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Extended24 human studies cited

Supervised Multi-Day Fasting

7 days without food · One episode of a week or longer, residential, with a refeeding period built in

Written by Reviewed Sep 2026

Also called: Therapeutic fasting, Water-only fasting, Buchinger fasting, 7 day fast

A week or more without food, in a clinic. The two largest cohorts report no deaths across 1,422 subjects and 768 visits, and they disagree with each other because one is water-only and the other supplies 200 to 250 kcal a day. The deaths in the case literature cluster in the refeeding period.

Overview

Past about a week, fasting stops being a thing people do and becomes a thing clinics run. The two largest published safety datasets are both residential and neither reports a death: a chart review of 768 medically supervised water-only fasting visits from 652 patients with a median fast of 7 days, and an observational cohort of 1,422 subjects fasting 4 to 21 days at a specialist clinic.

Those two datasets are not measuring the same intervention and the difference matters more than almost anything else on this page. The 1,422-subject cohort is not water fasting. It supplies 200 to 250 kcal a day including fruit juice and honey, providing 25 to 35 g of carbohydrate, with instructions to drink 3 L of water. The water-only cohort provides nothing. The water-only cohort recorded grade 3 events in 212 of 768 visits and one grade 4, a hyponatraemia on fasting day 9. The modified-fasting cohort recorded adverse effects in 0.7% of subjects with no grade 4 or 5 events. A small daily sodium and carbohydrate load is the design feature separating them.

The most important thing to carry out of this section is not about the fast. It is that in the historical case literature the deaths cluster in the refeeding period, and that serum testing does not see the deficit that makes refeeding dangerous.

How it works

Residential, with daily clinical contact, and a refeeding period planned before the fast starts.

The water-only protocol at the centre with published safety data defines the refeeding period as at least half the length of the fast. In practice a median 11 day fast was paired with a median 5 day refeed in a later cohort from the same setting, and the refeed itself is a low-fat, low-sodium, whole-plant-food diet.

The modified protocol runs differently and its details are published. A 600 kcal vegetarian day precedes the fast. The intestinal tract is emptied with a laxative, 20 to 40 g of sodium sulphate in 500 mL of water. During the fast, 3 L a day of water or non-caloric herbal tea, optional 20 g of honey, 250 mL of fresh juice at noon and 250 mL of vegetable soup in the evening, totalling 200 to 250 kcal and 25 to 35 g of carbohydrate, with an enema or mild laxative every second day and daily nurse and physician visits. Food is reintroduced across an average of 4 days, moving an ovo-lacto-vegetarian diet from 800 to 1,600 kcal a day. Bloods are drawn at baseline and at the end.

That clinic excludes cachexia and anorexia nervosa outright, alongside advanced kidney, liver or cerebrovascular insufficiency, dementia or other severely debilitating cognitive disease, pregnancy and lactation. That is the clearest published statement of what a large supervised fasting programme treats as an absolute exclusion.

What supervision actually buys, stated specifically: drug adjustment before the fast, baseline potassium, phosphate and magnesium in anyone with a low BMI or recent weight loss, thiamine before carbohydrate on refeeding, blood ketone rather than glucose measurement in anyone on an SGLT2 inhibitor or with type 1 diabetes, daily clinical contact, eating disorder screening, and graded refeeding with daily electrolytes. What it does not buy is a safe extended fast on lithium with fluid restriction, prevention of gallstones, or protection from euglycaemic ketoacidosis on a continued SGLT2 inhibitor.

What 7 days actually reaches

In 12 volunteers through a 7 day water-only fast with daily plasma proteomics across roughly 3,000 proteins, mean weight loss was 5.7 ± 0.8 kg and systemic proteomic changes became evident only after 3 days of complete caloric restriction. More than 1,000 proteins responded significantly, with nine distinct response profiles and strong enrichment for extracellular matrix proteins. The health-consequence estimates in that paper for 212 proteins across around 500 outcomes are proteogenomic inference rather than measured clinical outcomes. Glucose keeps falling and the fall is silent. Continuous glucose monitors on 12 healthy adults through a 7 day water-only fast recorded time below 70 mg/dL rising from 3.0 ± 7.1% to 66.0 ± 25.7% by day 5 and minimum daily glucose falling from 76 ± 14 to 50 ± 7 mg/dL, with no symptoms of hypoglycaemia reported. Sodium behaves worse on a true water fast than in the modified cohort. In 13 men on a 10 day complete fast, serum sodium fell 2.88% from day 6 and dropped below the 137 mmol/L lower limit on day 9, recovering by refeeding day 5, with chloride following. The authors' own conclusion is that no vitamins but a sodium chloride supplement should be considered. In the 1,422-subject modified cohort, sodium went from 140.1 ± 0.1 to 138.7 ± 0.1 mmol/L and stayed in range. Uric acid roughly doubles by day 7, from 5.9 ± 0.4 to 12.5 ± 1.0 mg/dL in 15 obese patients, with urate clearance falling from 5.5 ± 0.9 to 1.8 ± 0.2 mL/min, then falls back to 7.7 ± 1.3 mg/dL by day 28 despite continuing ketosis. The rise scales with the depth of ketosis rather than with dehydration. Inflammation goes the wrong way for the usual story. In the 1,422-subject cohort, CRP rose from 2.85 ± 0.14 to 4.30 ± 0.20 mg/L (p = 0.001), and that replicates: a study of 20 volunteers fasting a mean of 9.8 ± 3.1 days found elevated CRP, hepcidin, midkine and IL-8, increased platelet degranulation and upregulated complement and coagulation pathways, and its retrospective check of the same large cohort confirmed the CRP rise. Prolonged fasting is not an anti-inflammatory intervention while it is underway. On autophagy, the one human flux measurement in this duration range found nothing. Running human and mouse blood in the same study, no enhanced autophagy-associated lipidation was detectable in vivo in human leukocytes after up to 4 days of fasting; flux appeared only after ex vivo culture with leupeptin, and among human cell types only in neutrophils. This page therefore does not claim that a multi-day fast reaches autophagy, and it 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

    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.

  8. 3d onward

    Adapted, and the risk moves to the refeed

    Burning
    Fat and ketones, with glucose production splitting between liver and kidney as the fast lengthens
    Blood glucose
    3.5 ± 0.5 mmol/L (63.1 mg/dL) on day 4 in 11 lean adults, and 3.2 ± 0.2 mmol/L (57.7 mg/dL) on day 5 in 6 men
    Ketones (BHB)
    Production begins to plateau after about 5 days. Across a 21 day fast in 13 volunteers, blood ketones went from 0.1 ± 0.04 to 6.61 ± 1.25 mmol/L.
    Insulin
    Reaches its plateau by day 3 in obese subjects fasting for weeks, while plasma glucagon rises about two-fold to its own peak on the same day
    • In 8 healthy men at 72 hours, skeletal muscle mTOR Ser2448 phosphorylation fell about 50% and LC3B-II rose about 30%. p62 rose about 10% in the same biopsies, and the authors state that this makes autophagic flux impossible to read from these markers alone.
    • Continuous glucose monitors on 12 healthy adults through a 7 day water-only fast recorded time below 70 mg/dL rising from 3.0 ± 7.1% to 66.0 ± 25.7% by day 5, with minimum daily glucose falling from 76 ± 14 to 50 ± 7 mg/dL, and no symptoms of hypoglycaemia reported. Symptom-based self-monitoring stops working at this point.
    • Serum uric acid roughly doubled by day 7, from 5.9 ± 0.4 to 12.5 ± 1.0 mg/dL in 15 obese patients, then fell back to 7.7 ± 1.3 by day 28 while ketosis continued. Urate clearance fell proportionally more than creatinine clearance, so this is a specific tubular handling change.
    • Resting energy expenditure fell 20.3 ± 11.13% across a 21 day complete fast in 13 volunteers, which is where the eventual metabolic decline shows up.
    • Urinary nitrogen excretion decreases progressively for about 4 weeks and then holds at roughly 3 to 6 g per day. Nitrogen loss per kilogram of weight lost is inversely related to body fat, roughly 20 g of nitrogen per kg in non-obese people against roughly 10 g per kg in those carrying 50 kg or more of fat.
    • Mineral balance runs hundreds of milligrams per day negative while serum phosphorus and magnesium stay unchanged. That dissociation is why the deaths in the case literature cluster in the refeeding period rather than in the fast.

    Read the biologyKetogenesis and beta-hydroxybutyrateAutophagyRefeeding, the leucine threshold and refeeding syndrome

    Measured in people
    Vendelbo 2014 took muscle biopsies from 8 healthy men after 72 hours with forearm tracer kinetics. Kolnes 2026 monitored 12 healthy adults continuously through a 7 day water-only fast. Fox 1976 measured serum urate and urate clearance through 28 days in 15 obese patients. Dai 2024 measured resting energy expenditure and blood ketones across a 21 day fast in 13 volunteers. Owen 1969 measured urinary nitrogen fractions in 11 obese subjects through 5 to 6 weeks, and Forbes 1979 established the scaling of nitrogen loss with body fat. Licata 1981 measured mineral balance directly over 40 days on a metabolic ward and found mean daily balances of -104 mg calcium, -48 mg magnesium and -363 mg phosphorus while serum phosphorus and magnesium did not change at all.

Human evidence

Two large supervised safety datasets that disagree because they are different interventions, a 7 day proteomics study, two 10 day body composition studies, a 21 day physiological study, continuous glucose monitoring through a 7 day fast, and the historical case literature that establishes where the deaths occurred.

  • Finnell 2018, a chart review of medically supervised water-only fasting at a residential facility: 768 visits from 652 patients across 6,265 fasting days, median fast 7 days, shortest 2, longest 41, graded with CTCAE v4.03. The highest-grade adverse event per visit was grade 2 or lower in 555 visits, grade 3 in 212, grade 4 in 1 and grade 5 in none. Of 5,961 adverse events, 75.3% were grade 1. The two serious events were a dehydration event on fasting day 3 in a 73-year-old man and a grade 4 hyponatraemia on fasting day 9 in a 70-year-old man, both fully recovered. Highest-grade adverse event and fast duration were positively correlated (BMC Complementary and Alternative Medicine, 2018).
  • Wilhelmi de Toledo 2019, an observational cohort of 1,422 subjects fasting 4 to 21 days at 200 to 250 kcal a day, mean actual fast 8.2 ± 0.1 days, no control group. Systolic blood pressure 131.6 ± 0.7 to 120.7 ± 0.4 and diastolic 83.7 ± 0.4 to 77.9 ± 0.3 mmHg; glucose stabilised at an average of 4.7 mmol/L; uric acid 338.1 ± 2.3 to 495.2 ± 4.4 micromol/L; sodium 140.1 ± 0.1 to 138.7 ± 0.1 mmol/L; CRP 2.85 ± 0.14 to 4.30 ± 0.20 mg/L (p = 0.001). Adverse effects in 0.7% of subjects, no fatalities and no permanent adverse effects, with only two subjects having to interrupt the fast. 93.2% reported an absence of hunger (PLoS ONE, 2019).
  • Pietzner 2024, 12 volunteers through a 7 day water-only fast with plasma proteomics across roughly 3,000 proteins before, daily during and after. Mean weight loss 5.7 ± 0.8 kg. Systemic proteomic changes became evident only after 3 days, with more than 1,000 proteins responding across nine distinct profiles and strong enrichment for extracellular matrix proteins. The health-consequence estimates for 212 proteins are proteogenomic inference, not measured outcomes (Nature Metabolism, 2024).
  • Laurens 2021, 16 men, age 44 ± 14, BMI 26.2 ± 0.9, through a 10 day modified fast with up to 3 hours a day of low-intensity activity. Weight -7% (-5.9 ± 0.2 kg, p < 0.001) and basal metabolic rate -12% (p < 0.01). Fat mass -2.3 ± 0.18 kg, about 40% of weight lost; lean soft tissue -3.53 ± 0.13 kg, about 60%. That lean loss 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. Protein oxidation fell 41 ± 7% by day 5 and then held. Step count rose 60% and weight-bearing strength rose 33% (p < 0.001) (Journal of Cachexia, Sarcopenia and Muscle, 2021).
  • Dai 2022, 13 male adults through a 10 day complete water-only fast in a controlled facility with DXA at baseline, fasting day 6 and recovery day 5. Weight -7.28 kg (-9.8%). At day 6, fat mass -10.7% and lean mass -9.2%; by recovery day 5, fat mass -17.2% and lean mass back to baseline. Glucose nadir 3.508 ± 0.397 mmol/L on day 4. Serum sodium fell below 137 mmol/L on day 9 (Nutrients, 2022).
  • Dai 2024, 13 volunteers through a 34 day protocol of 3 day baseline, 21 days complete fasting, 5 days calorie restriction and 5 days full recovery. Body weight -14.96 ± 1.55%, blood ketones 0.1 ± 0.04 to 6.61 ± 1.25 mmol/L, blood uric acid 385.38 ± 57.78 to 866.31 ± 172.01 micromol/L, and resting energy expenditure -20.3 ± 11.13%. Complete blood count and liver indicators were mostly stable and within normal range and cardiac function showed functional adaptive changes without structural damage (Scientific Reports, 2024).
  • Scharf 2022, the attrition figure worth carrying: 48 enrolled and 26 completed, a 46% dropout, with a median fast of 17 days and an 8 day refeed. At the end of the fast, systolic blood pressure, abdominal circumference, LDL and high-sensitivity CRP were down while triglycerides and HOMA-IR were up (Nutrients, 2022).
  • Goldhamer 2001, 174 consecutive hypertensive inpatients treated with roughly 2 to 3 days of fruit and vegetables, roughly 10 to 11 days of medically supervised water-only fasting and roughly 6 to 7 days of refeeding on a low-fat, low-sodium vegan diet. Nearly 90% reached below 140/90 mmHg, with a mean reduction of 37/13 mmHg and 60/17 mmHg in stage 3 hypertension, and all subjects on antihypertensive medication at entry, 6.3% of the sample, discontinued it. This is an uncontrolled case series bracketed by two dietary interventions that independently lower blood pressure, so the fast itself cannot be isolated (Journal of Manipulative and Physiological Therapeutics, 2001).

What this does not tell you: Neither large safety dataset is randomised and neither has a control group. The 1,422-subject cohort is a single specialist clinic, is a 200 to 250 kcal modified fast rather than a water fast, measured ketones with a urine dipstick for acetoacetate rather than a blood beta-hydroxybutyrate assay, never measured serum phosphate, and reported no body composition at all. Its published record is not internally consistent: hyponatraemia appears as 3 cases in the adverse events table and 6 in the discussion, and the 5-day group is given as 695 in the methods and 659 in a table. The water-only chart review prints its serious adverse event rate as 0.002% where 2 of 768 visits is 0.26%, which appears to be a typographical error, so the raw counts are used here. Body composition figures come from studies of 13 and 16 men. Attrition in one prolonged water-only trial was 46%.

Reading the research record

The single most useful comparison in this section is between the two large cohorts, because reading either one as evidence about the other is the most common error in this literature.

The 1,422-subject cohort supplies 200 to 250 kcal a day, including 25 to 35 g of carbohydrate from fruit juice and honey, with instructions to drink 3 L of water. That is why its glucose stabilises around 4.7 mmol/L while day 5 of a true water fast reaches 3.2 ± 0.2 mmol/L, and it is a plausible part of why its sodium stayed in range while a 10 day complete fast dropped below the lower limit on day 9. Its ketone data are a semi-quantitative urine dipstick reacting to acetoacetic acid, self-measured in first morning urine. No blood beta-hydroxybutyrate was measured in that cohort at all, so any citation of it for blood ketone levels is a misreading. It also never measured serum phosphate, which is the electrolyte that matters most on refeeding, and it measured no body composition.

Two further things in that paper run against the way it is usually summarised. CRP went up rather than down, from 2.85 ± 0.14 to 4.30 ± 0.20 mg/L, and that has replicated in an independent water-only study. And the published record is not internally consistent, reporting hyponatraemia as 3 cases in its adverse events table and 6 in its discussion, and giving its 5-day group as 695 subjects in the methods and 659 in a table. Those are noted here rather than resolved.

The other correction that belongs here is about resting metabolic rate, because this is the duration where it genuinely falls. The popular claim that fasting does not slow metabolism rests on hood-measured resting expenditure rising 14.1% by day 3 in 11 lean subjects. Whole-day expenditure measured in a chamber fell 10.9% on day 2 and 5.5% on day 3, and across a 21 day fast resting energy expenditure fell 20.3 ± 11.13%, with basal metabolic rate down 12% by day 10 in a separate cohort. The early rise is real and it does not survive the week.

What happens to muscle

This is the duration where body composition has actually been measured, and the two studies that measured it best agree on the thing that matters: most of the acute lean mass signal is not tissue.

In 16 men on a 10 day modified fast, lean soft tissue fell 3.53 ± 0.13 kg, roughly 60% of total weight lost, which is the number usually quoted as evidence that extended fasting eats muscle. The decomposition, estimated from 10 day cumulated 24 hour urinary nitrogen, extracellular water and glycogen, was 44% extracellular water, 14% muscle and liver glycogen with its bound water, and 42% metabolically active lean tissue, which comes to 1.5 ± 0.1 kg, or 25% of total weight loss. Function did not track the scan: daily step count rose 60% (p < 0.001) and strength was maintained in non-weight-bearing muscles and rose 33% in weight-bearing ones (p < 0.001). At 3 month follow-up, body weight and lean soft tissue were still 2.3% and 3.2% below baseline (both p < 0.05).

In 13 men on a 10 day complete water fast with DXA at three timepoints, lean mass was down 9.2% at fasting day 6 and back to baseline by recovery day 5, while fat mass ended 17.2% down. That is the strongest available evidence that most acute lean mass loss during a fast is water and glycogen rather than myofibrillar protein. In 20 volunteers fasting a mean of 9.8 ± 3.1 days, proteomics indicated preservation of skeletal muscle and bone.

And the cleanest functional test of all, because it scanned and strength tested the same people. 13 healthy young adults, 6 women and 7 men, completed a seven day water only fast. DXA lean mass fell 4.6 ± 0.3 kg, an 8.0% drop, with 1.7 ± 0.5 kg of it from the arms and legs, against 1.4 ± 0.1 kg of fat. Muscle glycogen halved. Maximal isometric and isokinetic strength were unchanged. Peak oxygen uptake fell 13% and endurance capacity dropped, so the real cost lands on endurance rather than on force. AMPK activity was unaffected, which the authors say challenges its proposed role in muscle protein degradation, and they state plainly that whether the protein degradation includes contractile proteins is uncertain.

The nitrogen picture underneath: protein oxidation measured as total nitrogen excretion dropped 41 ± 7% by day 5 and then stayed stable through day 10, with plasma urea falling 41 ± 5% in parallel and 3-methylhistidine rising until day 5 and then falling. At 3 weeks, nitrogen balance was -5.9 g of nitrogen per day in 10 moderately obese women, with plasma leucine flux and urinary 3-methylhistidine both down 31% from control-diet values (p < 0.01). By 5 to 6 weeks, total urinary nitrogen was 4.66 ± 0.62 g per 24 hours with ammonia at 1.95 ± 0.22 and urea at 1.55 ± 0.60, an inversion of the fed-state pattern that is the signature of full adaptation.

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.

Who this applies to is the caveat that changes everything. Nitrogen loss per kilogram of weight lost is inversely related to body fat: 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. A lean person fasting loses roughly twice as much nitrogen per kilogram lost, and nearly all the classic prolonged starvation physiology was measured in obese subjects.

Doing it

Water-only and modified fasting are different interventions
200 to 250 kcal a day with 25 to 35 g of carbohydrate produces different glucose, different ketosis and different sodium handling from a water fast. The modified cohort held sodium in range; a 10 day complete fast dropped below the lower limit on day 9. Its authors concluded that a sodium chloride supplement should be considered.
Phosphate is the measurement nobody took
The 1,422-subject cohort measured sodium, potassium, calcium and magnesium only. Phosphate is the electrolyte that matters most on refeeding and the one the largest fasting cohort in the literature never measured. Every published refeeding protocol says to check it before calories are reintroduced and repeatedly afterwards.
The blood test will not show the deficit
Over 40 days on a metabolic ward, mean daily balances were -104 mg calcium, -48 mg magnesium and -363 mg phosphorus while serum phosphorus and magnesium did not change at all. Cumulative urinary losses exceeded intake by 58% for calcium, 75% for phosphorus and 500% for magnesium. A normal serum result during a fast is not evidence that the store is intact.
Thiamine before carbohydrate, not after
Wernicke encephalopathy is documented after a 40 day water-only fast and on day 53 of a 60 day therapeutic fast, the latter presenting in coma and respiratory failure. Thiamine demand rises sharply at the moment carbohydrate is reintroduced. The consensus recommendation is 100 mg before feeding or before any dextrose-containing intravenous fluid in an at-risk person, then 100 mg a day for 5 to 7 days or longer in severe starvation.
Uric acid and gout
Serum urate roughly doubles by day 7 and peaks somewhere around day 7 to day 15 before partly receding. The rise scales with ketosis rather than dehydration, because ketoacids and urate compete for the same proximal tubular transport, so drinking more does not fix it. In 1,422 supervised subjects there was one gout attack, in a 72-year-old man already on allopurinol with frequent prior attacks.
Gallstones
On a 500 kcal/day diet, the one-year risk of a symptomatic gallstone requiring hospital care was about 1.5% against 0.44% on 1,200 to 1,500 kcal, hazard ratio 3.4, number needed to harm 92, with most of the effect running through the rate of weight loss. Ursodeoxycholic acid is the intervention with randomised evidence, and dietary fat is the other lever because it triggers gallbladder contraction. A water fast contains no fat, so that second lever does not exist, and no cohort of comparable quality exists in water fasting to quantify it.
What supervision changes, specifically
Drug adjustment before the fast, baseline potassium, phosphate and magnesium, thiamine before carbohydrate, blood ketones rather than glucose in anyone on an SGLT2 inhibitor or with type 1 diabetes, daily clinical contact past about a week, eating disorder screening, and graded refeeding with daily electrolytes. The serious events in the published cohorts, a dehydration on day 3 and a hyponatraemia and a myocardial infarction both on day 9, were caught because someone was looking.

Breaking the fast

This is the dangerous part, and it is where a week-long fast differs in kind rather than degree from a two-day one. The pattern in the case literature is unambiguous. A death from lactic acidosis during refeeding after a 3 week fast. An apparently well young woman who fasted 210 days and died during realimentation, with gross fragmentation of cardiac myofibrils and a liver weighing 600 g at necropsy. Two of three crises in an inpatient series of 18 long fasts featured gross fluid retention on refeeding, one requiring diuretics. In the 382 day fast, the lowest potassium of the entire episode, 2.9 mEq/L, occurred on day 6 of refeeding, and magnesium fell to its lowest, around 1.1 mEq/L, in the first few days of refeeding as well. Both of that patient's lowest electrolyte values came after the fast ended. The mechanism is why serum testing misses it. Phosphate, potassium and magnesium are depleted from the body during a fast while serum values stay normal, because the cells shrink their contents alongside total body stores. Over 40 days on a metabolic ward, mean daily balances ran -104 mg calcium, -48 mg magnesium and -363 mg phosphorus with serum phosphorus and magnesium unchanged throughout. Reintroducing carbohydrate raises insulin, which drives glucose, phosphate, potassium and magnesium into cells at once, while phosphate is consumed further by the phosphorylation steps of glycolysis and by regenerating ATP and 2,3-DPG. Severe hypophosphataemia produces respiratory muscle failure, cardiac failure, rhabdomyolysis, haemolysis, seizures and delirium. Hypomagnesaemia makes hypokalaemia refractory to replacement. And the carbohydrate load consumes thiamine as the cofactor for pyruvate dehydrogenase, which is how a thiamine-depleted person given glucose is tipped into Wernicke encephalopathy. The published numbers for how often this happens: 21 of 62 ICU patients refed after 48 hours or more of starvation developed hypophosphataemia below about 2.0 mg/dL, at a mean of 1.9 ± 1.1 days after feeding started. In 69 adolescents started at 1,200 to 1,400 kcal a day, 19 of 69 needed phosphorus supplementation and 81% reached their phosphorus nadir within the first week. In 243 adults started on artificial nutrition support, 3 developed the full syndrome, with low baseline magnesium an independent predictor (p = 0.021). And the one randomised trial in this area found that restricting calories in people who had already developed refeeding hypophosphataemia improved survival: 149 of 164 alive at day 60 against 128 of 163, p = 0.002. What the guidelines say to do. Start at a maximum of 10 kcal/kg/day in a high-risk person, or 5 kcal/kg/day in extreme cases such as a BMI under 14 or negligible intake for more than 15 days, with continual cardiac rhythm monitoring in those cases, increasing to full needs over 4 to 7 days. Give thiamine 100 mg before feeding and 200 to 300 mg daily through the first 10 days in the high-risk, with a B complex and a multivitamin. Supplement potassium at roughly 2 to 4 mmol/kg/day, phosphate at 0.3 to 0.6 mmol/kg/day and magnesium at 0.2 mmol/kg/day intravenously or 0.4 orally. Check potassium, magnesium and phosphorus before starting and every 12 hours for the first 3 days in high-risk patients, and if they become difficult to correct or fall precipitously, cut the calories or dextrose by half and re-advance at about a third of goal every 1 to 2 days. What the fasting protocols themselves do: the water-only centre requires a refeeding period at least half the length of the fast, and the modified protocol reintroduces food across an average of 4 days, moving an ovo-lacto-vegetarian diet from 800 to 1,600 kcal a day. Community practice past five days is a ramp measured in days, broth first, then avocado and fermented food, then eggs, white fish or chicken, with normal portions not restored until day four or five and electrolytes continued through the whole refeed rather than stopped at the first meal. The last of those is practice rather than evidence; the guideline numbers above are not.

Citations

  1. Human2025
    Effects of seven days' fasting on physical performance and metabolic adaptation during exercise in humans

    Nature Communications

    13 healthy young adults completed a seven day water only fast. DXA lean mass fell 4.6 +/- 0.3 kg (8.0%), fat mass 1.4 +/- 0.1 kg, and muscle glycogen halved, yet maximal isometric and isokinetic strength were unchanged. Peak oxygen uptake fell 13%. AMPK activity was unaffected, which the authors say challenges its proposed role in muscle protein degradation.

  2. Human2018
    Is fasting safe? A chart review of adverse events during medically supervised, water-only fasting

    BMC Complementary and Alternative Medicine

    768 visits from 652 patients across 6,265 fasting days of medically supervised water-only fasting, median fast 7 days (IQR 6), shortest 2, longest 41, graded with CTCAE v4.03. Highest-grade adverse event per visit: grade 2 or lower in 555 visits, grade 3 in 212, grade 4 in 1, grade 5 in none. Of 5,961 adverse events, 75.3% were grade 1. The two serious events were a grade 3 dehydration on fasting day 3 in a 73-year-old man and a grade 4 hyponatraemia on fasting day 9 in a 70-year-old man, both fully recovered. The facility requires a refeeding period equal to half the fast length. The paper prints its serious adverse event rate as 0.002% where 2 of 768 is 0.26%, so the raw counts are used here.

  3. Human2019
    Safety, health improvement and well-being during a 4 to 21-day fasting period in an observational study including 1422 subjects

    PLoS ONE

    1,422 subjects fasting 4 to 21 days at a single specialist clinic on 200 to 250 kcal a day including 25 to 35 g of carbohydrate, mean fast 8.2 ± 0.1 days, no control group. Systolic 131.6 ± 0.7 to 120.7 ± 0.4 and diastolic 83.7 ± 0.4 to 77.9 ± 0.3 mmHg; uric acid 338.1 ± 2.3 to 495.2 ± 4.4 micromol/L; sodium 140.1 ± 0.1 to 138.7 ± 0.1 mmol/L; CRP 2.85 ± 0.14 to 4.30 ± 0.20 mg/L (p = 0.001), a rise. Adverse effects in 0.7%, no fatalities, two hospitalisations, only two subjects interrupting the fast, and 93.2% reporting an absence of hunger. Ketones were measured by urine dipstick reacting to acetoacetic acid, not blood beta-hydroxybutyrate; serum phosphate and body composition were not measured at all. The paper reports hyponatraemia as 3 cases in its table and 6 in its discussion, and its 5-day group as 695 in the methods and 659 in a table.

  4. Human2024
    Systemic proteome adaptions to 7-day complete caloric restriction in humans

    Nature Metabolism

    12 volunteers, 5 women and 7 men, through a 7 day water-only fast with plasma proteomics across roughly 3,000 proteins before, daily during and after. Mean weight loss 5.7 ± 0.8 kg. Systemic proteomic changes became evident only after 3 days of complete caloric restriction. More than 1,000 proteins responded significantly across nine distinct response profiles, strongly enriched for extracellular matrix proteins including large changes in the brain-specific protein tenascin-R. The health-consequence estimates for 212 proteins across around 500 outcomes are proteogenomic inference rather than measured clinical outcomes.

  5. Human2021
    Is muscle and protein loss relevant in long-term fasting in healthy men? A prospective trial on physiological adaptations

    Journal of Cachexia, Sarcopenia and Muscle

    16 men, age 44 ± 14, BMI 26.2 ± 0.9, through a 10 day modified fast at 200 to 250 kcal a day with up to 3 hours a day of low-intensity activity, followed through refeeding and a 3 month follow-up. Weight -7% (-5.9 ± 0.2 kg, p < 0.001), basal metabolic rate -12% (p < 0.01). Fat mass -2.3 ± 0.18 kg and lean soft tissue -3.53 ± 0.13 kg, the latter decomposing into 44% extracellular water, 14% glycogen with bound water and 42% metabolically active lean tissue, which is 1.5 ± 0.1 kg or 25% of total weight loss. Protein oxidation fell 41 ± 7% by day 5 and then held; plasma 3-methylhistidine rose to day 5 then fell. Step count +60% and weight-bearing strength +33% (both p < 0.001). At 3 months, weight and lean soft tissue were still 2.3% and 3.2% below baseline.

  6. Human2022
    Effects of 10-Day Complete Fasting on Physiological Homeostasis, Nutrition and Health Markers in Male Adults

    Nutrients

    13 male adults in a controlled facility through a 10 day complete water-only fast with DXA at baseline, fasting day 6 and recovery day 5. Weight -7.28 kg (-9.8%). At day 6, fat mass -10.7% and lean mass -9.2%; by recovery day 5, fat mass -17.2% and lean mass back to baseline. Glucose nadir 3.508 ± 0.397 mmol/L on day 4; beta-hydroxybutyrate 0.177 ± 0.044 to 1.308 ± 1.053 mmol/L by day 1, peaking day 5. Serum sodium fell 2.88% from day 6 and dropped below the 137 mmol/L lower limit on day 9, recovering by refeeding day 5. The authors conclude that a sodium chloride supplement should be considered.

  7. Human2024
    Analysis of physiological and biochemical changes and metabolic shifts during 21-Day fasting hypometabolism

    Scientific Reports

    13 volunteers through a 34 day protocol: 3 day baseline, 21 days complete fasting, 5 days calorie restriction, 5 days full recovery. Body weight -14.96 ± 1.55%, blood glucose -21.63 ± 0.058%, blood ketones 0.1 ± 0.04 to 6.61 ± 1.25 mmol/L, blood uric acid 385.38 ± 57.78 to 866.31 ± 172.01 micromol/L, resting energy expenditure -20.3 ± 11.13%. Respiratory quotient shifted toward fat. Complete blood count and liver indicators were mostly stable and within normal range, and cardiac function showed functional adaptive changes without structural damage. This is the clearest human measurement of the eventual fall in resting metabolic rate.

  8. Human2025
    Prolonged fasting promotes systemic inflammation and platelet activation in humans: A medically supervised, water-only fasting and refeeding study

    Molecular Metabolism

    20 middle-aged volunteers, mean fast 9.8 ± 3.1 days plus 5.3 ± 2.4 days of refeeding, 7.7% mean weight loss. Proteomics indicated preservation of skeletal muscle and bone. It also found an acute inflammatory response: elevated CRP, hepcidin, midkine and IL-8, increased platelet degranulation, and upregulated complement and coagulation pathways. A retrospective check of the 1,422-subject cohort confirmed CRP rising 2.8 ± 0.1 to 4.3 ± 0.2 mg/L. Amyloid beta 40 and 42 fell.

  9. Human2026
    Marked Increases in Continuous Glucose Monitor-Detected Hypoglycemia During a Seven-Day Water-Only Fast in Healthy Men and Women

    Journal of Diabetes Science and Technology

    12 healthy adults, 7 men, mean age 29.7, mean BMI 25.0, wearing continuous glucose monitors through a 7 day water-only fast. Time below 70 mg/dL rose from 3.0% ± 7.1% at baseline to 66.0% ± 25.7% by day 5 (P < .001) and minimum daily glucose fell from 76 ± 14 to 50 ± 7 mg/dL (P < .001). No symptoms of hypoglycaemia were reported, which makes symptom-based self-monitoring unreliable at this duration and removes the warning system entirely for anyone on a glucose-lowering drug.

  10. Human1976
    Renal excretion of uric acid during prolonged fasting

    Metabolism

    15 obese patients on prolonged therapeutic fasting. Serum uric acid 5.9 ± 0.4 mg/dL at day 0, 12.5 ± 1.0 at day 7 and 7.7 ± 1.3 at days 25 to 28, falling back despite sustained ketonaemia. Uric acid clearance 5.5 ± 0.9 to 1.8 ± 0.2 to 4.4 ± 1.5 mL/min against creatinine clearance 114 ± 11 to 80 ± 6 to 64 ± 6.3, so urate clearance fell proportionally more than filtration, making this a specific tubular handling change. There was no glycosuria, bicarbonaturia or increased phosphaturia to suggest general tubular dysfunction.

  11. Human1981
    Adverse effects of liquid protein fast on the handling of magnesium, calcium and phosphorus

    American Journal of Medicine

    6 obese subjects, 40 days on a metabolic ward. Cumulative urinary losses exceeded cumulative intake by 58% for calcium, 75% for phosphorus and 500% for magnesium, with mean daily balances of -104 mg calcium, -48 mg magnesium and -363 mg phosphorus, while serum phosphorus and magnesium did not change and serum calcium fell 0.5 mg/dL (p < 0.05). Serum bicarbonate fell 20% over the first 8 days. This is a protein-supplemented rather than a total fast, and the principle carries: a normal serum magnesium or phosphate during a fast does not mean the stores are intact, which is why refeeding is the dangerous phase.

  12. Review2020
    ASPEN Consensus Recommendations for Refeeding Syndrome

    Nutrition in Clinical Practice

    Defines refeeding syndrome as a fall in serum phosphorus, potassium and/or magnesium of 10% to 20% (mild), 20% to 30% (moderate) or more than 30% or with organ dysfunction (severe), occurring within 5 days of reintroducing calories. Lists prolonged fasting among the higher-risk conditions. Recommends initiating with 100 to 150 g of dextrose or 10 to 20 kcal/kg for the first 24 hours and advancing by a third of goal every 1 to 2 days, checking potassium, magnesium and phosphorus before starting and every 12 hours for the first 3 days in high-risk patients, and thiamine 100 mg before feeding then 100 mg a day for 5 to 7 days or longer in severe starvation. ASPEN states that the incidence of the syndrome is unknown and that quantification of risk is not currently possible.

  13. Human2015
    Restricted versus continued standard caloric intake during the management of refeeding syndrome in critically ill adults: a randomised, parallel-group, multicentre, single-blind controlled trial

    Lancet Respiratory Medicine

    339 critically ill adults who developed refeeding syndrome within 72 hours of starting nutrition, randomised to caloric restriction or standard feeding. Days alive after ICU discharge were 44.8 against 39.9, not significant (p = 0.19), but more patients were alive at day 60: 149 of 164 (91%) against 128 of 163 (78%), p = 0.002. The only randomised trial establishing that the rate of refeeding matters.

  14. Human1996
    Refeeding hypophosphatemia in critically ill patients in an intensive care unit. A prospective study

    Archives of Surgery

    62 ICU patients refed after 48 hours or more of starvation. Phosphorus fell by more than 0.16 mmol/L to below 0.65 mmol/L, about 2.0 mg/dL, in 21 of 62 (34%), and below about 1.0 mg/dL in 6 patients. The nadir came at a mean of 1.9 ± 1.1 days after feeding started.

  15. Human2015
    Malnutrition-induced Wernicke's encephalopathy following a water-only fasting diet

    Nutrition in Clinical Practice

    An adult woman developed Wernicke's encephalopathy after a 40 day water-only fasting diet, requiring parenteral thiamine and then oral thiamine at 100 to 800 mg a day for 6 months. Thiamine demand rises sharply at the moment carbohydrate is reintroduced, which is why the consensus recommendation is to give it before feeding rather than after.

  16. Human2015
    A Therapeutic Fast for Lymphoma Resulting in Wernicke Encephalopathy

    Journal of Emergency Medicine

    A 42-year-old man with non-Hodgkin lymphoma undertook a 60 day therapeutic water-only fast and arrived at the emergency department on day 53 in coma and respiratory failure, with Wernicke encephalopathy.

  17. Human2024
    Prolonged Water-Only Fasting Followed by a Whole-Plant-Food Diet Is a Potential Long-Term Management Strategy for Hypertension and Obesity

    Nutrients

    29 participants with stage 1 to 2 hypertension, median fast 11 days (range 7 to 40) and median refeed 5 days (range 3 to 17). Retention was 100% at the end of refeeding and 93% at 6 week follow-up, with 90% completing at least 7 days. All adverse events were mild grade 1 and transient, with no higher-grade or serious events.

  18. Human2022
    The Effects of Prolonged Water-Only Fasting and Refeeding on Markers of Cardiometabolic Risk

    Nutrients

    48 enrolled and 26 completed, a 46% attrition rate, with a median fast of 17 days and an 8 day refeed. At the end of the fast, systolic blood pressure, abdominal circumference, LDL and high-sensitivity CRP were down while triglycerides and HOMA-IR were up.

  19. Human2001
    Medically supervised water-only fasting in the treatment of hypertension

    Journal of Manipulative and Physiological Therapeutics

    174 consecutive hypertensive inpatients treated with roughly 2 to 3 days of fruit and vegetables, roughly 10 to 11 days of medically supervised water-only fasting, and roughly 6 to 7 days of refeeding on a low-fat, low-sodium vegan diet. Nearly 90% reached below 140/90 mmHg, mean reduction 37/13 mmHg and 60/17 mmHg in stage 3 hypertension, and all subjects on antihypertensive medication at entry, 6.3% of the sample, discontinued it. Uncontrolled case series, and the fast is bracketed by two dietary interventions that independently lower blood pressure, so the fast itself cannot be isolated.

  20. Human1970
    Prolonged starvation, a dangerous procedure?

    British Medical Journal

    18 obese patients fasting 60 days or more, range 62 to 249 days, losing 16.2% to 40.2% of initial body weight. One middle-aged man developed colicky umbilical pain on day 11 and died on day 13, with necropsy showing infarction of the small bowel due to an acute volvulus. Two other patients developed hyponatraemic shock or a renal sodium and potassium leak after 55 and 98 uneventful days, and both then developed pronounced fluid retention during refeeding. The authors' own conclusion was that prolonged therapeutic starvation is safe and effective only in hospital with full metabolic monitoring.

  21. Human1973
    Features of a successful therapeutic fast of 382 days' duration

    Postgraduate Medical Journal

    A 27-year-old man, admission weight 207 kg, fasted 382 days on non-caloric fluids with daily vitamin supplements, potassium only on days 93 to 162 and sodium chloride only on days 345 to 355, losing about 125 kg. Glucose sat around 30 mg/100 mL from month 4 and occasionally below 20 while he remained symptom-free and ambulant. His lowest potassium of the whole episode, 2.9 mEq/L, occurred on day 6 of refeeding, and magnesium fell further, to about 1.1 mEq/L, in the first few days of refeeding. Both of his lowest electrolyte values came after the fast ended.

  22. Review1982
    Fasting: the history, pathophysiology and complications

    Western Journal of Medicine

    The standing review of therapeutic fasting complications. Weight loss averaged 0.9 kg/day in the first week, slowing to 0.3 kg/day by the third, with the early rapid loss attributed primarily to negative sodium balance rather than tissue loss. Of the six fatal cases reviewed, two had severe pre-fast congestive heart failure that had initially appeared to improve with fasting and one had focal coronary artery stenosis at autopsy. The catalogue of non-fatal complications includes orthostatic hypotension, acute gout, urate nephrolithiasis, renal insufficiency, oedema, atrial flutter, anaemia, amenorrhoea, alopecia and vitamin deficiency. It also notes that most studies of fasting used obese persons and that the results may not always apply to lean persons.

  23. Human1990
    Protein metabolic effects of a prolonged fast and hypocaloric refeeding

    American Journal of Physiology

    10 moderately obese women fasted 3 weeks. Nitrogen balance at the end was -5.9 g of nitrogen a day, with plasma leucine flux and urinary 3-methylhistidine excretion both down 31% from control-diet values (p < 0.01), meaning whole-body protein turnover had slowed substantially. On refeeding with 80 g a day for one week, protein refeeding made nitrogen balance positive at +1.7 g/day (P < 0.05) while carbohydrate refeeding improved it only to -3.1 g/day.

  24. Human1969
    Liver and kidney metabolism during prolonged starvation

    Journal of Clinical Investigation

    11 obese subjects followed through 5 to 6 weeks of starvation. Daily nitrogen excretion decreased progressively for the first 4 weeks and then held at about 3 to 6 g a day. At 5 to 6 weeks in 5 subjects, total urinary nitrogen was 4.66 ± 0.62 g per 24 hours with ammonia at 1.95 ± 0.22 and urea at 1.55 ± 0.60, an inversion of the fed-state pattern. Free fatty acids, beta-hydroxybutyrate and acetoacetate did not plateau until after day 17, and total glucose production fell to approximately 86 g per 24 hours, roughly half hepatic and half renal. This is the verifiable source for late-starvation nitrogen figures.

  25. Human1979
    Loss of body nitrogen on fasting

    American Journal of Clinical Nutrition

    Total body nitrogen declines biexponentially during an extended fast, a small fraction with a half-life of a few days and the remainder with a half-life of many months. The ratio of nitrogen loss to weight loss is inversely related to body fat: approximately 20 g of nitrogen per kg of weight lost in non-obese people against approximately 10 g per kg in those with 50 kg or more of body fat. This is the caveat on generalising classic starvation physiology, all of which was measured in obese subjects, to lean people.

  26. Human2017
    Metabolic effects of fasting on human and mouse blood in vivo

    Autophagy

    Human and mouse blood studied in the same experiment, 4 to 5 subjects per condition. No enhanced autophagy-associated lipidation was detectable in vivo in human leukocytes after up to 4 to 5 days of fasting; flux appeared only after ex vivo culture with leupeptin, and among human cell types only in neutrophils. A 48 hour mouse fast, by contrast, produced LC3B lipidation across all leukocyte subpopulations. The paper also quantifies the dose mismatch: 48 hours of fasting causes approximately 20% weight loss in mice and less than 2% in humans over 4 days.

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.

  • Days three and four described as the low point for energy, followed by a reported lift
  • A reported high on days five and six, described as high mental energy and strong focus, although some accounts place that phase much later, around days twelve to fifteen
  • Persistent cold, described as wearing a coat indoors and being unable to get warm, which is near-universal in this tier
  • Weakness described as qualitatively different from fatigue, needing to plan movements, take stairs slowly and stop driving
  • Postural dizziness prominent enough to limit daily activity
  • Insomnia and night-time restlessness alongside daytime tiredness
  • Hunger staying gone, with a returning appetite read by the community itself as one of the signals to end the fast
  • From supervised clinic patient material rather than forums: nasal discharge, bloodshot eyes, skin rashes, sore throat, low back pain, gastric irritation, emotional disturbance, arrhythmia and palpitations, nausea and vomiting, hair loss, changed menstrual flow, and passage of kidney or gallstones

Sources: Uncontrolled self-report gathered from public fasting communities and from supervised clinic patient material, which is a different and somewhat more reliable class of report because a clinic describing its own patients has a denominator. Reports at this duration are the least representative on the site, coming disproportionately from clinic patients and long-form diarists. The community's own strong consensus is that fasting beyond roughly five to seven days belongs with supervision and bloodwork rather than self-management. Measured counterparts exist for several items: 93.2% of 1,422 supervised subjects reported an absence of hunger, and sleep disturbance was the most common self-reported symptom in that cohort at 14.94%, ahead of fatigue at 13.70%.

Who this is wrong for

  • Anyone doing this without supervision. Both large safety datasets are residential with daily clinical contact, and their serious events occurred on fasting days 3 and 9. There is no published trial establishing what electrolyte regimen or monitoring interval makes an unsupervised extended fast safe. The supervised protocols work by having a clinician present daily, not by a supplement schedule.
  • Anyone taking an SGLT2 inhibitor, unless a prescriber has stopped it at least 3 days ahead, or 4 for ertugliflozin. Supervision does not much change this risk: euglycaemic ketoacidosis on a continued SGLT2 inhibitor stays likely, the glucose reading stays normal-looking, and a urine strip is the wrong test because it measures acetoacetate rather than beta-hydroxybutyrate.
  • Anyone on insulin, a sulfonylurea or a meglitinide without prescriber-led adjustment, which is the single highest-yield intervention available and cannot be done any other way.
  • Anyone with type 1 diabetes, in whom exogenous insulin cannot be withdrawn the way endogenous insulin is suppressed, so the risk runs toward hypoglycaemia if the dose stays and toward ketoacidosis if it is cut too far.
  • Anyone with a BMI under 16, or under 18.5 with a second criterion, or who has lost more than 15% of body weight in the last 3 to 6 months, or has low potassium, phosphate or magnesium before feeding. Those are the NICE high-risk refeeding criteria and they identify risk on the way in, not only on the way out.
  • Anyone with cachexia or anorexia nervosa, which a large supervised fasting clinic excludes outright, alongside advanced kidney, liver or cerebrovascular insufficiency, dementia or other severely debilitating cognitive disease, pregnancy and lactation.
  • Anyone with a history of an eating disorder or current loss-of-control eating. Screening takes a few minutes and separates a materially different risk group in advance rather than after the fact.
  • Children and adolescents.
  • Anyone with known heart failure or coronary artery disease outside a hospital setting. Of the six fatal cases in the historical review of therapeutic fasting, two had severe pre-fast congestive heart failure that had initially appeared to improve with fasting, and one had focal coronary artery stenosis at autopsy. In the 1,422-subject cohort a 75-year-old man with known coronary disease had a non-ST-elevation myocardial infarction on fasting day 9.
  • Anyone with chronic kidney disease, and anyone on an ACE inhibitor, ARB or diuretic without those being reviewed first. Chronic kidney disease stage 4 to 5, dialysis and transplant sit in the discouraged group of the published diabetes-and-fasting risk stratification.
  • Anyone with primary adrenal insufficiency, established gout, sickle cell trait, or taking lithium alongside any fluid restriction.
  • Anyone who cannot commit to the refeeding period. The published water-only protocol defines it as at least half the length of the fast, and the case literature records deaths during realimentation rather than during the fast.

Questions

How safe is a fast of a week or more?
In supervised settings, safer than the historical case literature suggests and not without events. Across 768 medically supervised water-only fasting visits with a median length of 7 days, the highest-grade adverse event was grade 2 or lower in 555 visits and grade 3 in 212, with one grade 4 and no deaths. Across 1,422 subjects on a 200 to 250 kcal modified fast, adverse effects were recorded in 0.7% with no fatalities and two hospitalisations. Both datasets are residential with daily clinical contact, and their serious events happened on fasting days 3 and 9. Neither says anything about doing this alone.
Is the Buchinger data evidence about water fasting?
No, and this is the most common misreading in this literature. That cohort supplies 200 to 250 kcal a day including 25 to 35 g of carbohydrate from juice and honey, with 3 L of water. Its glucose stabilises around 4.7 mmol/L where day 5 of a true water fast reaches 3.2 ± 0.2. Its ketone data are a urine dipstick reading acetoacetic acid, not blood beta-hydroxybutyrate. It never measured serum phosphate and never measured body composition. Its published record also contains two internal inconsistencies, in its hyponatraemia count and in one group size, which are noted here rather than resolved.
Does fasting reduce inflammation?
Not while it is running, on the largest measurements available. CRP rose from 2.85 ± 0.14 to 4.30 ± 0.20 mg/L across the 1,422-subject cohort (p = 0.001), and an independent water-only study in 20 volunteers found the same acute-phase pattern with elevated CRP, hepcidin, midkine and IL-8 and increased platelet activation. A separate prolonged water-only trial did find high-sensitivity CRP down at the end of the fast alongside triglycerides and HOMA-IR up. The consistent finding is that the inflammatory picture during an extended fast is not the anti-inflammatory one usually described.
How much muscle does a week of fasting cost?
Less than the scan says. In 16 men on a 10 day modified fast, lean soft tissue fell 3.53 ± 0.13 kg, but 44% of that was extracellular water and 14% was glycogen with its bound water, leaving 1.5 ± 0.1 kg of metabolically active lean tissue, 25% of total weight loss. In 13 men on a 10 day water fast, lean mass was down 9.2% at day 6 and back to baseline after refeeding. Function moved the other way in the first study: step count rose 60% and weight-bearing strength rose 33%. A lean person loses roughly twice as much nitrogen per kilogram lost as an obese person, so these numbers do not transfer across body composition.
Why is refeeding more dangerous than the fast?
Because the deficit that matters is invisible on a blood test. Over 40 days on a metabolic ward, mean daily balances ran -104 mg calcium, -48 mg magnesium and -363 mg phosphorus while serum phosphorus and magnesium did not change at all. Reintroducing carbohydrate raises insulin, which drives glucose, phosphate, potassium and magnesium into cells simultaneously and consumes thiamine at the same moment. The case literature follows that mechanism: 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 in the 382 day fast, the lowest potassium and lowest magnesium of the whole episode both occurring in the first days of refeeding.
Does a long fast raise autophagy?
The one flux measurement in this duration range found nothing detectable. Running human and mouse blood in the same study, no enhanced autophagy-associated lipidation was detectable in vivo in human leukocytes after up to 4 to 5 days of fasting; a response appeared only after ex vivo culture with leupeptin and only in neutrophils, while a 48 hour mouse fast produced lipidation across all leukocyte subpopulations. That mouse fast also causes about 20% weight loss where 4 days of human fasting causes less than 2%. This page therefore does not claim a multi-day fast reaches autophagy.