim is Zauner 2000, which is a real study: 11 healthy lean subjects, resting energy expenditure measured by hood on days 1 to 4 of an 84 hour fast, rising from 3.97 ± 0.9 to 4.53 ± 0.9 kJ/min by day 3 (p < 0.05), a 14.1% rise, alongside noradrenaline going from 1716 ± 574 to 3728 ± 1636 pmol/L by day 4.

Mansell 1990 is routinely cited alongside it and does not measure the same thing. It is a 48 hour study in 6 non-diabetic men using a hyperinsulinaemic clamp, and what it measured was clamp-induced thermogenesis rather than resting metabolic rate during fasting. Its thermogenesis result points the other way: significant when fed at 0.27 ± 0.08 kJ/min (p < 0.01) and not significant after starvation at 0.11 ± 0.09.

The stronger design disagrees with the popular version. Andriessen 2023 put 12 healthy lean males in a respiration chamber for 60 continuous hours in a randomised crossover against a fed condition at energy balance, and 24 hour energy expenditure was significantly lower while fasting: 9.8 ± 0.2 against 11.0 ± 0.4 MJ on day 2, a fall of 10.9%, and 10.3 ± 0.3 against 10.9 ± 0.3 MJ on day 3, a fall of 5.5%, both p < 0.01. Hollstein found a decrease at exactly 24 hours by the same class of method.

Both sets of numbers are true and they are measuring different quantities. The resting component can rise while whole-day expenditure falls, because the thermic effect of food is removed and spontaneous activity drops. The honest statement is that resting metabolic rate measured by indirect calorimetry rises modestly through about day 3 in lean subjects while total 24 hour expenditure measured in a chamber falls over the same window. Resting rate does eventually fall: by 20.3 ± 11.13% across a 21 day fast in 13 volunteers.

What happens to muscle

Nobody has measured body composition across a single 24 hour fast, and it is not obvious what a scan would be measuring if they did.

What the longer studies establish is that the acute lean mass signal on a fast is mostly not tissue. In 13 men through a 10 day complete water fast with DXA at baseline, fasting day 6 and recovery day 5, total lean mass was down 9.2% at day 6 and back to baseline by recovery day 5 while fat mass was down 17.2%. In 16 men on a 10 day modified fast, lean soft tissue loss of 3.53 ± 0.13 kg decomposed into 44% extracellular water, 14% glycogen with its bound water, and 42% metabolically active lean tissue, which is 1.5 ± 0.1 kg, or 25% of total weight loss. The lean soft tissue compartment on a DXA is largely water, and glycogen binds roughly three times its own weight in water, so a scan taken at the end of a fast overstates lean loss.

Grams of nitrogen per day for days 1 to 3 of a total fast have not been published in any open primary human dataset. The figures usually attributed to Cahill's 1970 New England Journal of Medicine paper, roughly 12 g of nitrogen a day early falling to 3 to 4 g late, could not be verified against that paper and are not published here. The verifiable substitutes come from longer fasts: total urinary nitrogen of 4.66 ± 0.62 g per 24 hours at 5 to 6 weeks in obese subjects, with the paper's own statement of about 3 to 6 g a day after week 4, and nitrogen balance of -5.9 g per day at 3 weeks in 10 moderately obese women.

Doing it

Medication review is the whole safety story at this duration
The published table for intermittent fasting advises skipping a sulfonylurea on a 24 hour fast day, one third of usual basal insulin if controlled and half if not, skipping prandial insulin when no carbohydrate meal follows, and skipping an SGLT2 inhibitor. In the same paper, a 50% basal insulin reduction and a 70% prandial reduction each still produced significant hypoglycaemia rates.
Ketosis at 24 hours is the base rate
Among 1,535 non-diabetic patients fasted only for surgery, 59.4% had postoperative ketonuria and 29.0% of those with intraoperative blood gases showed metabolic acidosis. A positive urine strip after a day without food is not evidence that anything unusual has happened.
Salt, and nothing established beyond it
No published protocol monitors electrolytes for a fast under 24 hours. Sodium excretion rises during a fast because falling insulin removes its antinatriuretic effect, but in the metabolic ward study the turn came after roughly 48 hours. Community practice here is salt if symptomatic. There is no randomised evidence establishing an electrolyte regimen for any voluntary fast.
A fasting glucose taken now is not a normal fasting glucose
Glucose falls about 25% between 12 and 72 hours in healthy men, and at 24 hours it is on that slope. In healthy people the fall is largely asymptomatic because the brain is taking ketones, which also means symptoms are a poor monitor.
Expect the scale to move more than the tissue
Weight fell significantly in the randomised crossover and had not returned to baseline at 48 hours, alongside a haemoglobin, red cell and haematocrit rise that indicates plasma volume contraction. Early weight change on a fast is dominated by sodium, water and glycogen.
Standing up
Presyncope was recorded in 217 of 768 visits in the largest supervised water-only fasting chart review, all of it graded moderate. That is a longer-fast dataset, and it is the most consistently reported physical sign at any duration past a day.

Breaking the fast

Refeeding syndrome is not a concern after a single day in a well-nourished person. The NICE criteria that define refeeding risk are built around BMI, recent weight loss, prior electrolyte levels and little or no intake for more than 5 days as one of two criteria, or more than 10 days as a single sufficient one. A 24 hour fast is nowhere near either. What the community consistently reports is that the trouble at longer durations starts with the first meal rather than the fast, and the advice they give each other after a day is simply to make the first meal protein-forward, eat it slowly, and not break with something sugary. On the protein arithmetic, a large first meal is not wasted. In 36 recreationally active young men, 100 g of intrinsically labelled milk protein after whole-body resistance exercise produced a greater and more prolonged response than 25 g, lasting beyond 12 hours, with myofibrillar synthesis roughly 20% higher over 0 to 4 hours and roughly 40% higher over 4 to 12, and negligible effect on amino acid oxidation.

Citations

  1. Human2013
    Randomized cross-over trial of short-term water-only fasting: metabolic and cardiovascular consequences

    Nutrition, Metabolism and Cardiovascular Diseases

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

  2. Human2020
    Metabolic Responses to 24-Hour Fasting and Mild Cold Exposure in Overweight Individuals Are Correlated and Accompanied by Changes in FGF21 Concentration

    Diabetes

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

  3. Human1993
    Progressive alterations in lipid and glucose metabolism during short-term fasting in young adult men

    American Journal of Physiology

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

  4. Human1991
    Quantitation of hepatic glycogenolysis and gluconeogenesis in fasting humans with 13C NMR

    Science

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

  5. Human1996
    Contributions of gluconeogenesis to glucose production in the fasted state

    Journal of Clinical Investigation

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

  6. Human2022
    Intermittent fasting activates markers of autophagy in mouse liver, but not muscle from mouse or humans

    Nutrition

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

  7. Human2018
    Training state and skeletal muscle autophagy in response to 36 h of fasting

    Journal of Applied Physiology

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

  8. Human2024
    Perioperative urinary ketosis and metabolic acidosis in patients fasted for undergoing gynecologic surgery

    Acta Anaesthesiologica Scandinavica

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

  9. Human2005
    Increasing serum betahydroxybutyrate concentrations during the 72-hour fast: evidence against hyperinsulinemic hypoglycemia

    Journal of Clinical Endocrinology and Metabolism

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

  10. Review2019
    Clinical Management of Intermittent Fasting in Patients with Diabetes Mellitus

    Nutrients

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

  11. Human2026
    Hypoglycaemia and other risks of ramadan fasting in patients with primary adrenal insufficiency: A prospective controlled trial using 24-hour glucose monitoring

    Journal of Endocrinological Investigation

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

What people report

These are 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.

  • Hunger arriving in waves at former meal times rather than as a rising line, with hours 20 to 24 described as easier than hours 14 to 18
  • Salt in water as the standard first response to a headache, at roughly a quarter to half a teaspoon
  • A flat or low afternoon followed by a reported second wind in the evening
  • Feeling cold in the last few hours
  • In the one-meal-a-day version specifically, hunger at all three former meal times for the first three to seven days, easing over roughly two weeks

Sources: Uncontrolled self-report gathered from public fasting communities, with no control group, no denominator and no follow-up on the people who stopped posting. It is included because it describes what people experience, which is a different kind of information from a trial. The one item with a measured counterpart is hunger: in the largest supervised cohort, on 200 to 250 kcal a day, 93.2% reported an absence of hunger and 6.81% reported hunger as a recurring mild symptom.

Who this is wrong for

  • Anyone taking an SGLT2 inhibitor, unless a prescriber has stopped it first. The combination produces ketoacidosis at a normal-looking blood glucose, which a fingerstick will not warn you about. The FDA recommends stopping canagliflozin, dapagliflozin and empagliflozin at least 3 days and ertugliflozin at least 4 days before scheduled surgery for exactly this exposure, and a published case documents that the risk can persist after the drug is stopped.
  • Anyone on insulin, a sulfonylurea or a meglitinide without a prescriber adjusting the dose in advance. In the CREED cohort of 3,250 people with type 2 diabetes, hypoglycaemia incidence during Ramadan was 16.8% on insulin and 5.3% on oral agents, and the strongest predictor was a prior episode, odds ratio 7.80 (95% CI 5.31 to 11.45).
  • Anyone with type 1 diabetes outside a structured programme with glucose monitoring and an insulin plan. Exogenous insulin keeps working on its own schedule regardless of glucose, and cutting it too far removes the brake on ketogenesis, so the risk runs in both directions.
  • Anyone with primary adrenal insufficiency. In a randomised crossover in 30 treated patients who had all received therapeutic education, complications occurred in 67% during fasting against 23% outside it (P = 0.001) and 11 of 30 broke the fast because of one, while continuous glucose monitoring showed no significant difference in measured hypoglycaemia.
  • Pregnancy and breastfeeding. Lactation draws roughly 30 to 50 g of glucose a day out of the mother, and dieting or exercise to lose weight was the precipitant in 76% of 18 reported cases of lactation ketoacidosis.
  • Anyone with a history of an eating disorder, or current loss-of-control eating. A protocol scored by adherence to a restriction rule creates the conditions for rule-tightening, and the survey association is consistent across women, men and gender non-conforming participants.
  • Children and adolescents. There are no long-term growth or development outcome studies, and this is the age band where the eating disorder association is strongest.
  • Anyone with a BMI under 18.5, or who meets the NICE refeeding risk criteria on the way in.
  • Anyone with chronic kidney disease taking an ACE inhibitor or ARB. In 131 CKD patients, creatinine rose during fasting in 60.4% by day 7 and the rise was associated with taking a renin-angiotensin-aldosterone antagonist (relative risk 2, P = 0.002).
  • Anyone on lithium doing this without water. Lithium is reabsorbed alongside sodium in the proximal tubule, so volume depletion raises levels in a drug with a narrow therapeutic index. The risk is the water restriction rather than the food restriction.

Questions

Does a 24 hour fast turn on autophagy?
The two human measurements at exactly this duration point the other way in the tissue they sampled. A 24 hour fast raised p62/SQSTM1 in the vastus lateralis of 50 women, which is movement toward less autophagic degradation, and the only study with several within-fast timepoints found LC3I, LC3II and p62 all falling at 24 and 36 hours in untrained subjects. Neither measured flux, and static markers count autophagosomes rather than measuring how fast material moves through them. The blood cell measurements at 18 hours point up while the muscle measurements point down, which is the most useful thing to know about this literature.
Does one day of fasting slow my metabolism?
Whole-day measurement says it falls and hood-measured resting expenditure says it rises, and both are real. In a whole-room calorimeter at 24 hours, energy expenditure decreased in 20 volunteers. In a respiration chamber over 60 hours, 24 hour expenditure was 10.9% lower on day 2 and 5.5% lower on day 3 than at energy balance. Resting energy expenditure measured by hood rose 14.1% by day 3 in 11 lean subjects. The resting component can rise while the whole day falls, because the thermic effect of food is gone and spontaneous movement drops.
Is the growth hormone rise the one quoted as 1,300% and 2,000%?
No. Those percentages come from a 2011 press release about work presented at a conference, describing a 24 hour fast, and the same release reports that LDL cholesterol rose 14% during the fast. The peer-reviewed measurement at exactly 24 hours found growth hormone significantly higher than on a fed day (p = 1.1 x 10^-4) without printing the size of the change. The best-characterised figure in the literature is a five-fold rise in 24 hour growth hormone production on the second day of a fast in 9 men.
Will one day of fasting cost me muscle?
Nobody has measured body composition across a single 24 hour fast. What the longer studies show is that the acute lean mass signal is mostly water and glycogen: in a 10 day water fast, lean mass was down 9.2% at day 6 and back to baseline after refeeding, and in a 10 day modified fast, 58% of the measured lean soft tissue loss was extracellular water plus glycogen with its bound water. Grams of nitrogen per day for the first three days of a total fast have never been published.