Every fasting model, and what has actually been measured

Burning

Sugar from the meal you just ate. Insulin is high enough to keep fat locked away.

• Insulin is at its after-meal peak. While it is up there, the enzyme that releases fat from fat cells stays switched off, so fat is being stored, not released.
• The hour count on most fasting charts starts at the last bite. But the body is still digesting a mixed meal here, so the clock and what the body is doing are not yet the same thing.

Read the biologyInsulin and insulin sensitivity during a fast

Measured in people
The hour-by-hour fasting studies in people start later than this. Klein 1993 took its first sample at 12 hours, in 6 healthy men. Rothman 1991 gives its first figure for new sugar making as an average across the first 22 hours. This row is the fed state those measurements are compared against. No measurement from a long fast falls inside it.

Burning

A mix of leftover sugar and fat. The liver keeps the brain supplied with sugar.

Blood sugar

5.58 mmol/L (100.5 mg/dL) at the 12 hour mark, in 6 healthy men

Ketones

Total ketones around 0.20 mmol/L after an overnight fast. That figure is acetoacetate and BHB added together, not BHB on its own.

Insulin

64.6 pmol/L (9.3 microunits/mL) at 12 hours, in the same 6 men

• At 12 hours, glycerol is released at 2.08 and palmitic acid at 1.63 micromol/kg/min. These are two markers of fat being released. They are the starting numbers everything later in the fast is compared against.
• Making new sugar from scratch already supplies most of the body's sugar at this point. It is not a backup waiting to be called on. This is the first place the popular "switch" story breaks down.

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

Measured in people
Klein 1993 measured blood sugar, insulin and fat release with tracers at 12 hours, in 6 healthy men. Féry 1983 measured total ketones in people after an overnight fast and put the starting level at 0.20 mmol/L. Rothman 1991 measured liver glycogen again and again with a special MRI scan through a 68 hour fast in healthy adults. It worked out that new sugar making supplied 64% of all the body's sugar across the first 22 hours.

Burning

Fat use rising. More and more of the body's sugar is being made fresh, not pulled from storage.

Ketones

A middle change of 0% between 12 and 18 hours, measured every 6 hours in 34 adults

• BHB, the main ketone, does not move in this window. In 34 adults sampled every 6 hours through the standard 72 hour fast doctors use for testing, the middle change from 12 to 18 hours was zero.
• Making new sugar supplies 64% of the body's sugar across the first 22 hours. A second, unrelated method put it at about 47% by 14 hours. The body does not switch from stored sugar to fat at a set hour.
• In 11 overweight adults, activity of the autophagy gene LC3A in blood cells was 22% higher after an 18 hour fast than after a 12 hour fast. In the same people, activity of the MTOR gene also rose, by 9%.

Read the biologyGlycogen depletion and gluconeogenesisKetogenesis and beta-hydroxybutyrateAutophagy

Measured in people
Service 2005 measured BHB every 6 hours in 34 adults whose 72 hour test fasts came back clear for a rare insulin-making tumour. Rothman 1991 measured the share of new sugar making with a special MRI scan. Jamshed 2019 measured gene activity in blood cells in an 11-person, 4 day study where each person tried two eating windows: 8am to 2pm, and 8am to 8pm. That is one gene's activity in blood cells. It is not the real speed of autophagy, and it is not muscle. No study in any species has tested a 16 hour time point for autophagy.

Burning

Fat and glycerol, with ketone making starting its climb

Ketones

Climbing. The middle rise from 18 to 36 hours was +333%, the steepest stretch of a 72 hour fast.

Insulin

70% of the whole drop seen across a 72 hour fast has already happened by 24 hours

• The single biggest jump in fat release across a whole 72 hour fast comes between 18 and 24 hours. 60% of the total rise in fat release happens between 12 and 24 hours. The authors put that down to falling insulin, not to any change in blood sugar.
• In a study of 30 healthy adults, growth hormone was clearly higher after a 24 hour water-only fast than after a normal eating day. Each person did both. The size of that rise is not in the published summary, so it is not given here.
• In the same study, haemoglobin, red blood cell count and haematocrit all rose together at 24 hours. That pattern means the fluid part of the blood shrank. It does not mean new red blood cells were made.
• In 20 volunteers, energy burned over 24 hours, measured in a sealed room, fell during a 24 hour fast. How much it fell varied from person to person.

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

Measured in people
Horne 2013 had 30 seemingly healthy adults do both a 24 hour water-only fast and a normal eating day, in random order. It measured growth hormone, blood counts, cholesterol, triglycerides, bicarbonate and weight. Klein 1993 measured fat release with tracers. Hollstein 2020 measured energy burned over 24 hours in a sealed room in 20 volunteers. The exact percentage drop is not in the published summary, so it is not given here. On autophagy, a 24 hour fast raised p62 (also called SQSTM1) in the thigh muscle of 50 women. That points toward less recycling in muscle, not more.

Burning

Fat. The remaining sugar is built from glycerol, lactate and protein building blocks.

Ketones

1.308 mmol/L on average after day 1, in 13 men starting a 10 day water fast, up from 0.177 at the start. People varied a lot: the spread around that average (1.053) was nearly as big as the average itself.

• Making new sugar supplies 82% of the body's sugar between 22 and 36 hours.
• In 8 healthy men, the thyroid hormone T3 fell from 2.30 to 1.84 nmol/L by 30 hours. TSH, the hormone that tells the thyroid to work, fell below 1 mU/L, and its usual night-time peak disappeared. Blood cortisol did not change.
• So the thyroid slows down within the first 30 hours, well before anyone can measure a drop in resting metabolic rate.
• Salt loss has not turned yet. In 9 women with obesity who kept eating their usual amount of salt while fasting, salt in the urine started to rise after about 48 hours and then went above what they took in.

Read the biologyGlycogen depletion and gluconeogenesisKetogenesis and beta-hydroxybutyrateAutophagy

Measured in people
Rothman 1991 measured the share of new sugar making with a special MRI scan in healthy adults. Hugues 1984 fasted 8 healthy men for 30 hours and tested their thyroid hormones again and again. Dai 2022 measured blood BHB daily in 13 men fasting in a supervised facility. Sigler 1975 collected urine in 3 hour blocks from 9 women with obesity on a hospital ward. On autophagy, the only study in people with several time points in one fast took thigh muscle samples at 2, 12, 24 and 36 hours. The markers LC3I, LC3II and p62 all fell, but only in untrained people. The authors sum it up as muscle recycling being only mildly affected by 36 hours of fasting.

Burning

Fat, with ketones now carrying a real share of the load. Nearly all the body's sugar is now made fresh.

Ketones

A middle rise of +210% between 36 and 54 hours, in 34 adults

• In 9 healthy men, daily growth hormone output rose from 78 to 371 micrograms per litre on the second day of a fast. That is a five-fold rise. The number of bursts went from 14 to 32 a day, and the amount per burst went from 6.3 to 11 micrograms.
• How long growth hormone lasted in the blood did not change, at 18 against 20 minutes. So the body was truly making more, not just clearing it more slowly.
• Blood IGF-1 had not changed at 56 hours. It does not fall until around day 5, when it dropped from 1.31 to 0.77 U/mL in 6 men.
• From 36 hours on, making new sugar supplies 96% of the body's sugar.
• At 48 hours, in a clamp test where insulin is dripped in, the body's sugar use fell from 39.8 to 24.1 micromol/kg/min. Sugar burned for energy crashed from 21.8 to 3.9. Sugar stored did not change. That is a change in which fuel the body burns. It is not the kind of sugar problem seen in diabetes.
• Energy burned over 24 hours, measured in a sealed room, was 10.9% lower on day 2 of a fast than on a day of normal eating (9.8 against 11.0 MJ).

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

Measured in people
Hartman 1992 took blood every 5 minutes for 24 hours in 9 healthy men, on a normal day and on the second day of a fast, and worked out the true output with maths. IGF-1 was measured at 56 hours. Ho 1988 measured IGF-1 (as somatomedin C) through a 5 day fast in 6 men. Mansell 1990 did clamp tests on 6 men without diabetes at 48 hours without food. Andriessen 2023 put 12 healthy lean men in a sealed room for 60 hours straight, once fasting and once eating. No study in people has measured autophagy in any tissue between 36 and 72 hours. That is why autophagy is not linked from this row.

Burning

Fat and ketones. The brain now gets a measurable share of its fuel from BHB.

Blood sugar

4.14 mmol/L (74.6 mg/dL) at 72 hours in 6 healthy men, about 25% below the 12 hour value

Ketones

74% of people whose test fast came back clear went above 2.7 mmol/L before the 72 hour mark. The middle rise from 54 to 72 hours was +167%.

Insulin

30.1 pmol/L (4.3 microunits/mL) at 72 hours, about half the 12 hour value

• Brain BHB was measured directly with a strong MRI scanner aimed at the back of the brain. It rose from 0.05 mmol/L before fasting to 0.60 after the second day and 0.98 after the third. Brain lactate rose from 0.69 to 1.47 mmol/L.
• Blood and brain BHB rose closely together. The brain level rose about 0.26 times as much as the blood level did.
• Over a 48 hour fast, fat built up in the livers of men and in the muscles of women. Sugar handling and fuel burning, in the whole body and the liver, were the same in both sexes.
• The two ways of measuring energy burned split apart here, and both are real. Resting energy burned, measured with a hood, rose 14.1% by day 3 in 11 lean people. Energy burned over the whole day, measured in a sealed room, was still 5.5% below a normal eating day on day 3 (10.3 against 10.9 MJ). The resting part can rise while the whole day falls.
• The 72 hour supervised fast is a routine hospital test that doctors have used for decades to check hormone problems. That is the strongest evidence there is that healthy people handle this length of fast when they are being watched.

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

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

Burning

Fat and ketones. As the fast gets longer, sugar making is split between the liver and the kidneys.

Blood sugar

3.5 mmol/L (63.1 mg/dL) on day 4 in 11 lean adults, and 3.2 mmol/L (57.7 mg/dL) on day 5 in 6 men

Ketones

Ketone making starts to level off after about 5 days. Across a 21 day fast in 13 volunteers, blood ketones went from 0.1 to 6.61 mmol/L.

Insulin

Levels off by day 3 in people with obesity fasting for weeks. Glucagon, which raises blood sugar, about doubles and peaks on the same day.

• In 8 healthy men at 72 hours, muscle samples showed mTOR activity down about 50% and the recycling marker LC3B-II up about 30%. p62 rose about 10% in the same samples. The authors say that makes the real speed of recycling impossible to read from these markers alone.
• 12 healthy adults wore sugar monitors through a 7 day water-only fast. By day 5, their time spent below 70 mg/dL rose from 3% to 66%, and their lowest daily sugar fell from 76 to 50 mg/dL. None of them reported symptoms of low blood sugar. So checking how you feel stops working as a safety check at this point.
• In 15 patients with obesity, blood uric acid about doubled by day 7, from 5.9 to 12.5 mg/dL. It then fell back to 7.7 by day 28, while ketosis carried on. The kidneys cleared uric acid less well, more so than they cleared other waste. So this is a specific change in how the kidneys handle uric acid.
• In 13 volunteers, resting energy burned fell 20.3% across a 21 day complete fast. This is where the slowdown in metabolism finally shows up.
• Nitrogen in the urine, a sign of protein loss, falls steadily for about 4 weeks and then holds at about 3 to 6 g a day. How much protein is lost per kilogram of weight lost depends on body fat. It is roughly 20 g of nitrogen per kg in people without obesity, and roughly 10 g per kg in people carrying 50 kg or more of fat.
• The body runs hundreds of milligrams a day short on minerals, while blood phosphorus and magnesium stay the same. That gap is why the deaths in the medical case reports cluster in the period of eating again, not during the fast.

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

Measured in people
Vendelbo 2014 took muscle samples from 8 healthy men after 72 hours, with tracer tests in the forearm. Kolnes 2026 monitored 12 healthy adults around the clock through a 7 day water-only fast. Fox 1976 measured blood uric acid and how well the kidneys cleared it through 28 days in 15 patients with obesity. Dai 2024 measured resting energy burned and blood ketones across a 21 day fast in 13 volunteers. Owen 1969 measured nitrogen in the urine in 11 people with obesity through 5 to 6 weeks. Forbes 1979 worked out how nitrogen loss scales with body fat. Licata 1981 measured mineral balance directly over 40 days on a hospital ward. On average, people ran 104 mg of calcium, 48 mg of magnesium and 363 mg of phosphorus a day short, while blood phosphorus and magnesium did not change at all.