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Intermittent7 human studies cited

Alternate-Day Fasting

1 day without food · Every other day

Written by Reviewed Sep 2026

Also called: ADF, Modified alternate-day fasting, Every other day fasting

A fast day every other day, usually at 25% of energy needs rather than zero. It matched daily calorie restriction on weight over a year, had the highest dropout of any arm in that trial, and raised LDL cholesterol relative to daily restriction by month 12.

Overview

Alternate-day fasting alternates a fast day with an unrestricted or feast day. Most trials use a modified version at about 25% of energy needs on the fast day, roughly 500 to 600 kcal, with 125% prescribed on the alternating day. A few use zero calories.

It is the fasting protocol with the longest randomised follow-up outside of the time-restricted trials, and the one where the results are most double-edged. Over a year it matched daily calorie restriction on weight. It also had the highest dropout of any arm in that trial at 38%, and by month 12 LDL cholesterol was significantly elevated relative to daily restriction.

The most interesting trial in the set is not the longest one. Templeman 2021 was built specifically to separate fasting from energy deficit, with one arm fasting without a net deficit, and it found that when the deficit was removed almost nothing happened.

How it works

Fast day, normal day, repeat. The prescriptions used in trials: 25% of energy needs on fast days and 125% on feast days in Trepanowski, 600 kcal on the fast day with an ad libitum feast day in Ezpeleta, 500 kcal fast days in the pooled sex-difference analysis, and zero calories in Catenacci's pilot.

Adherence behaves in a specific and repeatable way that anyone considering this should know: participants ate more than prescribed on fast days and less than prescribed on feast days. In the pooled analysis of three 12-week trials, intake on fast days exceeded the prescription by roughly 400 to 500 kcal in every group with no differences between them. The protocol self-corrects toward a smaller deficit than the paper describes.

Dropout is the other repeatable finding. In the one-year trial it was 38% in the alternate-day arm against 29% on daily restriction and 26% in the no-intervention control.

What 1 day actually reaches

What has been measured in humans on alternate-day fasting is a rise in beta-hydroxybutyrate that persists even on non-fasting days, a reduction in trunk fat, an improved fat-to-lean ratio, and in people who had practised it for more than six months, reduced sICAM-1, reduced LDL and reduced triiodothyronine. The trial reporting those has an erratum published the following year and should be read alongside it. Beyond that, the honest position is that the fasting-specific physiology has been tested and mostly did not appear. Templeman 2021 ran three arms in lean healthy adults: 24-hour fasting with 150% intake on alternate days to produce a net deficit, matched continuous restriction at 75% of intake daily, and fasting without a net deficit at 200% intake on alternate days. Postprandial cardiometabolic indices, gut hormones and expression of key genes in subcutaneous adipose tissue did not differ between groups (all P > 0.05). The arm that fasted without a deficit lost 0.52 kg of body mass and 0.12 kg of fat. There is no human measurement of autophagy or mitophagy on an alternate-day protocol. The nearest is a 24-hour fast performed three days a week with muscle biopsies, where the authors concluded that autophagy markers in human muscle were reduced and attributed it to weight loss. This page does not claim ADF reaches either mechanism.

Fig. 1 · what has been measured, hour by hour
  1. 0h to 4h

    Still absorbing

    Burning
    Glucose from the meal just eaten, with insulin high enough to hold fat release down
    • Insulin is at its post-meal peak, and while it is there, adipose tissue lipase stays suppressed and fat is being stored rather than released.
    • The hour count on most fasting charts starts at the last bite. Absorption of a mixed meal is still running here, so the clock and the physiology are not yet the same thing.

    Read the biologyInsulin and insulin sensitivity during a fast

    Measured in people
    The human hour-by-hour fasting series begin later than this. Klein 1993 took its first sample at 12 hours in 6 healthy men, and Rothman 1991 reports its first gluconeogenesis figure as an average across the first 22 hours. This row is the fed state those measurements are compared against, and no extended-fasting measurement exists inside it.

  2. 4h to 12h

    Post-absorptive

    Burning
    A mix of remaining glucose and fatty acids, with the liver covering the brain's glucose demand
    Blood glucose
    5.58 ± 0.08 mmol/L (100.5 mg/dL) at the 12 hour mark, in 6 healthy men
    Ketones (BHB)
    Total ketone bodies around 0.20 mmol/L in overnight-fasted subjects. That figure is acetoacetate plus beta-hydroxybutyrate, not BHB on its own.
    Insulin
    64.6 ± 12.9 pmol/L (9.3 microunits/mL) at 12 hours, in the same 6 men
    • Glycerol release runs at 2.08 ± 0.22 and palmitic acid release at 1.63 ± 0.20 micromol/kg/min at 12 hours. Those are the numbers everything later in the fast is measured against.
    • Gluconeogenesis is already the majority contributor to glucose production rather than a reserve waiting to be called on, which is the first place the popular switch model breaks.

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

    Measured in people
    Klein 1993 measured glucose, insulin and stable-isotope lipid kinetics at 12 hours in 6 healthy men. Féry 1983 measured total ketone body turnover in overnight-fasted subjects and put the baseline at 0.20 mmol/L. Rothman 1991 measured hepatic glycogen serially by 13C nuclear magnetic resonance through a 68 hour fast in healthy adults and calculated that gluconeogenesis supplied 64 ± 5% of total glucose production across the first 22 hours.

  3. 12h to 18h

    Ketones have not moved yet

    Burning
    Fatty acids rising, with glucose production increasingly made rather than released from store
    Ketones (BHB)
    Median change of 0% between 12 and 18 hours, sampled every 6 hours in 34 adults
    • Beta-hydroxybutyrate does not move in this window. In 34 adults sampled every 6 hours through the standard diagnostic 72 hour fast, the median change from 12 to 18 hours was zero.
    • Gluconeogenesis is supplying 64 ± 5% of glucose production across the first 22 hours, confirmed by an orthogonal method at about 47% by 14 hours. Glucose production does not switch from glycogen to fat at a threshold hour.
    • Whole blood LC3A messenger RNA was 22 ± 5% higher at 18 hours than in a 12 hour control arm in 11 overweight adults (p = 0.001). In the same subjects MTOR expression also rose, by 9 ± 3%.

    Read the biologyGlycogen depletion and gluconeogenesisKetogenesis and beta-hydroxybutyrateAutophagy

    Measured in people
    Service 2005 measured beta-hydroxybutyrate every 6 hours in 34 adults whose 72 hour fasts were negative for insulinoma. Rothman 1991 measured the gluconeogenic share by 13C NMR. Jamshed 2019 measured whole blood messenger RNA in an 11-person 4 day randomised crossover of an 08:00 to 14:00 eating window against 08:00 to 20:00. That is one gene's expression in blood cells, not autophagic flux and not muscle. No study in any species has tested a 16 hour timepoint for autophagy.

  4. 18h to 1d

    The steepest hours for fat release

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

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

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

  5. 1d to 36h

    Thyroid drops before metabolic rate does

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

    Read the biologyGlycogen depletion and gluconeogenesisKetogenesis and beta-hydroxybutyrateAutophagy

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

Human evidence

One 12-month randomised trial with DXA every 6 months, one 3-week trial designed to isolate fasting from energy deficit, a zero-calorie pilot, a 4-week trial in healthy non-obese adults, and a 3-month trial combining it with exercise in fatty liver disease.

  • Trepanowski 2017, one year, 100 metabolically healthy adults with obesity randomised to alternate-day fasting at 25% of energy needs on fast days, daily calorie restriction at 75%, or no-intervention control, with a 6-month weight-loss phase and a 6-month maintenance phase. Weight versus control at month 12 was -6.0% for fasting and -5.3% for daily restriction. Dropout was 38% in the fasting arm, the highest of the three. No between-intervention differences in blood pressure, heart rate, triglycerides, fasting glucose, fasting insulin, insulin resistance, CRP or homocysteine at month 6 or 12. HDL rose in the fasting arm at month 6 (+6.2 mg/dL; 95% CI 0.1 to 12.4) but not at 12, and LDL was significantly elevated in the fasting arm by month 12 (+11.5 mg/dL; 95% CI 1.9 to 21.1) (JAMA Internal Medicine, 2017).
  • Templeman 2021, 3 weeks, lean healthy adults, 12 per arm, designed to separate fasting from energy deficit. Body mass fell 1.91 kg on matched continuous restriction, 1.60 kg on fasting with a deficit (P = 0.46 against continuous) and 0.52 kg on fasting without a deficit. Fat mass told a different story: 1.75 kg on continuous restriction, meaning almost all of the loss was fat, against 0.74 kg on fasting with a deficit (P = 0.01) despite similar total weight loss, and 0.12 kg without a deficit. The authors concluded that alternate-day fasting less effectively reduces body fat mass than a matched degree of daily energy restriction, without evidence of fasting-specific effects (Science Translational Medicine, 2021).
  • Catenacci 2016, 8 weeks of zero-calorie alternate-day fasting in 14 adults against daily restriction at minus 400 kcal/day in 12, then 24 weeks unsupervised. 93% completed the 8-week protocol with no adverse effects attributed to it. Fasting achieved a 376 kcal/day greater energy deficit yet weight change did not differ significantly (-8.2 kg versus -7.1 kg), and neither did body composition, lipids or insulin sensitivity (Obesity, 2016).
  • Stekovic 2019, 4 weeks of strict alternate-day fasting in healthy non-obese adults, with a longer-term component in people who had already practised it for more than six months. Average calorie reduction was 37%. Cardiovascular markers improved, fat mass fell particularly trunk fat, the fat-to-lean ratio improved, and beta-hydroxybutyrate rose even on non-fasting days. After long-term practice, sICAM-1, LDL and triiodothyronine were reduced. No adverse effects occurred even after more than six months. An erratum was published in 2020 and should be read with it (Cell Metabolism, 2019).
  • Ezpeleta 2023, 3 months in 80 adults with obesity and non-alcoholic fatty liver disease, four arms. Intrahepatic triglyceride content fell 5.48% in the combined fasting plus exercise arm against 1.30% for exercise alone (P = 0.02) and 0.17% for control (P < 0.01); against fasting alone at 2.25% the difference was P = 0.05, so the combination was not clearly better than fasting by itself. Lean mass, AST, HbA1c, blood pressure, plasma lipids and liver fibrosis score did not differ between groups (Cell Metabolism, 2023).
  • Lin 2021, a secondary analysis pooling 75 men and women from three 12-week trials at 500 kcal on fast days. Weight fell 4.6% in premenopausal women, 6.5% in postmenopausal women and 6.2% in men, with no group by time interaction, and fat mass, lean mass, fasting insulin, insulin resistance and blood pressure fell similarly across all three. Fast-day intake exceeded the prescription by roughly 400 to 500 kcal in every group. LDL fell more in postmenopausal than premenopausal women (P = 0.01) (Nutrition, Metabolism and Cardiovascular Diseases, 2021).

What this does not tell you: The one-year trial randomised 100 people and lost 38% of the fasting arm. Templeman ran 3 weeks with 12 per arm in lean healthy adults. Catenacci is a 26-person pilot. The pooled sex analysis is secondary, from overlapping trial infrastructure at one research group. Diet was self-reported in all of them and fast-day intake consistently exceeded what was prescribed. No randomised trial of this protocol was designed or powered for cardiovascular events or mortality.

Reading the research record

There are two real disagreements here and one piece of public confusion.

The first disagreement is about fat mass. Templeman found alternate-day fasting reducing body fat less effectively than a matched daily deficit, 0.74 kg against 1.75 kg (P = 0.01), despite similar total weight loss. Catenacci found fat mass loss identical between zero-calorie fasting and daily restriction. Trepanowski found no separation on weight over a year. The trials differ in length, in population and in whether the fast days were zero-calorie or modified, and none is large.

The second is about lipids. LDL cholesterol was significantly elevated in the fasting arm relative to daily restriction by month 12 in the one-year trial (+11.5 mg/dL; 95% CI 1.9 to 21.1), while long-term practitioners in Stekovic showed reduced LDL, and postmenopausal women in the pooled analysis showed LDL falling more than premenopausal women did. These are different designs measuring different populations, and the year-long randomised result is the one with the strongest design behind it. It should not be omitted from any summary of the protocol.

The public confusion is the eating-duration mortality analysis. Published in 2025, it reports a cardiovascular mortality hazard ratio of 2.35 (95% CI 1.39 to 3.98) for a derived eating duration under 8 hours against 12 to 14 hours, from 19,831 NHANES adults with a median follow-up of 8.1 years, and null results for all-cause and cancer mortality, with the all-cause association failing many sensitivity analyses. Its exposure is derived from two 24-hour dietary recalls standing in for years of habit, and the abstract version that generated the 91% headline was never peer reviewed at the time it was covered. A formal rebuttal signed by a large group of circadian and nutrition researchers, several of whom report industry relationships, was published in the same journal. It is an observational association in a small exposed subgroup that its own authors decline to interpret causally, and it is not about alternate-day fasting, which does not produce a short daily eating window.

What happens to muscle

Alternate-day fasting has better body composition data than 5:2 and the picture is not flattering.

Catenacci 2016 used DXA over 8 weeks in adults with obesity. Fat mass loss was identical between arms at 3.7 kg (p = 0.995). Lean mass loss was 3.2 kg on fasting and 2.6 kg on daily restriction (p = 0.539). Derived from those DXA compartments, lean mass was roughly 46% of the loss on fasting and 41% on daily restriction, which is a high fat-free-mass fraction for an 8-week intervention in this population. That derivation is arithmetic on the reported compartments rather than a figure the authors published, and the paper's own table footnote warns that scale weight change does not match the sum of the DXA fat and lean changes.

Trepanowski 2017 measured lean mass by DXA every six months in 100 people over a year. At month 6, lean mass versus control was -1.5 kg (95% CI -3.2 to 0.2) on fasting and -2.1 kg (95% CI -3.8 to 0.4) on daily restriction; at month 12, -0.9 kg and -1.4 kg. Every one of those confidence intervals crosses zero, so the trial did not demonstrate a lean mass difference in either direction.

Templeman's result sits alongside these. Fasting with a deficit lost similar total weight to daily restriction but significantly less fat, which by inference means a larger non-fat share, in lean healthy adults over 3 weeks.

Against that, a 32-person 12-week trial in people with a BMI of 20 to 29.9 reported weight down 5.2 kg and fat mass down 3.6 kg with fat-free mass unchanged versus controls, and a scoping review of 20 randomised trials in 1653 adults aged 45 and over found between-group differences in total fat-free mass generally small and comparable to continuous energy restriction. Those same authors note that DXA and bioimpedance measures of total fat-free mass may not reflect skeletal muscle change, and that the evidence does not permit definitive conclusions.

What the protein literature adds is that none of these trials controlled protein intake or included resistance training, and that during an energy deficit the proportion of weight lost as fat-free mass was lower at 1.6 and 2.4 g/kg/day than at 0.8 g/kg/day, while a slower rate of loss preserved more lean tissue for the same total loss.

Doing it

What a fast day contained
25% of energy needs in the one-year trial, 600 kcal in the fatty liver trial, 500 kcal in the pooled analysis. Only one small pilot used zero calories, and it found no significant weight or body composition advantage over daily restriction despite a 376 kcal/day larger deficit.
Check LDL cholesterol
LDL was significantly elevated in the fasting arm relative to daily calorie restriction by month 12 in the one-year randomised trial (+11.5 mg/dL; 95% CI 1.9 to 21.1). That is a real finding from the strongest design in this section.
Protein and training
No alternate-day trial controlled protein or supervised resistance training. In the wider literature, resistance training is present in every trial where lean mass was maintained or gained during a deficit, and the proportion of loss coming from fat-free mass was lower at 1.6 and 2.4 g/kg/day than at 0.8.
Electrolytes and fluid on fast days
Falling insulin drives sodium excretion early in a fast and the natriuresis and diuresis peak in the 24 to 72 hour window, which is where orthostatic symptoms are most reported. No published protocol monitors electrolytes for fasts this short, and a 500 kcal day carrying salt and fluid is a different exposure from a dry one.
Expect the deficit to shrink
Participants ate more than prescribed on fast days and less than prescribed on feast days in the year-long trial, and fast-day intake ran 400 to 500 kcal over prescription across three pooled trials. The achieved deficit is smaller than the protocol on paper.
Plan for the twelve-month mark
Dropout in the fasting arm of the one-year trial was 38%, against 29% on daily restriction and 26% in the no-intervention control. Whatever else is true, this is the protocol people leave.

Breaking the fast

The trials ended each fast day by returning to a normal or feast day without a prescribed refeed, and the observed behaviour was that people ate less than prescribed on those days. The measurement worth carrying is from prolonged fasting rather than a single day: after a three-week fast in 10 moderately obese women, one week of 80 g of protein a day moved nitrogen balance from -5.9 g/day to +1.7 g/day (P < 0.05), while 80 g of carbohydrate moved it only to -3.1 g/day. Refeeding syndrome is not a risk at a 24-hour fast day unless a person already met the NICE high-risk criteria before starting.

Citations

  1. Human2017
    Effect of Alternate-Day Fasting on Weight Loss, Weight Maintenance, and Cardioprotection Among Metabolically Healthy Obese Adults: A Randomized Clinical Trial

    JAMA Internal Medicine

    100 adults with obesity, one year, three arms, DXA every 6 months. Weight versus control at month 12: -6.0% fasting, -5.3% daily restriction. Dropout 38% fasting, 29% restriction, 26% control. Lean mass versus control at month 6 was -1.5 kg (95% CI -3.2 to 0.2) fasting and -2.1 kg (95% CI -3.8 to 0.4) restriction, every interval crossing zero. LDL was significantly elevated in the fasting arm versus restriction by month 12 (+11.5 mg/dL; 95% CI 1.9 to 21.1). Participants ate more than prescribed on fast days and less on feast days.

  2. Human2021
    A randomized controlled trial to isolate the effects of fasting and energy restriction on weight loss and metabolic health in lean adults

    Science Translational Medicine

    3 weeks, lean healthy adults, 12 per arm: fasting with a net deficit, matched continuous restriction, and fasting without a net deficit. Body mass -1.91 kg continuous, -1.60 kg fasting with deficit (P = 0.46), -0.52 kg fasting without deficit. Fat mass -1.75 kg continuous versus -0.74 kg fasting with deficit (P = 0.01) and -0.12 kg without. No differences in postprandial cardiometabolic indices, gut hormones or adipose gene expression (all P > 0.05).

  3. Human2016
    A randomized pilot study comparing zero-calorie alternate-day fasting to daily caloric restriction in adults with obesity

    Obesity (Silver Spring)

    8 weeks of zero-calorie alternate-day fasting (n = 14) versus daily restriction at minus 400 kcal/day (n = 12), then 24 weeks unsupervised. 93% completed the fasting protocol with no attributed adverse effects. Fasting achieved a 376 kcal/day greater deficit yet weight change did not differ (-8.2 versus -7.1 kg). DXA fat mass loss was identical at -3.7 kg in both (p = 0.995); lean mass loss -3.2 kg fasting versus -2.6 kg restriction (p = 0.539). At 24 weeks, changes in percent fat and lean mass were more favourable in the fasting arm.

  4. Human2019
    Alternate Day Fasting Improves Physiological and Molecular Markers of Aging in Healthy, Non-obese Humans

    Cell Metabolism

    4 weeks of strict alternate-day fasting in healthy middle-aged adults, plus a longer-term component in people practising it for more than 6 months. Average calorie reduction 37%. Cardiovascular markers improved, fat mass fell particularly trunk fat, the fat-to-lean ratio improved and beta-hydroxybutyrate rose even on non-fasting days. Long-term practice showed reduced sICAM-1, LDL and triiodothyronine. No adverse effects after more than 6 months. An erratum was published in Cell Metabolism in 2020 and applies to this paper.

  5. Human2023
    Effect of alternate day fasting combined with aerobic exercise on non-alcoholic fatty liver disease: A randomized controlled trial

    Cell Metabolism

    80 adults with obesity and non-alcoholic fatty liver disease, 3 months, four arms. Intrahepatic triglyceride content -5.48% for fasting plus exercise versus -1.30% exercise alone (P = 0.02) and -0.17% control (P < 0.01); versus fasting alone at -2.25% the difference was P = 0.05. Lean mass, AST, HbA1c, blood pressure, plasma lipids, fibrosis score and the hepatokines fetuin-A, FGF-21 and selenoprotein P did not differ between groups.

  6. Human2021
    Does the weight loss efficacy of alternate day fasting differ according to sex and menopausal status?

    Nutrition, Metabolism and Cardiovascular Diseases

    Secondary analysis pooling 75 men and women from three 12-week trials at 500 kcal on fast days alternating with ad libitum feast days. Weight fell 4.6% in premenopausal women, 6.5% in postmenopausal women and 6.2% in men (main effect of time P < 0.001) with no group by time interaction. Fat mass, lean mass, fasting insulin, insulin resistance and blood pressure fell similarly. Fast-day intake exceeded the prescription by roughly 400 to 500 kcal in all groups. LDL fell more in postmenopausal women (P = 0.01).

  7. Human2025
    Association of eating duration less than 8 h with all-cause, cardiovascular, and cancer mortality

    Diabetes and Metabolic Syndrome

    Observational. 19,831 NHANES adults 2003 to 2018, mortality through December 2019, median follow-up 8.1 years, eating duration derived from two 24-hour dietary recalls. Cardiovascular mortality hazard ratio 2.35 (95% CI 1.39 to 3.98) for under 8 hours versus 12 to 14 hours. Not significant for all-cause or cancer mortality; the all-cause association did not survive many sensitivity analyses. Included here because readers will have seen the 91% headline from the earlier conference abstract, which was not peer reviewed at the time.

  8. Review2025
    Short-term 24h dietary recalls from observational studies cannot support claims on mortality

    Diabetes and Metabolic Syndrome

    Published commentary in the same journal, signed by a large group of circadian and nutrition researchers, contesting the use of two days of dietary recall as an exposure for mortality claims. Several signatories are principal investigators on trials cited across this site and several report industry relationships in the competing-interests statement.

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 more than planned on fast days, which was measured at roughly 400 to 500 kcal above prescription across three trials
  • Not compensating on feast days, since participants also ate less than prescribed on those
  • Fast days getting easier after the first two weeks
  • Dropping out later rather than early, with 38% of the fasting arm gone by twelve months in the year-long trial

Sources: Uncontrolled self-report, except for the fast-day and feast-day intake figures and the dropout rate, which were measured outcomes in Trepanowski 2017 and the pooled 75-participant analysis. 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 without a prescriber stopping it first. A 24-hour fast day repeated every other day is the exposure most likely on this page to produce ketoacidosis at a blood glucose that looks normal.
  • Anyone on insulin, a sulfonylurea or a meglitinide without a prescriber writing a fast-day plan. In the published guidance for non-religious intermittent fasting, a sulfonylurea is skipped on a 24-hour fast day and basal insulin cut to a third or a half of the usual dose, and one study that reduced basal insulin by 50% on fasting days still reported significant hypoglycaemia.
  • Anyone with type 1 diabetes outside a structured programme with glucose monitoring.
  • Pregnancy and breastfeeding.
  • Anyone with a history of an eating disorder or current loss-of-control eating. Alternating restriction with unrestricted eating is the clearest structural match on this page to the cycle clinicians screen for.
  • Children and adolescents.
  • Anyone with a BMI under 18.5 or meeting the NICE refeeding risk criteria.
  • Anyone with chronic kidney disease taking an ACE inhibitor or ARB, where volume loss plus blocked efferent arteriolar compensation removes both halves of the kidney's autoregulation.
  • Anyone with adrenal insufficiency, where a fixed replacement dose cannot rise the way endogenous cortisol does.
  • Anyone with raised LDL cholesterol who is not monitoring it, given the year-long randomised finding above.
  • Anyone with gout or on urate-lowering treatment, since ketosis reduces urate excretion and the effect is not corrected by drinking more water.

Questions

Does alternate-day fasting beat daily calorie restriction?
Over one year in 100 people it did not, on weight or on any cardiometabolic measure, and it had the highest dropout of the three arms at 38%. In a three-week trial designed to isolate fasting from energy deficit, fasting with a deficit lost similar total weight to matched daily restriction but significantly less fat, 0.74 kg against 1.75 kg (P = 0.01).
Why does LDL cholesterol come up in every discussion of this protocol?
Because of a specific randomised result: by month 12, LDL was significantly elevated in the alternate-day arm relative to daily calorie restriction (+11.5 mg/dL; 95% CI 1.9 to 21.1). HDL had risen in the same arm at month 6 but that difference was gone by month 12. Long-term practitioners in a separate trial showed reduced LDL, so the picture is not uniform, but the year-long randomised finding is the strongest design in the set.
Does fasting every other day protect muscle better than dieting every day?
The trials do not show that. In an 8-week DXA pilot, fat mass loss was identical between arms while lean mass loss was 3.2 kg on fasting and 2.6 kg on daily restriction, a difference that was not significant. In the one-year trial every lean mass confidence interval crossed zero. None of these trials controlled protein intake or included resistance training, which are the two levers with the clearest evidence behind them.
Do fast days have to be zero calories?
Almost none of the trials used zero. The standard is 25% of energy needs, roughly 500 to 600 kcal. The one small zero-calorie pilot achieved a 376 kcal/day larger deficit than daily restriction and still produced no significant difference in weight, body composition, lipids or insulin sensitivity.