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Time-restricted10 human studies cited

16:8 Time-Restricted Eating

16 hours without food · Daily

Written by Reviewed Sep 2026

Also called: 8-hour eating window, 16-hour fast

All food inside an 8-hour window, most often noon to 8pm, with no calories for the other 16 hours. It is the most heavily trialled fasting pattern, and the trials disagree about whether the window itself does anything once calories are accounted for.

Overview

16:8 is the pattern most people mean when they say intermittent fasting. Eat within an 8-hour window, drink non-caloric fluids the rest of the time, repeat daily. Nothing is prescribed about what goes in the window.

It has been tested in more randomised trials than any other fasting protocol, in people with obesity, type 2 diabetes, type 1 diabetes, PCOS and in resistance-trained athletes, over durations from 4 weeks to 12 months. That volume of evidence is the reason to take it seriously and also the reason the picture is complicated: the trials that let people eat freely and the trials that controlled calories reached different conclusions, and both sets are still being published.

The consistent finding across the self-directed trials is that people eat several hundred fewer calories per day without being told to count anything. Cienfuegos measured roughly 550 kcal/d less in short-window arms with no calorie instruction. Lin measured 425 kcal/d less over 12 months. Whether anything happens beyond that reduction is the contested part.

How it works

Pick an 8-hour window and hold it. In the trials the window was usually 12:00 to 20:00, which is the schedule used by the Varady group at the University of Illinois Chicago across Lin 2023, Pavlou 2023 and Runchey 2026. Moro used 13:00 to 21:00 in trained men. Liu used 08:00 to 16:00.

The rule the trials enforced was calories, not food. TREAT participants abstained from calories outside the window, and TIMET counted any caloric entry more than 15 minutes outside the personalised window as a day off protocol. Water, black coffee and tea were not what the trials were policing.

Adherence in the randomised trials has been consistently high for a self-directed diet: 6.1 days per week over 12 months in Lin 2023, the same in Pavlou 2023, and 83.5% of days in TREAT. In the trials that also had a calorie-counting arm, the calorie arm adhered less well, 61% of participants meeting their goal in Lin and 68% in Pavlou.

One caution from Oldenburg 2025: when the window was self-selected rather than fixed, the TRE arm actually achieved 9.8 hours, not 8. A prescribed window and an achieved window are different things.

What 16 hours actually reaches

A 16-hour fast, repeated daily, keeps a person in the post-absorptive state for a few hours longer each day than a normal eating pattern does, and lowers 24-hour glucose exposure. That much has been measured. Pavlou 2023 lowered HbA1c by 0.91% relative to control in type 2 diabetes over 6 months, and Runchey 2026 lowered HbA1c relative to calorie restriction in type 1 diabetes with no increase in ketoacidosis or severe hypoglycaemia in 32 randomised adults. Parr 2023 measured a reduced 24-hour glucose area under the curve on an 8-hour window (p = 0.001). The claim that autophagy switches on at 16 hours has no human study behind it. No trial has measured autophagy in a person at a fixed clock hour of fasting, and the human measurements that exist point in different directions depending on which tissue was sampled. The nearest measurement is a 6-month randomised trial in 121 people with obesity that measured autophagic flux in blood cells and found the intermittent fasting plus time-restricted eating arm differed from standard care (P = 0.04, post hoc), with no significant rise from baseline inside that arm and no effect in the calorie restriction arm. That trial combined intermittent fasting with time-restricted eating, so it does not isolate a 16-hour window.

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

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

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

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

  5. 1d to 36h

    Thyroid drops before metabolic rate does

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

    Read the biologyGlycogen depletion and gluconeogenesisKetogenesis and beta-hydroxybutyrateAutophagy

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

Human evidence

More randomised trials than any other fasting protocol, including two 12-month trials and trials in type 1 diabetes, type 2 diabetes and PCOS. Weight loss relative to unrestricted eating is consistent. Superiority over an equivalent calorie prescription is not.

  • Lin 2023, 12 months, 90 randomised and 77 completed at the University of Illinois Chicago. An 8-hour window with no calorie counting lost 4.61 kg relative to control (95% CI -7.37 to -1.85), calorie restriction lost 5.42 kg, and the difference between them was 0.81 kg (P = 0.68). Fasting glucose, fasting insulin, insulin resistance and HbA1c were not significantly changed relative to control (Annals of Internal Medicine, 2023).
  • Liu 2022, 12 months, 139 randomised with both arms prescribed the same calorie targets. Time-restricted eating plus calorie restriction lost 8.0 kg and calorie restriction alone lost 6.3 kg, a net difference of 1.8 kg that was not significant (95% CI -4.0 to 0.4; P = 0.11). Body fat, lean mass, blood pressure and metabolic risk factors all tracked the primary result (New England Journal of Medicine, 2022).
  • Lowe 2020, the TREAT trial, 12 weeks and 141 randomised. The TRE arm lost 0.94 kg and the structured three-meal control lost 0.68 kg, a between-group difference of 0.26 kg that was not significant (P = .63). The trial's most cited result is its body composition finding, covered below (JAMA Internal Medicine, 2020).
  • Pavlou 2023, 6 months in 75 adults with type 2 diabetes and a mean baseline HbA1c of 8.1%. Weight fell 3.56% relative to control (P = .004) and HbA1c fell 0.91%, with no difference from 25% calorie restriction on HbA1c. No serious adverse events (JAMA Network Open, 2023).
  • Moro 2016, 8 weeks in 34 resistance-trained men with energy and macronutrients matched between arms and a supervised training programme. Fat mass fell in the TRE arm relative to normal diet (time by diet interaction P = 0.0448) while fat-free mass, limb muscle area and maximal strength were maintained in both. Total testosterone and IGF-1 fell significantly in the TRE arm only (P = 0.0476 and P = 0.0397) and triiodothyronine fell (Journal of Translational Medicine, 2016).
  • Oldenburg 2025, 12 weeks in 88 adults with a hyperinsulinaemic euglycaemic clamp and MRI. Weight, body composition, caloric intake and glycaemic measures were similar across TRE, 15% calorie restriction and unrestricted eating, and the TRE arm achieved a 9.8 hour window rather than the prescribed 8 (Obesity, 2025).

What this does not tell you: Two of the three largest trials measured body composition as a secondary outcome in a subgroup, and none was powered as a body composition trial. Diet was self-reported in every free-living trial. The trials with the strongest weight results compared against unrestricted eating or standard care, which answers a different question from the trials that matched the calorie prescription. Nobody has run a randomised trial of any fasting protocol with cardiovascular events or mortality as the endpoint.

Reading the research record

The live disagreement is whether the window adds anything once calorie intake is controlled, and two trials that both controlled calories reached opposite conclusions.

Liu 2022 in the New England Journal of Medicine prescribed 1500 to 1800 kcal/d for men and 1200 to 1500 kcal/d for women in both arms and ran for 12 months in 139 adults with obesity in Guangzhou. Adding an 08:00 to 16:00 window produced 1.8 kg more weight loss, which was not significant, and no separation on body fat, lean mass or metabolic risk factors. The authors concluded the window was not more beneficial than daily calorie restriction.

Jamshed 2022 in JAMA Internal Medicine gave both arms identical energy restriction and identical weight-loss counselling and ran for 14 weeks in 90 adults at a US weight loss clinic. A 07:00 to 15:00 window produced 2.3 kg more weight loss (95% CI -3.7 to -0.9; P = .002) and 4 mmHg better diastolic blood pressure. Fat loss did not separate (-1.4 kg; P = .09), and neither did the ratio of fat loss to weight loss.

The two trials differ in duration, population, and in what the control arm was doing: Liu's control was actively calorie counting, Jamshed's was eating over 12 or more hours with the same energy prescription. Lowe 2020 is not directly comparable to either, because nothing about calories was prescribed in it at all. This page does not pick a winner between them. The largest registered trial capable of settling the placement question, a 162-person three-arm comparison of early TRE, late TRE and daily calorie restriction at the University of Colorado Denver, has a primary completion date of March 2027.

What happens to muscle

This is where the trials diverge most sharply, and the divergence is informative rather than noise.

In TREAT's in-person subgroup of 50 people with DXA, the TRE arm lost 1.70 kg of body weight, of which 1.10 kg was lean mass (95% CI -1.73 to -0.48; P < .001) and only 0.51 kg was fat mass. The authors put the lean share at approximately 65% of weight lost and contrasted it with the 20% to 30% they describe as typical. Appendicular lean mass index fell 0.22 kg/m2, with a between-group difference of 0.16 kg/m2 (P = .005).

Chow 2020 found the same direction in 20 people over 12 weeks: weight -3.7%, fat mass -4%, lean mass -3.0%, with lean mass falling significantly compared with the non-TRE arm.

Lin 2023 found the opposite over 12 months in 90 people. Fat mass, waist circumference and BMI fell relative to control while lean mass, visceral fat, bone mineral density and bone mineral content did not differ from control.

Moro 2016 preserved fat-free mass completely, in trained men eating 1.93 g/kg/day of protein with calories matched and three supervised sessions a week. The same cohort followed to 12 months tells the other half of the story: at 12 months the TRE arm's arm muscle cross-sectional area fell 4.31% (p = 0.003) and thigh area fell 2.90% (p = 0.03) while the normal-diet arm gained 11.87% and 6.94%, and the control arm gained fat-free mass while TRE did not (interaction p = 0.002). Energy intake in the TRE arm had drifted down 6.4% by then. Bench press and leg press one-rep max rose equally in both arms (p = 0.791 and p = 0.740), so the cross-sectional area loss did not show up as a strength deficit. Limb area in both Moro papers was measured by anthropometry rather than imaging, which is a real weakness, although the direction is hard to explain as a skinfold artefact since the group that lost fat is the group whose measured area went down.

The variable that tracks the split is not the window. Every trial that preserved lean mass ran protein at 1.6 to 2.2 g/kg/day with resistance training. The trial that measured integrated daily myofibrillar protein synthesis directly, Parr 2023, ran an 8-hour window at 1.0 g/kg/day for 10 days and found synthesis unchanged at 1.28 versus 1.26 %/day (p = 0.82) while lean mass still fell more in the TRE arm (-1.0 versus -0.2 kg; p = 0.01). The mechanism and the outcome disagreed inside one trial, and protein intake is the difference between that trial and the ones that preserved lean mass.

Doing it

Protein target
Every TRE trial that held lean mass ran 1.6 to 2.2 g/kg/day. Parr 2023 ran 1.0 g/kg/day and lost more lean mass. Morton 2018, across 49 studies and 1863 participants, found no further fat-free mass gain above a total intake of about 1.62 g/kg/day in energy balance.
Training
Resistance training is present in every trial in this literature where lean mass was maintained or gained during an energy deficit. There is no trial showing lean mass gain in a deficit without it. Moro's trained men trained from 16:00 to 18:00, inside their feeding window.
What breaks the fast
The trials policed calories, not intake. TREAT participants abstained from calories from 20:00 to 12:00. TIMET counted a caloric entry more than 15 minutes outside the window as a day off protocol. No trial here tested a threshold below which a small amount of calories is neutral.
Window placement
Trials have used 08:00 to 16:00, 12:00 to 20:00 and 13:00 to 21:00 and no head-to-head trial at 8 hours has separated them. The one crossover that compared early and delayed 9-hour windows found no difference between the two conditions.
Medication review first
Insulin, sulfonylureas, meglitinides, SGLT2 inhibitors, ACE inhibitors, ARBs and diuretics all behave differently against a compressed intake. In one review of intermittent fasting a 50% basal insulin reduction on fasting days still produced significant hypoglycaemia rates.
Expect the window to drift
The self-selected arm in Oldenburg 2025 achieved 9.8 hours against a prescribed 8. If the window matters to you, it has to be logged, which is what the higher-adherence trials did.

Breaking the fast

Nothing in the randomised record prescribes a particular first meal for a 16-hour fast, and no trial tested one. What is measured is the protein arithmetic. A single 20 g dose of isolated protein maximally stimulated muscle protein synthesis over 4 hours in young men in two studies, one with 6 participants and one with 48, but the ceiling those studies established does not hold at larger doses: 100 g of milk protein after whole-body resistance exercise produced a larger and longer response than 25 g, lasting beyond 12 hours, with negligible change in amino acid oxidation. Older adults need more per meal, with 40 g beating 20 g after exercise. The practical consequence is that a person eating 150 g of protein across three meals inside an 8-hour window is not, on the human evidence, wasting the surplus.

Citations

  1. Human2023
    Time-Restricted Eating Without Calorie Counting for Weight Loss in a Racially Diverse Population: A Randomized Controlled Trial

    Annals of Internal Medicine

    90 randomised, 77 completed, 12 months. An 8-hour window with no calorie counting lost 4.61 kg versus control (95% CI -7.37 to -1.85), calorie restriction lost 5.42 kg, TRE versus CR 0.81 kg (P = 0.68). Energy intake fell 425 kcal/d in TRE. Lean mass, visceral fat and bone density did not differ from control. Adherence 6.1 days per week over 12 months.

  2. Human2022
    Calorie Restriction with or without Time-Restricted Eating in Weight Loss

    New England Journal of Medicine

    139 adults with obesity, both arms on the same prescribed calorie targets for 12 months. TRE plus calorie restriction -8.0 kg (95% CI -9.6 to -6.4), calorie restriction alone -6.3 kg, net difference -1.8 kg (95% CI -4.0 to 0.4; P = 0.11). Waist, BMI, body fat, lean mass, blood pressure and metabolic risk factors were all consistent with the primary result.

  3. Human2020
    Effects of Time-Restricted Eating on Weight Loss and Other Metabolic Parameters in Women and Men With Overweight and Obesity: The TREAT Randomized Clinical Trial

    JAMA Internal Medicine

    141 randomised, 12 weeks, remote with a Bluetooth scale. TRE -0.94 kg versus control -0.68 kg, between-group -0.26 kg (P = .63). In the 50-person in-person DXA subgroup the TRE arm lost 1.70 kg, of which 1.10 kg was lean mass (P < .001) and 0.51 kg fat mass, approximately 65% of weight lost as lean by the authors' own reading. Protein intake was not measured.

  4. Human2023
    Effect of Time-Restricted Eating on Weight Loss in Adults With Type 2 Diabetes: A Randomized Clinical Trial

    JAMA Network Open

    75 adults with type 2 diabetes, mean baseline HbA1c 8.1%, 6 months. An 8-hour window lost 3.56% of body weight versus control (P = .004) and lowered HbA1c 0.91%, matching 25% calorie restriction on HbA1c. Time in euglycaemic range, medication effect score, blood pressure and lipids did not differ. No serious adverse events.

  5. Human2016
    Effects of eight weeks of time-restricted feeding (16/8) on basal metabolism, maximal strength, body composition, inflammation, and cardiovascular risk factors in resistance-trained males

    Journal of Translational Medicine

    34 resistance-trained men, 8 weeks, energy and macronutrients matched between arms, protein 1.93 versus 1.89 g/kg/day, supervised training. Fat mass fell in TRE relative to normal diet (interaction P = 0.0448); fat-free mass, limb area and maximal strength maintained in both. Testosterone and IGF-1 fell in TRE only (P = 0.0476 and P = 0.0397); triiodothyronine fell.

  6. Human2021
    Twelve Months of Time-restricted Eating and Resistance Training Improves Inflammatory Markers and Cardiometabolic Risk Factors

    Medicine and Science in Sports and Exercise

    The Moro cohort extended to 12 months, 20 analysed. TRE arm muscle cross-sectional area -4.31% (p = 0.003) and thigh -2.90% (p = 0.03) while the normal-diet arm gained 11.87% and 6.94%; fat-free mass interaction p = 0.002 favouring normal diet. Strength rose equally in both arms. Energy intake in TRE fell 6.4% spontaneously. The widely quoted 18.8% visceral fat reduction was not significant (p = 0.518). Limb area by anthropometry, not imaging.

  7. Human2023
    Eight-hour time-restricted eating does not lower daily myofibrillar protein synthesis rates: A randomized control trial

    Obesity (Silver Spring)

    18 men, mean age 46, BMI 30, isoenergetic diet at about 1.0 g/kg/day protein, 10 days of a 10:00 to 18:00 window versus 08:00 to 20:00, integrated myofibrillar synthesis by deuterated water. Synthesis did not differ (1.28 versus 1.26 %/day, p = 0.82). 24-hour glucose area under the curve fell (p = 0.001). Lean mass loss was greater in TRE (-1.0 versus -0.2 kg, p = 0.01).

  8. Human2025
    Time-restricted eating, caloric reduction, and unrestricted eating effects on weight and metabolism: a randomized trial

    Obesity (Silver Spring)

    88 enrolled, 81 completed, 12 weeks, with DXA, MRI and a hyperinsulinaemic euglycaemic clamp. Weight, body composition, caloric intake and glycaemic measures were similar across self-selected 8-hour TRE, 15% calorie restriction and unrestricted eating. The TRE arm achieved a 9.8 hour window (95% CI 9.0 to 10.6). Metabolic flexibility was lower in TRE than in calorie restriction.

  9. Human2022
    Effectiveness of Early Time-Restricted Eating for Weight Loss, Fat Loss, and Cardiometabolic Health in Adults With Obesity: A Randomized Clinical Trial

    JAMA Internal Medicine

    90 adults, 14 weeks, both arms on identical energy restriction and weight-loss counselling. A 07:00 to 15:00 window produced 2.3 kg more weight loss (95% CI -3.7 to -0.9; P = .002) and 4 mmHg better diastolic blood pressure. Fat loss did not separate (-1.4 kg; P = .09) and neither did the fat-to-weight-loss ratio (P = .43). Included here as the calorie-controlled trial that disagrees with Liu 2022.

  10. Human2020
    Time-Restricted Eating Effects on Body Composition and Metabolic Measures in Humans who are Overweight: A Feasibility Study

    Obesity (Silver Spring)

    20 participants, 12 weeks, DXA and 2-week continuous glucose monitoring, 8-hour window versus non-TRE. Within the TRE group weight fell 3.7%, fat mass 4% and lean mass 3.0%, with visceral fat down 11.1%. Lean mass fell significantly compared with the non-TRE arm. The achieved window was 9.9 hours.

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.

  • Eating less without counting anything, which the trials measured as roughly 425 to 550 kcal/d
  • Hunger in the first weeks, then a reported drop in desire to eat
  • Finding the rule easier to keep than a calorie target, which matches the adherence gap in the trials
  • Difficulty hitting a protein target inside the window
  • Social friction around breakfast and early dinners

Sources: Uncontrolled self-report. The items above are participant-reported outcomes collected inside the randomised trials cited on this page, not survey data, and self-reported diet is a known weak point in every free-living fasting trial. A separate review of what fasting communities report is still being compiled and nothing here is drawn from it.

Who this is wrong for

  • Anyone taking an SGLT2 inhibitor, unless a prescriber has stopped it first. The combination can produce ketoacidosis at a normal-looking blood glucose, which a fingerstick reading will not warn you about. The FDA advises stopping canagliflozin, dapagliflozin and empagliflozin at least 3 days and ertugliflozin at least 4 days before scheduled surgery for exactly this reason, and a planned fast is the same metabolic exposure.
  • Anyone on insulin, a sulfonylurea or a meglitinide without a prescriber adjusting the dose in advance. In the CREED cohort of 3250 people with type 2 diabetes, hypoglycaemia incidence during Ramadan was 16.8% on insulin and 5.3% on oral agents, and the strongest predictor was having had an episode before (odds ratio 7.80).
  • Anyone with type 1 diabetes outside a structured programme with glucose monitoring and an insulin plan. The one randomised trial in type 1 diabetes was 32 people over 6 months with no increase in ketoacidosis or severe hypoglycaemia, which is encouraging and is not a mandate to do this unsupervised.
  • Pregnancy and breastfeeding. Lactation draws roughly 30 to 50 g of glucose a day out of the mother, and dieting or exercise to lose weight was the precipitant in 76% of 18 reported cases of lactation ketoacidosis.
  • Anyone with a history of an eating disorder, or current loss-of-control eating. A protocol scored by adherence to a restriction rule creates the conditions for rule-tightening.
  • Children and adolescents. There are no long-term growth or development outcome studies, and this is the age band where the eating disorder association in survey data is strongest.
  • Anyone with a BMI under 18.5, or who meets the NICE refeeding risk criteria on the way in.
  • Anyone with chronic kidney disease taking an ACE inhibitor or ARB, where volume loss and blocked efferent arteriolar compensation remove both halves of the kidney's autoregulation.

Questions

Does 16 hours of fasting turn on autophagy?
No human study establishes that number, or any clock hour. Autophagy runs continuously and changes rate rather than switching on at a threshold, and the human measurements that exist disagree with each other depending on which tissue was sampled. The strongest human result is a 6-month randomised trial in 121 people that measured autophagic flux in blood cells and found a fasting arm differing from standard care in a post hoc comparison, with no significant rise from baseline inside that arm.
Will I lose muscle on 16:8?
The trials split, and protein intake is what separates them. TREAT reported about 65% of the weight lost in its in-person subgroup as lean mass at an unmeasured protein intake. Lin 2023 found no lean mass change over 12 months. Moro 2016 preserved fat-free mass entirely at 1.93 g/kg/day with supervised resistance training. Parr 2023 at 1.0 g/kg/day lost more lean mass than the control window while muscle protein synthesis was identical between arms.
Is 16:8 better than just counting calories?
The two 12-month trials that matched calorie prescriptions came out differently. Liu 2022 found adding an 8-hour window to calorie restriction produced 1.8 kg more loss, which was not significant. Jamshed 2022 found an early window plus identical energy restriction produced 2.3 kg more loss than a 12-hour window plus the same restriction. What is consistent is that the window arms adhered better than the calorie-counting arms in the trials that measured both.
Does black coffee break the fast?
The trials did not test that question. What they enforced was calories: TREAT participants abstained from calories outside the window, and TIMET treated any caloric entry more than 15 minutes outside the window as a day off protocol. No randomised trial identified here established a threshold below which a small caloric intake is neutral.