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Intermittent8 studies in people cited

The 5:2 Diet

1 day without food · Two non-consecutive days a week

Written by Reviewed Sep 2026

Also called: Intermittent energy restriction, Two fast days a week, Modified fasting

Two very-low-calorie days a week, usually 500 to 600 calories, and normal eating on the other five. In five studies lasting 6 to 12 months, it gave about the same weight loss and heart and blood sugar changes as cutting calories every day. The steady difference is how well people stick with it and how hungry they feel, not what happens in the body.

Overview

5:2 is not a true fast. In every study that tested it, people ate some food on fast days. It was 500 to 600 calories in Carter. It was 400 calories for women and 600 for men in Sundfor. It was about 2710 kJ (kilojoules, a metric unit of food energy) in Harvie. In the HELENA study it was 75% fewer calories than normal on those days. The studies differ in how much people ate, not whether they ate.

5:2 was built to answer one question. Does packing a week's calorie cut into two days do something that spreading it out does not? Five studies lasting 6 to 12 months have now compared the two. On weight, long-term blood sugar (HbA1c), blood fats and blood pressure, they keep landing in the same place.

The more useful results are about behaviour. In a year-long study, people felt hungrier on 5:2. In a 300-person real-world study, the share of people sticking with it fell from 74% at 6 weeks to 22% at one year. And in the longest study with the most people, only 146 of 332 finished.

How it works

Two very-low-calorie days a week, not back to back, and five days of normal eating. Here are the exact numbers the studies used. Carter's study in type 2 diabetes used 500 to 600 calories on fast days, against a group eating 1200 to 1500 calories every day. Sundfor used 400 calories for women and 600 for men. Headland used 2100 kJ for women and 2520 kJ for men. HELENA cut calories by 75% on the two days, which came to about 20% less across the week, the same as the daily group.

Notice what these studies have in common. Both groups got the same total calories across the week. In these studies, 5:2 is a way of scheduling a calorie cut. It is not an extra calorie cut.

Hajek 2021 is the only study that tested 5:2 the way most people meet it: as self-help instructions, with or without six weekly group support sessions. That is where the drop-off numbers come from.

What 1 day actually reaches

A 24-hour stretch on 500 to 600 calories is not a water fast, and the body's response shows that. There was one place where 5:2 beat daily calorie cutting in a head-to-head test, and that was insulin. In 107 women who had not reached menopause, both groups cut calories by 25% for 6 months. The 5:2 group had bigger drops in fasting insulin and insulin resistance. Everything else in that study moved about the same in both groups. That included leptin, a male hormone measure, a marker of inflammation (CRP), cholesterol, blood fats and blood pressure. On autophagy (the cell's clean-up process), the closest measurement in people comes from 50 women, average age 51. They fasted 24 hours on three days a week, not back to back, for 8 weeks. Researchers took thigh muscle samples after 12-hour and 24-hour fasts. The 24-hour fast raised one marker (SQSTM1). In the lower-calorie group, the genes BECLIN1, SQSTM1 and LAMP2 turned down after an overnight fast. The authors concluded that clean-up markers in human muscle went down, and blamed weight loss. In mice, the same paper found intermittent fasting raised clean-up markers in the liver. Liver is where the effect is clearest in animals, and nobody takes liver samples from healthy volunteers. So this page does not claim 5:2 reaches autophagy. The one measurement in people with this kind of plan pointed the other way in the tissue that was tested.

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

Five studies of 6 to 12 months compared 5:2 with cutting calories every day. One was in people with type 2 diabetes and asked whether 5:2 was about as good. Another, with 300 people, measured how well people stuck with it in real life.

  • Headland 2019, 332 people for one year, split into daily calorie cutting, a week-on week-off plan, and 5:2. Only 146 of 332 finished, 44%. That is the single most useful number in the study. Among those who finished, weight fell 6.6 kg, 5.1 kg and 5.0 kg, and the groups did not really differ. Good HDL cholesterol rose 7% and blood fats fell 13%, with no difference between groups.
  • Hajek 2021, 300 adults followed for one year. They got normal brief advice, 5:2 self-help instructions, or 5:2 self-help plus six weekly group sessions. The share sticking with 5:2 was 74% at 6 weeks, 31% at 6 months and 22% at one year. At one year, weight was down 1.9 kg on 5:2 and 1.8 kg on normal advice, so 5:2 did no better. People rated 5:2 more highly. It did not make them eat worse or move less.
  • The HELENA study (Schubel 2018), 150 overweight non-smokers. It ran 12 weeks of dieting, 12 weeks of holding steady, then 26 weeks of follow-up. During the diet, weight fell 7.1% on 5:2, 5.2% on daily cutting and 3.3% with no diet. The 5:2 edge was not clearly real. By week 50 the two diet groups were level at 5.2% and 4.9%. There was no difference in 82 fat-tissue genes the researchers picked in advance, or in blood markers.
  • Carter 2018, 137 adults with type 2 diabetes for 12 months. One group ate 500 to 600 calories on two days a week, not back to back. The other ate 1200 to 1500 calories every day. Long-term blood sugar (HbA1c) fell 0.5% on daily cutting and 0.3% on 5:2. That 0.2% gap was small enough to count 5:2 as about as good. Weight fell 5.0 kg and 6.8 kg. That 1.8 kg gap was too big for the study to call them equal, and so were the gaps in fat and lean mass. Both things are true and both belong in any summary.
  • Sundfor 2018, 112 adults with belly fat, for one year. Weight fell 8.0 kg on 5:2 and 9.0 kg on daily cutting, with 1.1 kg and 0.4 kg regained, and the groups did not really differ. Waist size, blood pressure, blood fats and HDL cholesterol did not differ between groups either. People felt hungrier on 5:2: 4.7 against 3.6 on a hunger score.
  • Harvie 2011, 107 overweight women who had not reached menopause, for 6 months. Both groups cut calories by 25%, either on two days or every day. Weight fell 6.4 kg on 5:2 and 5.6 kg daily, which was not a real difference. Fasting insulin and insulin resistance fell a little in both groups, but more on 5:2. Leptin, a male hormone measure, CRP, cholesterol, blood fats and blood pressure changed about the same.

What this does not tell you: Every study here compared 5:2 with daily calorie cutting or brief advice, not with eating freely. So they answer a question about scheduling, not about fasting. What people ate was self-reported throughout. Body make-up was reported in only some studies, and never as the main question. Only 44% finished the biggest study. No study of this plan was built to track heart attacks, strokes or deaths.

Reading the research record

The studies here agree with each other more than most nutrition studies do. The same weekly calories, eaten in two lean days or spread over seven, give about the same weight and about the same heart and blood sugar changes. Harvie's insulin result is the main exception. The later, longer studies did not repeat it. In those, fasting blood sugar, fasting insulin and insulin resistance did not separate.

The disagreement worth reading closely is not between these studies. It is between them and a study of survey data that reached the public before other scientists had checked it.

In March 2024, a short research summary shown at an American Heart Association meeting said people who ate within less than 8 hours a day had a 91% higher risk of dying from heart disease. It got heavy press. It was a meeting summary, which the American Heart Association itself calls early work that has not been checked by other scientists.

It has since been published in full. The 2025 paper covers 19,831 adults from a US health survey (NHANES) from 2003 to 2018, with deaths tracked through December 2019. People were 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. That is bigger than the 91% the summary suggested, and the range of likely values was wide. There was no clear link with death from any cause or from cancer, and the any-cause link fell apart in many re-checks. The heart death link held in 8 subgroups and 14 re-checks. The authors themselves say more research is needed to tell whether the short eating window causes the risk, or whether something else about those people does.

A large group of body clock and nutrition researchers wrote a reply in the same journal. It includes the lead researchers on several studies cited across this site. They argue that a couple of days of people recalling what they ate cannot support claims about death. Several of them list ties to industry in that paper.

The eating window in question came from two days of people recalling their food, standing in for their usual habits across 8 years on average. Critics of the early summary pointed out that the short-window group was different from the start in ways that predict heart risk, and likely included shift workers rather than people fasting on purpose. This page reports the published number, the results that found no link, and the reply. It does not treat any of it as evidence from a study that assigned people to fast.

What happens to muscle

5:2 has been measured less carefully for body make-up than the eating window plans. Where it was measured, the "they're the same" claim people assume was not shown.

Carter 2018 is the clearest case. Lean mass means the non-fat part of you, mostly muscle and water. The study was set up to test whether 5:2 was about as good as daily cutting. The fat gap between groups was 1.3 kg and the lean mass gap was 0.5 kg. Neither was small enough for the study to call them equal. That does not mean 5:2 lost more lean mass. It means the study could not show the two were the same on body make-up. That is a different and more honest statement than the one usually made.

Headland 2019 said body fat loss did not really differ across its three groups, among people who finished. Harvie and Sundfor reported similar heart and blood sugar changes without giving lean mass as a main number.

The muscle sample study closest to this plan, 50 women fasting 24 hours on three days a week for 8 weeks, found clean-up markers in human muscle went down, not up. The authors blamed weight loss.

The wider protein research adds a general rule that applies here as much as anywhere. 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. Every study where people kept or gained lean mass while eating less had them lift weights. And losing weight more slowly kept more lean mass for the same total loss. None of the 5:2 studies controlled protein, and none had a supervised training program.

Doing it

What a fast day actually contained
500 to 600 calories in the type 2 diabetes study. 400 calories for women and 600 for men in the one-year Norwegian study. 75% fewer calories on the two days in HELENA. None of these studies tested a zero-calorie day.
Protein on fast days
No 5:2 study controlled protein. The closest evidence on protein during heavy calorie cutting is a study of 17 women with obesity on two 500-calorie diets. At 1.5 g of protein per kg of ideal body weight, they held their protein balance level. At 0.8 g per kg, they lost 2 g of nitrogen a day, a sign of losing protein. Note that this is per kg of ideal body weight, not lean body mass.
Electrolytes and fluid
No published plan checks salts and minerals for fasts under 24 hours. The body sheds the most salt and water between 24 and 72 hours into a fast. That is when dizziness on standing up is most reported. A 500-calorie day that includes salt and fluids is a different thing from a dry or zero-calorie day.
Space the days out
Every study used two days that were not back to back. Nobody has tested two days in a row against two days apart.
Plan for month six, not week six
In the real-world study, 74% were sticking with it at 6 weeks, 31% at 6 months and 22% at one year. The group-support boost early on was gone by twelve months. In the biggest study, 146 of 332 finished.
Medication timing on fast days
One published guide for fasting outside of religious practice says: skip a sulfonylurea on a 24-hour fast day, halve it on a partial day, skip mealtime insulin when no carbs follow, and cut long-acting insulin to a third or half the usual dose. That is your prescriber's call, not something to change on your own.

Breaking the fast

In the studies, people broke each fast day by eating normally the next day, with no special refeeding plan. The science adds one thing. After a truly long fast, eating protein swings the body's protein balance back to positive, and eating carbs does not. In 10 women with moderate obesity after a three-week fast, 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. That was measured after a three-week fast, not a 24-hour day. Refeeding syndrome is not a worry with 5:2 unless you already met the NICE high-risk list before you started.

Sources

  1. Study in people2011
    The effects of intermittent or continuous energy restriction on weight loss and metabolic disease risk markers: a randomized trial in young overweight women

    International Journal of Obesity

    107 women with overweight or obesity, before menopause, cut calories by 25% for 6 months, either every day or with 2 very low-calorie days a week. Weight loss was about the same, 6.4 kg against 5.6 kg. Fasting insulin and insulin resistance fell a little more with the 2-day plan. Cholesterol, blood fats, blood pressure and other blood markers were about the same.

    107 overweight or obese premenopausal women, 6 months, matched 25% energy restriction delivered intermittently (about 2710 kJ/day on 2 days a week) or continuously. Weight -6.4 kg versus -5.6 kg (P = 0.4). Fasting insulin and insulin resistance fell more with intermittent restriction, between-group differences -1.2 microU/mL and -1.2 microU mmol/L (both P = 0.04). Leptin, free androgen index, CRP, cholesterol, triglycerides and blood pressure comparable.

  2. Study in people2018
    Effects of intermittent and continuous calorie restriction on body weight and metabolism over 50 wk: a randomized controlled trial

    American Journal of Clinical Nutrition

    150 non-smoking adults with overweight or obesity tried 5:2 (2 days a week at a 75% cut), a daily cut of the same weekly size, or no diet. During the diet phase they lost 7.1%, 5.2% and 3.3% of body weight, and the gap between the two diets was not a clear difference. By week 50 the two diets were level, 5.2% against 4.9%. Fat tissue genes and blood markers did not differ between the diets.

    150 overweight and obese non-smokers randomised to 5:2 (2 days at a 75% deficit, net weekly deficit about 20%), continuous restriction at the same weekly deficit, or control. Intervention phase weight -7.1% versus -5.2% versus -3.3% (intermittent versus continuous P = 0.053). At week 50, -5.2% versus -4.9% (P = 0.89). No significant differences between restriction arms in 82 preselected adipose genes or circulating biomarkers.

  3. Study in people2018
    Effect of Intermittent Compared With Continuous Energy Restricted Diet on Glycemic Control in Patients With Type 2 Diabetes: A Randomized Noninferiority Trial

    JAMA Network Open

    137 adults with type 2 diabetes spent 12 months on a daily calorie cut or an on-and-off one. Long-term blood sugar fell about the same, 0.5% against 0.3%, close enough to count as a match. The on-and-off group lost 6.8 kg against 5.0 kg, but that result was too uncertain to call, and so were fat and lean mass. Low and high blood sugar episodes in the first 2 weeks were similar.

    137 adults with type 2 diabetes, mean baseline HbA1c 7.3%, 12 months. HbA1c -0.5% continuous versus -0.3% intermittent (P = .65), difference 0.2% (90% CI -0.2% to 0.5%), meeting the equivalence margin. Weight -5.0 versus -6.8 kg, difference -1.8 kg (90% CI -3.7 to 0.07), not meeting the 2.5 kg margin; fat mass -1.3 kg and fat-free mass -0.5 kg also did not meet their margins. Hypoglycaemic or hyperglycaemic events in the first 2 weeks were similar between arms.

  4. Study in people2018
    Effect of intermittent versus continuous energy restriction on weight loss, maintenance and cardiometabolic risk: A randomized 1-year trial

    Nutrition, Metabolism and Cardiovascular Diseases

    112 adults with belly fat spent 6 months losing weight on an on-and-off calorie cut or a daily one, then 6 months keeping it off. Weight loss was about the same, 8.0 kg against 9.0 kg, and so was regain, 1.1 kg against 0.4 kg. Waist, blood pressure and blood fats improved the same in both. People felt hungrier on the on-and-off plan.

    112 adults with abdominal obesity, 6-month weight-loss phase then 6-month maintenance. Weight -8.0 kg intermittent versus -9.0 kg continuous (P = 0.6); regain 1.1 kg versus 0.4 kg (P = 0.6). Waist, blood pressure, triglycerides and HDL improved with no between-group difference. Hunger was rated higher on intermittent restriction, 4.7 versus 3.6 (P = 0.002).

  5. Study in people2019
    Effect of intermittent compared to continuous energy restriction on weight loss and weight maintenance after 12 months in healthy overweight or obese adults

    International Journal of Obesity

    332 healthy adults with overweight or obesity tried a daily calorie cut, a week-on week-off cut, or 5:2 for one year. Only 146 finished, and dropout was about the same in all three. Those who finished lost 6.6 kg, 5.1 kg and 5.0 kg, with no real difference, and good cholesterol rose 7% and blood fats fell 13% in all groups. A correction to this paper was later published.

    332 randomised to continuous restriction, week-on week-off restriction or 5:2 for one year. 146 of 332 completed, a 44% completion rate, with discontinuation not differing across arms (P = 0.4). Among completers, weight -6.6 kg, -5.1 kg and -5.0 kg (P = 0.2 for time by diet), with body fat loss similarly non-different. HDL rose 7% and triglycerides fell 13% with no group differences. An erratum was published for this paper.

  6. Study in people2021
    A randomised controlled trial of the 5:2 diet

    PLoS ONE

    300 adults with obesity got brief standard advice, 5:2 self-help instructions, or 5:2 plus six weekly group sessions, and were followed for one year. Only 22% were still doing 5:2 at one year. Weight loss at one year matched standard advice, 1.9 kg against 1.8 kg, and the early boost from group support was gone by then. People did rate 5:2 more highly.

    300 adults with obesity randomised to standard brief advice, 5:2 self-help instructions, or 5:2 self-help plus six weekly group support sessions, followed one year. Adherence 74% at 6 weeks, 31% at 6 months, 22% at one year. Weight at one year -1.9 kg versus -1.8 kg on standard advice (P = 0.79). Group support gave more loss at 6 weeks (P = 0.02) but the difference was gone by one year (P = 0.37). 5:2 received higher participant ratings.

  7. Study in people2022
    Intermittent fasting activates markers of autophagy in mouse liver, but not muscle from mouse or humans

    Nutrition

    50 women, average age 51, fasted 24 hours on 3 days a week for 8 weeks, eating 70% or 100% of their calorie needs. The authors concluded that markers of the cell clean-up process (autophagy) in their muscle went down, not up, and put it down to weight loss. In mice, the same fasting raised clean-up markers in the liver.

    50 women, mean age 51, randomised to a 24-hour fast on 3 non-consecutive days a week at 70% or 100% of energy requirements for 8 weeks, with vastus lateralis biopsies after 12 and 24 hour fasts. The 24-hour fast raised SQSTM1; BECLIN1, SQSTM1 and LAMP2 mRNA fell in the 70% arm after an overnight fast. The authors concluded autophagy markers in human muscle were reduced and attributed this to weight loss. The same intervention raised autophagy markers in mouse liver.

  8. 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 people eating over 12 to 14 hours, but not more likely to die of cancer or any cause. Eating time came from just two days of food recall, and the authors say other factors may explain the link.

    Observational, not a trial. 19,831 NHANES adults from 2003 to 2018 with 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, consistent across 8 subgroups and 14 sensitivity analyses. Not significant for all-cause or cancer mortality, and the all-cause association did not survive many sensitivity analyses. The authors state further research is needed to separate the exposure from residual confounding.

  9. 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 an eating window worked out from two days of food recall cannot support claims about death. Several signers run eating-window 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, arguing that an eating window derived from two days of dietary recall cannot support mortality claims. Several signatories are principal investigators on the intervention 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.

  • More hunger than with daily calorie cutting. One year-long study measured it at 4.7 against 3.6 on a hunger score.
  • Preferring two hard days to seven restricted ones. That shows in the higher ratings people gave 5:2 in the real-world study.
  • Sticking with it fading over months, rather than failing at the start.
  • Fast days getting easier after the first few weeks.
  • Eating more than planned on fast days. Studies of alternate-day fasting measured this, and it is the same habit.

Sources: These are self-reports with no comparison group, except for the hunger score and how well people stuck with it. Those were set in advance and measured in Sundfor 2018 and Hajek 2021. 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 is what has caused ketoacidosis (a dangerous acid build-up) in published cases. While it happens, blood sugar looks normal or only a little high.
  • Anyone on insulin, a sulfonylurea or a meglitinide, unless a prescriber writes a fast-day plan. In a group of 1378 people on sulfonylureas, low blood sugar with symptoms hit 25.6% on glibenclamide, 16.8% on glimepiride and 14.0% on gliclazide. Overall, 6.7% had severe episodes.
  • Anyone with type 1 diabetes outside a set program. The risk runs both ways: low blood sugar if long-acting insulin is not cut, and ketoacidosis if it is cut too far.
  • Pregnancy and breastfeeding. Making milk pulls about 30 to 50 g of sugar a day out of the mother. In 18 reported cases of ketoacidosis while breastfeeding, dieting or exercise to lose weight set off 76% of them.
  • Anyone with a past eating disorder, or who now has episodes of eating they cannot control. Holding back, then eating freely, is the same shape as 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. One item on that list is eating little or nothing for more than 5 days.
  • Anyone with chronic kidney disease on an ACE inhibitor or ARB. In 131 people with kidney disease, a blood marker of kidney strain (creatinine) rose during fasting in 60.4% by day 7. People on this family of blood pressure drugs were twice as likely to have the rise.
  • Anyone with adrenal insufficiency, where the body cannot make enough cortisol. In a study of 30 people on replacement hormones who tried both fasting and not fasting, 67% had problems while fasting against 23% when not fasting. 37% broke the fast because of a problem, even though glucose monitors showed no clear difference in low blood sugar.
  • Anyone with gout or on drugs to lower uric acid. Ketones compete with uric acid for removal by the kidneys, so uric acid in the blood goes up. Drinking more water does not fix that.

Questions

Is 5:2 better than eating less every day?
In five studies of 6 to 12 months, the two gave about the same weight loss and the same heart and blood sugar changes. One study found bigger drops in fasting insulin and insulin resistance with 5:2. Later, longer studies did not repeat that. The type 2 diabetes study found 5:2 about as good for long-term blood sugar (HbA1c). It could not show the two were equal for weight or body make-up, in either direction.
What about the study saying short eating windows raise heart death risk?
It started as a short meeting summary reporting a 91% higher risk, and was published in full in 2025. The paper says 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. The eating window came from two days of people recalling what they ate, standing in for their habits across 8 years on average. A large group of researchers published a formal reply in the same journal. The paper's own authors do not claim short eating windows cause the deaths.
Do fast days have to be zero calories?
No study here tested that. Fast days had 500 to 600 calories in the type 2 diabetes study, 400 calories for women and 600 for men in the one-year Norwegian study, and 75% fewer calories in HELENA. All the results on this page belong to those numbers.
Will I stick to it?
The real-world study that measured this in 300 people found 74% sticking with it at 6 weeks, 31% at 6 months and 22% at one year. In the biggest study, 146 of 332 people finished the twelve months. People did rate 5:2 more highly than normal advice. In a separate year-long study, people felt clearly hungrier on 5:2 than on daily calorie cutting.