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Intermittent7 studies in people 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 eating about 25% of your normal needs rather than nothing. Over a year it matched daily calorie cutting on weight. It also had the most people quit of any group in that study, and by month 12 it raised LDL cholesterol compared with daily cutting.

Overview

Alternate-day fasting swaps back and forth between a fast day and a normal or "feast" day. Most studies use a softer version. People eat about 25% of their energy needs on the fast day, roughly 500 to 600 calories, and are told to eat 125% on the next day. A few studies use zero calories.

Outside the eating window plans, this is the fasting plan with the longest studies. It is also the one with the most two-sided results. Over a year it matched daily calorie cutting on weight. But it also had the most people quit in that study, 38%. And by month 12, LDL cholesterol was clearly higher than with daily cutting.

The most interesting study here is not the longest one. Templeman 2021 was built to separate fasting from simply eating less. One group fasted but ate enough on other days to make up for it. When the calorie cut was taken away, almost nothing happened.

How it works

Fast day, normal day, repeat. Here is what the studies used. Trepanowski used 25% of energy needs on fast days and 125% on feast days. Ezpeleta used 600 calories on fast days and let people eat freely on feast days. The pooled study comparing men and women used 500-calorie fast days. Catenacci's small early study used zero calories.

How people stick with it follows a clear pattern you should know about. People ate more than planned on fast days and less than planned on feast days. When three 12-week studies were pooled, fast-day eating ran about 400 to 500 calories over plan in every group. So the real calorie cut ends up smaller than the plan on paper says.

Quitting is the other pattern that repeats. In the one-year study, 38% quit the alternate-day group, against 29% on daily cutting and 26% in the group that made no change.

What 1 day actually reaches

Here is what has been measured in people doing alternate-day fasting. Ketones (beta-hydroxybutyrate), which the body makes when burning fat, rose and stayed up even on eating days. Fat around the trunk went down. The balance of fat to lean mass got better. In people who had done it for more than six months, three things were lower: a marker of blood vessel inflammation (sICAM-1), LDL cholesterol, and active thyroid hormone. The study reporting those published a correction the next year, and should be read with it. Beyond that, the honest answer is that fasting-only effects were tested and mostly did not show up. Templeman 2021 ran three groups of lean, healthy adults. One fasted 24 hours and ate 150% on the next day, for a net calorie cut. One cut calories to 75% every day, for the same cut. One fasted but ate 200% on the next day, so there was no net cut. Blood sugar and blood fat responses to meals, gut hormones, and key genes in belly fat did not differ between groups. The group that fasted with no net cut lost 0.52 kg of body weight and 0.12 kg of fat. Nobody has measured autophagy or mitophagy (the cell's clean-up processes) in people on alternate-day fasting. The closest is a 24-hour fast done three days a week with muscle samples. The authors concluded that clean-up markers in human muscle went down, and blamed weight loss. This page does not claim alternate-day fasting reaches either process.

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

    Still digesting

    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.

  2. 4h to 12h

    Meal digested, overnight fast

    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.

  3. 12h to 18h

    Ketones have not moved yet

    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.

  4. 18h to 1d

    The biggest hours for fat release

    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.

  5. 1d to 36h

    Thyroid drops before metabolic rate does

    past this window
    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.

What studies in people found

There is one 12-month study with body scans every 6 months, one 3-week study built to separate fasting from eating less, a small early study of zero-calorie fast days, a 4-week study in healthy adults without obesity, and a 3-month study that added exercise in people with fatty liver.

  • Trepanowski 2017, 100 healthy adults with obesity for one year. They fasted at 25% of energy needs on fast days, cut calories to 75% every day, or made no change. There were 6 months of weight loss, then 6 months of holding steady. At month 12, compared with no change, weight was down 6.0% on fasting and 5.3% on daily cutting. 38% quit the fasting group, the most of the three. The two diets did not differ on blood pressure, heart rate, blood fats, fasting blood sugar, fasting insulin, insulin resistance, or inflammation markers (CRP and homocysteine) at month 6 or 12. Good HDL cholesterol rose in the fasting group at month 6 (6.2 mg/dL) but not at 12. By month 12, LDL cholesterol was clearly higher in the fasting group (11.5 mg/dL).
  • Ezpeleta 2023, 80 adults with obesity and fatty liver for 3 months, in four groups. Liver fat fell 5.48% with fasting plus exercise, against 1.30% with exercise alone and 0.17% with no change. Fasting alone lowered it 2.25%, and the combination was not clearly better than fasting by itself. Lean mass, a liver enzyme (AST), long-term blood sugar, blood pressure, blood fats and liver scarring score did not differ between groups.
  • Lin 2021, a pooled look at 75 men and women from three 12-week studies with 500-calorie fast days. Weight fell 4.6% in women before menopause, 6.5% in women after menopause and 6.2% in men, and the groups did not differ. Fat, lean mass, fasting insulin, insulin resistance and blood pressure fell about the same in all three. Fast-day eating ran about 400 to 500 calories over plan in every group. LDL cholesterol fell more in women after menopause than before.
  • Catenacci 2016, a small early study. 14 adults did zero-calorie fast days every other day for 8 weeks, and 12 cut 400 calories every day. Then everyone went 24 weeks on their own. 93% finished the fasting plan, with no side effects blamed on it. The fasting group cut 376 more calories a day, yet weight loss did not really differ: 8.2 kg against 7.1 kg. Neither did body make-up, blood fats or insulin handling.
  • Templeman 2021, lean healthy adults, 12 per group, for 3 weeks. It was built to separate fasting from eating less. Weight fell 1.91 kg on daily cutting, 1.60 kg on fasting with a calorie cut, and 0.52 kg on fasting without one. Fat told a different story. Daily cutting lost 1.75 kg of fat, so almost all its loss was fat. Fasting with a calorie cut lost only 0.74 kg of fat, despite similar total weight loss. Fasting without a cut lost 0.12 kg. The authors concluded alternate-day fasting takes off body fat less well than the same daily calorie cut, with no sign of any fasting-only benefit.
  • Stekovic 2019, 4 weeks of strict alternate-day fasting in healthy adults without obesity. It also looked at people who had already done it for more than six months. On average they ate 37% fewer calories. Heart markers improved. Fat fell, especially around the trunk, and the fat-to-lean balance got better. Ketones rose even on eating days. After long-term practice, a blood vessel inflammation marker (sICAM-1), LDL cholesterol and active thyroid hormone were lower. There were no side effects even after more than six months. A correction was published in 2020 and should be read with it.

What this does not tell you: The one-year study had 100 people, and 38% of the fasting group quit. Templeman ran 3 weeks with 12 lean healthy adults per group. Catenacci was a small early study of 26 people. The pooled men-and-women study was a second look at data from linked studies run by one research group. What people ate was self-reported in all of them, and fast-day eating always ran over plan. No study of this plan was built to track heart attacks, strokes or deaths.

Reading the research record

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

The first disagreement is about fat. Templeman found alternate-day fasting took off less body fat than the same daily calorie cut: 0.74 kg against 1.75 kg, despite similar total weight loss. Catenacci found fat loss was the same with zero-calorie fasting and daily cutting. Trepanowski found no difference in weight over a year. The studies differ in length, in who took part, and in whether fast days were zero calories or low calories. None is large.

The second is about cholesterol. In the one-year study, LDL cholesterol was clearly higher in the fasting group than with daily cutting by month 12 (11.5 mg/dL). But long-term fasters in Stekovic had lower LDL. And in the pooled study, LDL fell more in women after menopause than before. These are different kinds of studies in different people. The year-long study has the strongest design, and its result should not be left out of any summary of this plan.

The public confusion is about a study of eating-window length and death, published in 2025. It used a US health survey (NHANES) of 19,831 adults, followed for 8.1 years on average. People who ate within less than 8 hours a day had 2.35 times the rate of heart disease death, compared with 12 to 14 hours. It found no clear link with death from any cause or from cancer, and the any-cause link fell apart in many re-checks. The eating window came from two days of people recalling what they ate, standing in for years of habits. The early meeting summary behind the "91%" headline had not been checked by other scientists when the press covered it.

A large group of body clock and nutrition researchers published a formal reply in the same journal. Several of them list ties to industry. The finding is a link seen in a small group of people, and its own authors do not claim it proves cause. It is also not about alternate-day fasting, which does not give you a short daily eating window.

What happens to muscle

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

Lean mass means the non-fat part of you, mostly muscle and water. Catenacci 2016 used body scans over 8 weeks in adults with obesity. Fat loss was the same in both groups, 3.7 kg. Lean mass loss was 3.2 kg on fasting and 2.6 kg on daily cutting, which was not a real difference. Working it out from those scan numbers, lean mass was about 46% of the loss on fasting and 41% on daily cutting. That is a high lean share for 8 weeks in this group of people. We did that math ourselves from the reported numbers; the authors did not publish it. The paper's own table also warns that scale weight change does not match the scan's fat and lean changes added together.

Trepanowski 2017 scanned lean mass every six months in 100 people over a year. At month 6, compared with no change, lean mass was down 1.5 kg on fasting and 2.1 kg on daily cutting. At month 12 it was down 0.9 kg and 1.4 kg. None of those results was clear enough to rule out no change at all, so the study did not show a lean mass difference either way.

Templeman's result sits next to these. Fasting with a calorie cut lost about the same weight as daily cutting but clearly less fat. That means more of its loss was non-fat, in lean healthy adults over 3 weeks.

On the other side, a 12-week study of 32 people with a BMI of 20 to 29.9 found weight down 5.2 kg and fat down 3.6 kg, with lean mass unchanged compared with the comparison group. A review of 20 studies in 1653 adults aged 45 and over found lean mass differences between groups were usually small, and about the same as daily calorie cutting. Those same authors note that body scans and body fat scales may not show changes in actual muscle, and that the evidence is not strong enough for firm conclusions.

The protein research adds this. None of these studies controlled protein or had people lift weights. While eating less, the share of weight lost as lean mass was lower at 1.6 and 2.4 g of protein per kg of body weight a day than at 0.8 g. And losing weight more slowly kept more lean mass for the same total loss.

Doing it

What a fast day contained
25% of energy needs in the one-year study, 600 calories in the fatty liver study, and 500 calories in the pooled study. Only one small early study used zero calories. It found no clear edge over daily cutting on weight or body make-up, even with a 376-calorie-a-day bigger cut.
Check LDL cholesterol
By month 12 of the one-year study, LDL was clearly higher in the fasting group than with daily calorie cutting (11.5 mg/dL). That is a real result from the strongest study here.
Protein and training
No alternate-day study controlled protein or had people lift weights with supervision. In the wider research, every study where people kept or gained lean mass while eating less had them lift weights. And the share of loss from lean mass was lower at 1.6 and 2.4 g of protein per kg of body weight a day than at 0.8.
Electrolytes and fluid on fast days
Falling insulin makes you pass more salt early in a fast. The body sheds the most salt and water between 24 and 72 hours in, which is when dizziness on standing is most reported. No published plan checks salts and minerals for fasts this short. A 500-calorie day with salt and fluids is a different thing from a dry one.
Expect the calorie cut to shrink
In the year-long study, people ate more than planned on fast days and less than planned on feast days. Across three pooled studies, fast-day eating ran 400 to 500 calories over plan. The real calorie cut is smaller than the plan on paper.
Plan for the twelve-month mark
In the one-year study, 38% quit the fasting group, against 29% on daily cutting and 26% in the group that made no change. Whatever else is true, this is the plan people walk away from.

Breaking the fast

In the studies, each fast day simply ended with a normal or feast day, with no special refeeding plan. And people actually ate less than planned on those days. One measurement is worth knowing, though it comes from a much longer fast. After a three-week fast in 10 women with moderate obesity, one week of 80 g of protein a day moved their protein balance from losing 5.9 g of nitrogen a day to gaining 1.7 g. 80 g of carbs only moved it to losing 3.1 g a day. Refeeding syndrome is not a risk with a 24-hour fast day unless you already met the NICE high-risk list before starting.

Sources

  1. Study in people2017
    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 spent a year on alternate-day fasting, a daily calorie cut, or no change, with body scans every 6 months. Compared with no change, fasting lost 6.0% of body weight and daily cutting 5.3%, but more people quit fasting, 38% against 29% and 26%. Lean mass changes were too uncertain to call, and LDL (bad) cholesterol rose on fasting compared with daily cutting. People ate more than planned on fast days and less on feast days.

    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. Study in people2021
    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

    Lean healthy adults, 12 per group, spent 3 weeks fasting with a calorie gap, cutting the same calories daily, or fasting with no calorie gap. Weight loss was about the same for daily cutting and fasting with a gap, 1.91 kg against 1.60 kg. But daily cutting lost more fat, 1.75 kg against 0.74 kg, and fasting with no gap barely moved weight. Blood markers after meals, gut hormones and fat tissue genes did not differ.

    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. Study in people2016
    A randomized pilot study comparing zero-calorie alternate-day fasting to daily caloric restriction in adults with obesity

    Obesity (Silver Spring)

    Adults with obesity did 8 weeks of zero-calorie fasting every other day (14 people) or ate 400 fewer calories a day (12 people), then 24 weeks on their own. 93% finished the fasting plan with no side effects blamed on it. Fasting made a bigger calorie gap, yet weight loss was about the same, 8.2 kg against 7.1 kg, and fat loss was 3.7 kg in both. By 24 weeks the fasting group's body make-up looked better.

    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. Study in people2019
    Alternate Day Fasting Improves Physiological and Molecular Markers of Aging in Healthy, Non-obese Humans

    Cell Metabolism

    Healthy, non-obese middle-aged adults did strict alternate-day fasting for 4 weeks, and people who had done it for more than 6 months were also studied. They ate 37% fewer calories on average. Heart markers improved, fat fell, mostly around the middle, and ketones rose even on eating days. Long-time fasters had lower LDL cholesterol, a thyroid hormone and an inflammation marker, with no side effects, and a correction to the paper was later published.

    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. Study in people2023
    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 fatty liver spent 3 months on alternate-day fasting plus exercise, fasting alone, exercise alone, or no change. Liver fat fell 5.48% with fasting plus exercise, against 1.30% with exercise alone and 0.17% with no change. Against fasting alone, at 2.25%, the edge was borderline. Lean mass, liver enzymes, long-term blood sugar, blood pressure, blood fats and liver scarring scores did not differ between groups.

    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. Study in people2021
    Does the weight loss efficacy of alternate day fasting differ according to sex and menopausal status?

    Nutrition, Metabolism and Cardiovascular Diseases

    A pooled look at 75 men and women from three 12-week studies of alternate-day fasting, with 500 calories on fast days. Weight fell 4.6% in women before menopause, 6.5% in women after menopause and 6.2% in men, and the groups did not really differ. Fat, lean mass, insulin and blood pressure fell alike, and everyone ate about 400 to 500 calories more than planned on fast days. LDL cholesterol fell more in women after menopause.

    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. Study in people2025
    Association of eating duration less than 8 h with all-cause, cardiovascular, and cancer mortality

    Diabetes and Metabolic Syndrome

    Not a trial. Researchers looked back at 19,831 US adults from a national health survey, followed for about 8.1 years. People whose eating fit into under 8 hours were more likely to die of heart disease than those eating over 12 to 14 hours, but not more likely to die of cancer or any cause. It is here because many readers saw a 91% headline from an earlier conference talk that other scientists had not checked at the time.

    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. Review of studies2025
    Short-term 24h dietary recalls from observational studies cannot support claims on mortality

    Diabetes and Metabolic Syndrome

    A letter in the same journal from a large group of body clock and nutrition researchers. They argue that two days of food recall is not enough to base claims about death on. Several signers run trials cited on this site, and several list ties to industry.

    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 things people say about their own fasting. Nobody checked them, so they are not evidence. They are here because they tell you what to expect and what to watch for, which the studies do not. They cannot tell you whether anything works.

  • Eating more than planned on fast days. Three studies measured this at about 400 to 500 calories over plan.
  • Not making up for it on feast days, since people also ate less than planned on those days.
  • Fast days getting easier after the first two weeks.
  • Quitting later rather than early. In the year-long study, 38% of the fasting group had quit by twelve months.

Sources: These are self-reports with no comparison group, except for fast-day and feast-day eating and the quit rate. Those were measured in Trepanowski 2017 and the pooled study of 75 people. A separate review of what fasting communities report is still being put together, and nothing here comes from it.

Who this is wrong for

  • Anyone taking an SGLT2 inhibitor, unless a prescriber stops it first. A 24-hour fast day every other day is the plan on this site most likely to cause ketoacidosis (a dangerous acid build-up) while blood sugar looks normal.
  • Anyone on insulin, a sulfonylurea or a meglitinide, unless a prescriber writes a fast-day plan. Published advice for fasting outside religious practice says to skip a sulfonylurea on a 24-hour fast day and cut long-acting insulin to a third or half the usual dose. One study that cut long-acting insulin by 50% on fasting days still saw a lot of low blood sugar.
  • Anyone with type 1 diabetes outside a set program with glucose monitoring.
  • Pregnancy and breastfeeding.
  • Anyone with a past eating disorder, or who now has episodes of eating they cannot control. Switching between holding back and eating freely is the closest match on this site to the cycle doctors screen for.
  • Children and teens.
  • Anyone with a BMI under 18.5, or who meets the UK NICE risk list for refeeding problems.
  • Anyone with chronic kidney disease who takes an ACE inhibitor or ARB. Losing fluid while on these drugs takes away both of the ways the kidneys protect their own blood flow.
  • Anyone with adrenal insufficiency. A fixed replacement dose cannot rise the way the body's own cortisol would.
  • Anyone with high LDL cholesterol who is not keeping an eye on it, given the year-long result above.
  • Anyone with gout or on drugs to lower uric acid. Being in ketosis makes the kidneys clear less uric acid, and drinking more water does not fix that.

Questions

Does alternate-day fasting beat daily calorie restriction?
Over one year in 100 people, it did not, on weight or on any heart or blood sugar measure. It also had the most people quit of the three groups, 38%. In a three-week study built to separate fasting from eating less, fasting with a calorie cut lost about the same weight as the same daily cut, but clearly less fat: 0.74 kg against 1.75 kg.
Why does LDL cholesterol come up in every discussion of this protocol?
Because of one specific result. By month 12 of the year-long study, LDL was clearly higher in the alternate-day group than with daily calorie cutting (11.5 mg/dL). Good HDL cholesterol had risen in the same group at month 6, but that was gone by month 12. Long-term fasters in a separate study had lower LDL, so the picture is not all one way. But the year-long result comes from the strongest study in the set.
Does fasting every other day protect muscle better than dieting every day?
The studies do not show that. In an 8-week study with body scans, fat loss was the same in both groups. Lean mass loss was 3.2 kg on fasting and 2.6 kg on daily cutting, which was not a real difference. In the one-year study, none of the lean mass results was clear either way. None of these studies controlled protein or had people lift weights, and those are the two things with the clearest evidence behind them.
Do fast days have to be zero calories?
Almost none of the studies used zero. The usual amount is 25% of energy needs, about 500 to 600 calories. The one small zero-calorie study cut 376 more calories a day than daily cutting, and still found no clear difference in weight, body make-up, blood fats or insulin handling.