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Time-restricted10 studies in people cited

16:8 Time-Restricted Eating

16 hours without food · Daily

Written by Reviewed Sep 2026

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

You eat all your food inside 8 hours, most often noon to 8pm. For the other 16 hours you take in no calories. This is the most tested fasting pattern. The studies still disagree on whether the window itself helps once you account for eating less.

Overview

16:8 is what most people mean by intermittent fasting. You eat inside an 8-hour window. The rest of the day you drink only things with no calories. You do it every day. Nothing says what you have to eat in the window.

It has been tested on more people than any other fasting plan. That includes people with obesity, type 2 diabetes, type 1 diabetes and PCOS, and men who lift weights. Studies ran from 4 weeks to 12 months. That much testing is a reason to take it seriously. It is also why the picture is messy. Studies that let people eat freely and studies that set a calorie target came to different answers, and both kinds are still coming out.

One result shows up again and again. People on 16:8 eat a few hundred fewer calories a day without being told to count anything. In one study (Cienfuegos) the short-window groups ate about 550 fewer calories a day. In a 12-month study (Lin) they ate 425 fewer. Whether 16:8 does anything beyond that is the part people argue about.

How it works

Pick an 8-hour window and stick to it. Most studies used noon to 8pm (12:00 to 20:00). That is the schedule one research group in Chicago used in Lin 2023, Pavlou 2023 and Runchey 2026. Moro used 1pm to 9pm (13:00 to 21:00) in men who lift weights. Liu used 8am to 4pm (08:00 to 16:00).

The studies policed calories, not types of food. In the TREAT study, people took in no calories outside the window. The TIMET study counted any calories more than 15 minutes outside the window as a day off the plan. The studies were not policing water, black coffee or tea.

People stuck with it well for a plan they ran on their own. They kept to it 6.1 days a week over 12 months in Lin 2023, the same in Pavlou 2023, and on 83.5% of days in TREAT. Some studies also had a calorie-counting group. That group found it harder: 61% of people met their calorie goal in Lin and 68% in Pavlou.

One warning, from Oldenburg 2025. When people picked their own window instead of being given one, their real window was 9.8 hours, not 8. The window you plan and the window you actually keep are not the same thing.

What 16 hours actually reaches

Fasting 16 hours a day keeps you a few hours longer each day in the state after your last meal has been fully digested. It also lowers your blood sugar across the day. Those two things have been measured in people. In 6 months, in people with type 2 diabetes, Pavlou 2023 lowered long-term blood sugar (HbA1c) by 0.91% compared with a group that ate as usual. In 32 adults with type 1 diabetes, Runchey 2026 lowered long-term blood sugar more than calorie cutting did. It did not raise the rate of ketoacidosis (a dangerous acid build-up in the blood) or severe low blood sugar. In Parr 2023, an 8-hour window lowered total blood sugar across 24 hours. You will often hear that autophagy, the cell's clean-up process, switches on at 16 hours. No study in people backs that up. No study has measured autophagy in a person at a set hour of fasting. The measurements that do exist point different ways depending on which body tissue was tested. The closest thing is a 6-month study of 121 people with obesity. It measured the clean-up process in blood cells. The group doing intermittent fasting plus a daily eating window came out different from the usual-care group, but only when researchers looked back after the fact. Inside that group, the clean-up process did not clearly rise from where it started. The calorie-cutting group showed no effect. That study mixed intermittent fasting with a daily eating window, so it cannot tell you what a 16-hour window does on its own.

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

    past this window
    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

It has been tested on more people than any other fasting plan. That includes two 12-month studies, and studies in type 1 diabetes, type 2 diabetes and PCOS. People lose weight compared with eating freely, and that result is steady. Losing more than people who cut the same calories is not.

  • The TREAT study (Lowe 2020), 141 people for 12 weeks. The window group lost 0.94 kg. A group eating three set meals a day lost 0.68 kg. The gap of 0.26 kg was too small to count as a real difference. The study is best known for what happened to muscle, covered below.
  • Liu 2022, 139 people with obesity for 12 months. Both groups had the same calorie target. The window plus calorie cutting lost 8.0 kg. Calorie cutting alone lost 6.3 kg. The gap of 1.8 kg was too small to count as a real difference. Body fat, lean mass, blood pressure and heart and blood sugar markers all told the same story.
  • Lin 2023, 90 people in Chicago for 12 months (77 finished). An 8-hour window with no calorie counting lost 4.61 kg more than people who ate as usual. Calorie cutting lost 5.42 kg. The 0.81 kg gap between the window and calorie cutting was too small to count as a real difference. Fasting blood sugar, fasting insulin, insulin resistance and long-term blood sugar (HbA1c) did not change compared with eating as usual.
  • Oldenburg 2025, 88 adults for 12 weeks, with a lab test of how the body handles insulin and MRI scans. Weight, body make-up, calories eaten and blood sugar came out about the same with the window, with 15% calorie cutting, and with eating freely. The window group actually kept a 9.8 hour window, not the 8 hours they were given.
  • Pavlou 2023, 75 adults with type 2 diabetes for 6 months. Their long-term blood sugar (HbA1c) started at 8.1% on average. Weight fell 3.56% more than in people who ate as usual. Long-term blood sugar fell 0.91%, the same as with 25% calorie cutting. There were no serious side effects.
  • Moro 2016, 34 men who already lifted weights, for 8 weeks. Both groups ate the same calories and the same mix of foods, and both trained under supervision. Fat fell more in the window group. Muscle, limb size and top strength held steady in both groups. Testosterone and IGF-1, a growth signal, fell only in the window group. So did active thyroid hormone.

What this does not tell you: Two of the three biggest studies only checked body make-up in part of the group, as a side question. None was built to answer the muscle question. In every study where people lived at home, what they ate was self-reported. The studies with the best weight results compared 16:8 with eating freely or usual care. That is a different question from studies that gave both groups the same calorie target. No study of any fasting plan has tracked heart attacks, strokes or deaths.

Reading the research record

The open question is whether the window adds anything once calories are held steady. Two studies that both held calories steady came to opposite answers.

Liu 2022 ran for 12 months in 139 adults with obesity in Guangzhou, China. Both groups had the same calorie target: 1500 to 1800 a day for men and 1200 to 1500 for women. Adding an 8am to 4pm window (08:00 to 16:00) gave 1.8 kg more weight loss, which was too small to count as a real difference. Body fat, lean mass and heart and blood sugar markers did not separate either. The authors concluded the window was no better than daily calorie cutting.

Jamshed 2022 ran for 14 weeks in 90 adults at a US weight loss clinic. Both groups cut the same calories and got the same weight loss coaching. A 7am to 3pm window (07:00 to 15:00) gave 2.3 kg more weight loss. It also gave 4 mmHg better lower blood pressure (the bottom number). Fat loss did not clearly separate (1.4 kg), and neither did the share of weight lost as fat.

The two studies differ in length, in who took part, and in what the comparison group did. Liu's comparison group was counting calories. Jamshed's was eating over 12 or more hours on the same calorie target. Lowe 2020 cannot be compared directly with either, because it set no calorie target at all. This page does not pick a winner.

The biggest study that could settle the timing question is at the University of Colorado Denver. It has 162 people in three groups: an early window, a late window, and daily calorie cutting. Its main results are due in March 2027.

What happens to muscle

This is where the studies split hardest. The split tells you something. It is not just noise.

Lean mass means the non-fat part of you, mostly muscle and water. In TREAT, 50 people came in for body scans. The window group lost 1.70 kg. Of that, 1.10 kg was lean mass and only 0.51 kg was fat. The authors put the lean share at about 65% of the weight lost. They said 20% to 30% is typical. Lean mass in the arms and legs fell 0.22 kg/m2, which was 0.16 kg/m2 more than the comparison group.

Chow 2020 found the same pattern in 20 people over 12 weeks. Weight fell 3.7%, fat fell 4% and lean mass fell 3.0%. Lean mass fell clearly more than in the group without a window.

Lin 2023 found the opposite over 12 months in 90 people. Fat, waist size and BMI fell compared with eating as usual. Lean mass, belly fat, and bone density did not change compared with eating as usual.

Moro 2016 kept lean mass fully. Those were men who already lifted weights, ate 1.93 g of protein per kg of body weight a day, ate the same calories as the other group, and trained with supervision three times a week.

The same men, followed to 12 months, tell the other half of the story. By then, arm muscle size in the window group fell 4.31% and thigh size fell 2.90%. In the normal-diet group, arm size rose 11.87% and thigh size rose 6.94%. The normal-diet group gained lean mass and the window group did not. Calories in the window group had drifted down 6.4% on their own.

Strength told a different story. Bench press and leg press maxes rose the same in both groups, so the smaller limbs did not show up as lost strength. Both Moro papers measured limb size with tape and skinfolds, not scans. That is a real weakness. Still, the group that lost fat is the group whose measured limbs got smaller, which is hard to blame on the skinfold method.

The thing that tracks the split is not the window. Every study that kept lean mass fed people 1.6 to 2.2 g of protein per kg a day and had them lift weights. One study measured daily muscle building directly (Parr 2023). It used an 8-hour window at 1.0 g of protein per kg a day for 10 days. Muscle building did not change: 1.28 against 1.26 %/day. Yet lean mass still fell more in the window group, 1.0 kg against 0.2 kg. The muscle-building measure and the lean mass result disagreed inside one study. Protein intake is what sets that study apart from the ones that kept lean mass.

Doing it

Protein target
Every window study that kept lean mass fed people 1.6 to 2.2 g of protein per kg of body weight a day. Parr 2023 used 1.0 g per kg a day and people lost more lean mass. Morton 2018 pooled 49 studies with 1863 people. In people not losing or gaining weight, it found no extra muscle gain above about 1.62 g per kg a day in total.
Training
Every study where people kept or gained lean mass while eating less had them lift weights. No study shows lean mass gain while eating less without it. The trained men in Moro's study lifted from 4pm to 6pm (16:00 to 18:00), inside their eating window.
What breaks the fast
The studies policed calories, not what you drank. TREAT people took in no calories from 8pm to noon (20:00 to 12:00). TIMET counted any calories more than 15 minutes outside the window as a day off the plan. No study here tested whether a tiny amount of calories is harmless.
Window placement
Studies have used 8am to 4pm, noon to 8pm and 1pm to 9pm (08:00 to 16:00, 12:00 to 20:00 and 13:00 to 21:00). No study has put 8-hour windows head to head. The one study that swapped the same people between an early and a late 9-hour window found no difference.
Medication review first
Some drugs act differently when you eat in a short window. That includes insulin, sulfonylureas, meglitinides, SGLT2 inhibitors, ACE inhibitors, ARBs and water pills. In one review of intermittent fasting, people who cut their long-acting insulin by 50% on fasting days still often had low blood sugar.
Expect the window to drift
When people in Oldenburg 2025 picked their own window, they kept 9.8 hours instead of 8. If the window matters to you, write it down. The studies where people stuck with it best had people log it.

Breaking the fast

No study tells you what your first meal after a 16-hour fast should be, and none tested one. What has been measured is protein. One 20 g serving of protein powder gave the most muscle building over 4 hours in young men, in two studies, one with 6 people and one with 48. But that ceiling does not hold at bigger servings. After a full-body weight workout, 100 g of milk protein gave a bigger and longer muscle-building response than 25 g. It lasted more than 12 hours, and very little of the extra protein was burned off. Older adults need more per meal: after exercise, 40 g beat 20 g. So if you eat 150 g of protein across three meals in an 8-hour window, the evidence in people says you are not wasting the extra.

Sources

  1. Study in people2023
    Time-Restricted Eating Without Calorie Counting for Weight Loss in a Racially Diverse Population: A Randomized Controlled Trial

    Annals of Internal Medicine

    In 90 people over 12 months, an 8-hour eating window with no calorie counting lost 4.61 kg more than eating as usual. Calorie counting did about as well, and the gap between the two plans was too small to count as a real difference. The window group ate 425 fewer calories a day without counting. Lean mass, belly fat and bone density did not change compared with eating as usual.

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

  2. Study in people2022
    Calorie Restriction with or without Time-Restricted Eating in Weight Loss

    New England Journal of Medicine

    139 adults with obesity were all given the same calorie target for 12 months, some with an eating window on top. The window group lost 8.0 kg and the calorie-only group lost 6.3 kg. That gap was too small to count as a real difference. Waist, body fat, lean mass, blood pressure and blood markers told the same story.

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

  3. Study in people2020
    Effects of Time-Restricted Eating on Weight Loss and Other Metabolic Parameters in Women and Men With Overweight and Obesity: The TREAT Randomized Clinical Trial

    JAMA Internal Medicine

    141 people spent 12 weeks on an eating window or in a comparison group, weighing themselves at home. Both lost very little, 0.94 kg against 0.68 kg, and the gap was too small to count as a real difference. In 50 people who got body scans, the window group lost 1.70 kg, and 1.10 kg of that was lean mass, not fat. The study did not track how much protein people ate.

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

  4. Study in people2023
    Effect of Time-Restricted Eating on Weight Loss in Adults With Type 2 Diabetes: A Randomized Clinical Trial

    JAMA Network Open

    75 adults with type 2 diabetes tried an 8-hour eating window for 6 months. They lost 3.56% more body weight than people who ate as usual. Their long-term blood sugar (HbA1c) fell 0.91%, as much as with a 25% calorie cut. Blood pressure and blood fats did not change, and there were no serious side effects.

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

  5. Study in people2016
    Effects of eight weeks of time-restricted feeding (16/8) on basal metabolism, maximal strength, body composition, inflammation, and cardiovascular risk factors in resistance-trained males

    Journal of Translational Medicine

    34 men who already lifted weights spent 8 weeks eating the same calories and protein, with or without a daily eating window, and trained under supervision. Fat fell more in the window group. Muscle, limb size and top strength held steady in both groups. Testosterone and IGF-1, a growth signal, fell only in the window group, and so did a thyroid hormone.

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

  6. Study in people2021
    Twelve Months of Time-restricted Eating and Resistance Training Improves Inflammatory Markers and Cardiometabolic Risk Factors

    Medicine and Science in Sports and Exercise

    The same weight-lifting men were followed to 12 months, and 20 were measured. In the window group arm muscle size fell 4.31% and thigh size 2.90%, while the normal-diet group gained 11.87% and 6.94%. Strength rose the same in both groups, and the window group ate 6.4% less without being told to. A widely quoted drop in belly fat was not a real difference, and limbs were measured with tape, not scans.

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

  7. Study in people2023
    Eight-hour time-restricted eating does not lower daily myofibrillar protein synthesis rates: A randomized control trial

    Obesity (Silver Spring)

    18 men, average age 46 and BMI 30, ate the same calories with about 1.0 g of protein per kg a day for 10 days. One group ate from 10:00 to 18:00 and the other from 08:00 to 20:00. Daily muscle building was the same in both, and blood sugar across the day fell with the short window. But the short window group lost more lean mass, 1.0 kg against 0.2 kg.

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

  8. Study in people2025
    Time-restricted eating, caloric reduction, and unrestricted eating effects on weight and metabolism: a randomized trial

    Obesity (Silver Spring)

    88 adults spent 12 weeks on a self-chosen 8-hour window, a 15% calorie cut, or eating freely, with body scans and a detailed insulin test. Weight, body make-up, calories eaten and blood sugar came out about the same in all three. The window group really ate over 9.8 hours, not 8. Their bodies were also less able to switch between burning fat and sugar than the calorie-cutting group's.

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

  9. Study in people2022
    Effectiveness of Early Time-Restricted Eating for Weight Loss, Fat Loss, and Cardiometabolic Health in Adults With Obesity: A Randomized Clinical Trial

    JAMA Internal Medicine

    90 adults with obesity spent 14 weeks on the same calorie cut and the same weight loss coaching. Eating only from 07:00 to 15:00 gave 2.3 kg more weight loss and 4 mmHg better lower blood pressure. Fat loss did not clearly differ, and neither did the share of weight lost as fat. It is here because it disagrees with Liu 2022, another study that held calories steady.

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

  10. Study in people2020
    Time-Restricted Eating Effects on Body Composition and Metabolic Measures in Humans who are Overweight: A Feasibility Study

    Obesity (Silver Spring)

    20 people with overweight spent 12 weeks on an 8-hour window or no window, with body scans and glucose monitors. In the window group weight fell 3.7%, fat 4%, lean mass 3.0% and belly fat 11.1%. Lean mass fell clearly more than in the no-window group. People actually ate over 9.9 hours, not 8.

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

What people report

These are 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 less without counting anything. The studies measured this at about 425 to 550 fewer calories a day.
  • Hunger in the first weeks, then less urge to eat.
  • Finding the rule easier to keep than a calorie target. That matches how much better people stuck with the window in the studies.
  • Trouble getting enough protein inside the window.
  • Social friction around breakfast and early dinners.

Sources: These are self-reports with no comparison group. They are what people in the studies on this page said about their own experience. They are not survey data. What people say they ate is a known weak spot in every fasting study done at home. 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 has stopped it first. Together with fasting, it can cause ketoacidosis (a dangerous acid build-up) while blood sugar looks normal. A finger-prick test will not warn you. The FDA says to stop canagliflozin, dapagliflozin and empagliflozin at least 3 days, and ertugliflozin at least 4 days, before planned surgery for this exact reason. A planned fast puts the body under the same strain.
  • Anyone on insulin, a sulfonylurea or a meglitinide, unless a prescriber changes the dose first. In CREED, a study of 3250 people with type 2 diabetes fasting for Ramadan, low blood sugar hit 16.8% of people on insulin and 5.3% on pills. The biggest warning sign was having had low blood sugar before: those people were 7.80 times as likely to have it again.
  • Anyone with type 1 diabetes outside a set program with glucose monitoring and an insulin plan. The one study in type 1 diabetes had 32 people over 6 months and saw no rise in ketoacidosis or severe low blood sugar. That is encouraging. It is not a green light to do this on your own.
  • 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. A plan judged by how well you keep a food rule sets you up to keep tightening the rule.
  • Children and teens. There are no long-term studies on growth or development. It is also the age group where surveys most strongly link fasting with eating disorders.
  • Anyone with a BMI under 18.5, or who already 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.

Questions

Does 16 hours of fasting turn on autophagy?
No study in people backs up that number, or any set hour. Autophagy, the cell's clean-up process, runs all the time. It speeds up or slows down. It does not flip on at a set point. The measurements in people disagree with each other depending on which tissue was tested. The best result in people is a 6-month study of 121 people that measured the process in blood cells. A fasting group came out different from usual care, but only when researchers looked back after the fact. Inside that group, the process did not clearly rise from where it started.
Will I lose muscle on 16:8?
The studies split, and protein is what separates them. In TREAT, about 65% of the weight lost in the group that got body scans was lean mass. Nobody measured how much protein they ate. Lin 2023 found no lean mass change over 12 months. Moro 2016 kept lean mass fully in men eating 1.93 g of protein per kg of body weight a day and lifting weights with supervision. Parr 2023, at 1.0 g per kg a day, lost more lean mass than the longer-window group, even though muscle building measured the same in both groups.
Is 16:8 better than just counting calories?
The studies that gave both groups the same calorie target came out differently. Liu 2022, over 12 months, found that adding an 8-hour window to calorie cutting gave 1.8 kg more loss, too small to count as a real difference. Jamshed 2022, over 14 weeks, found an early window plus the same calorie cut gave 2.3 kg more loss than a 12-hour window plus that cut. One thing is steady: in studies that tested both, people stuck with the window better than with calorie counting.
Does black coffee break the fast?
The studies did not test that. What they policed was calories. TREAT people took in no calories outside the window. TIMET counted any calories more than 15 minutes outside the window as a day off the plan. No study found here showed that a small amount of calories is harmless.