Every fasting model, and what has actually been measured

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.

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.

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.

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.

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.

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.

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.

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.