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Protein-forward14 studies in people cited

Protein-Forward Fasting

16 hours without food · Daily

Written by Reviewed Sep 2026

Also called: High-protein time-restricted eating, Muscle-sparing fasting

A short eating window done on purpose with lots of protein and weight training. The main lesson: when people lose muscle on fasting plans, the cause is low protein plus a short window plus time. It is not the window by itself.

Overview

Every eating window study that kept lean mass fed people 1.6 to 2.2 g of protein per kg of body weight a day and had them lift weights with supervision. Lean mass means the non-fat part of you, mostly muscle and water. Tinsley 2019 used 1.6 g per kg. Stratton 2020 used 1.83, with both groups cutting calories by the same 25%. Moro 2016 used 1.93, with the same calories and food mix in both groups. Blake 2025 used 2.2, while eating 10% more than needed. In all four, lean mass and strength came out the same on the short window as in the comparison group.

The study that lost more lean mass used 1.0 g per kg a day. In Parr 2023, 18 men ate enough to hold their weight for 10 days, on an 8-hour window or a 12-hour window. Researchers measured daily muscle building directly, using a tracer and repeated small muscle samples. Muscle building was the same in both groups: 1.28 against 1.26 %/day. Yet lean mass still fell more on the short window: 1.0 kg against 0.2 kg.

That is the one study that measured both the muscle-building process and the end result at once, and they disagreed. The window did not slow muscle building. Something else took the lean mass. The most obvious suspect is that people ate about half the daily protein of the studies that kept lean mass.

The honest limit: no study has tested a short window with low protein over months, looking mainly at lean mass. For anyone who lifts, that is the biggest gap in this research.

How it works

This is a normal short eating window with two things added on purpose.

First, a protein target high enough that the window is not what holds you back. The studies that kept lean mass used 1.6 to 2.2 g per kg of body weight a day. A review that pooled 49 studies with 1863 people, who were not losing or gaining weight, found extra protein gave no more lean mass above a total of about 1.62 g per kg a day.

Second, weight training. Every study in this field where people kept or gained lean mass while eating less had them lift weights. Not one left it out.

How you spread protein across the window matters less than people think. Short lab studies often find a difference between spreading protein evenly and loading it into one meal. Longer studies of body make-up mostly do not. The best-run study that found no difference used two tracer methods in 24 adults aged 65 to 80. Muscle building was 2.16 %/day with protein spread evenly and 2.23 %/day with it loaded into one meal. One review of the field says you cannot separate the effect of spreading from the effect of amount. It says adults already eating 0.8 to 1.3 g per kg a day do better having at least one meal big enough to max out muscle building than following any set pattern.

How fast you lose weight is the third lever. Top athletes losing 0.7% of body weight a week gained 2.1% lean mass. Those losing 1.4% a week lost 0.2%, for the same total weight lost. The slower group also got more total training time.

What 16 hours actually reaches

Fasting does not switch muscle building off. When healthy adults have not eaten for a while, muscle building ticks along at about 0.03 to 0.05 %/h, and that rate is the same in young and older people. Eating gives it a boost on top. A short window changes how many chances you get for that boost. At 72 hours of fasting, the forearm muscles start leaking protein building blocks. A key muscle-building switch (mTOR) drops by about 50%, along with related signals (4EBP1, rpS6 and ULK1). Two muscle-breakdown enzymes (MAFbx and MURF1) do not change. So at that point, muscle loss comes more from building slowing down than from breakdown speeding up. A 16-hour daily fast is nowhere near that state. The growth hormone claim is usually told wrong, so here it is carefully. Growth hormone goes up about three to five times within two to five days of fasting, while IGF-1 goes down. When researchers blocked growth hormone during a 40-hour fast, protein waste in the urine (urea) rose by about 50% and muscle breakdown went up. So it really does help the body save protein. But when fat release was blocked, adding growth hormone did nothing to protein breakdown. The authors concluded that growth hormone mainly saves protein by releasing fat, so the body burns fat instead of protein. It does not build muscle directly. In people fully adapted to long starvation, it failed outright: urea in the urine fell 50%, but ammonia rose 50%, so total protein loss did not change.

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

There are four studies of short eating windows with 1.6 to 2.2 g of protein per kg a day and supervised weight training. There is one study at 1.0 g per kg a day that measured both muscle building and body make-up. And there are protein-amount studies that show what the targets mean.

  • Tinsley 2019, 40 women who already lifted weights, for 8 weeks of supervised training three days a week. The window groups ate across about 7.5 hours, and the comparison group across 13.2 hours. All groups ate 1.6 g of protein per kg a day. Body make-up was measured with a detailed four-part method plus ultrasound. Lean mass rose 2% to 3% in all groups, with no difference between them. Muscle thickness and performance improved in all groups. Fat loss looked better on the window only when counting just the people who followed the plan closely, not when counting everyone.
  • Longland 2016, 40 young men for 4 weeks, eating about 40% less than they needed. They did weight training plus interval training six days a week, on 1.2 or 2.4 g of protein per kg a day. Lean mass rose 1.2 kg on more protein and 0.1 kg on less. Fat fell 4.8 kg on more protein and 3.5 kg on less. Lean mass went up during a 40% calorie cut, not just held steady.
  • Moro 2016, 34 men who already lifted weights, for 8 weeks. Calories and food mix were the same in both groups, with 1.93 against 1.89 g of protein per kg a day. They trained with supervision inside their eating window. Fat fell more on the window. Lean mass, limb size and top strength held steady in both groups. Testosterone and IGF-1 fell only on the window. At 12 months, in 20 of the men, arm muscle size on the window was down 4.31% and thigh size down 2.90%, while the comparison group gained. The window group had drifted to eating 6.4% fewer calories on their own. Strength rose the same in both groups.
  • Stratton 2020, 26 people for 4 weeks of supervised training. Both groups were told to cut calories by the same 25% and eat 1.8 g of protein per kg a day, and both did (1.83 g in each). Body make-up was measured with a four-part method plus ultrasound. Both groups changed over time, but the window made no difference to any body make-up or strength measure. Lean mass held steady equally. The stress hormone cortisol rose only in the normal-diet group.
  • Mettler 2010, 20 athletes who lifted weights, for 2 weeks at 60% of their usual calories, on about 1.0 or about 2.3 g of protein per kg a day. Lean mass loss was 1.6 kg on less protein and 0.3 kg on more. Fat loss and performance did not differ. The summary also says the high-protein group rated their tiredness higher.
  • Parr 2023, 18 men with an average age of 46 and a BMI of 30, eating enough to hold their weight on about 1.0 g of protein per kg a day. For 10 days they ate from 10am to 6pm (10:00 to 18:00) or from 8am to 8pm (08:00 to 20:00). Daily muscle building did not differ: 1.28 against 1.26 %/day. Blood sugar across 24 hours was lower on the short window. Total weight fell about the same (1.6 kg against 1.1 kg). But lean mass loss was bigger on the short window: 1.0 kg against 0.2 kg.
  • Blake 2025, 17 well-trained men and women, for 8 weeks of supervised training four days a week, with the weights going up over time. Both groups ate 10% more calories than needed, with 2.2 g of protein per kg a day. Lean mass rose in both: 2.67 kg on the window against 1.82 kg. The comparison group put on 1.4 kg more fat. Squat strength gains were 4.0 kg lower on the window, which matches doing less total work: 6960 reps against 7334. This is the only study here where the window group trained on an empty stomach.

What this does not tell you: The window-plus-training studies had between 17 and 26 people analysed, and the only 12-month study had 20. Every one is too small to reliably spot a lean mass difference between groups. So "no difference" here is weak evidence, not proof that the two are the same. The studies ran from 4 weeks to 12 months, and only one reached 12 months. Two of them measured limb muscle size with tape and skinfolds, not scans. No study has compared a short window at 1.0 against 1.8 g of protein per kg a day, looking mainly at lean mass, for six months or longer.

Reading the research record

The disagreements here are real, and the answer changes depending on what you hold steady.

On fat loss, Moro 2016 found the short window beat the same diet eaten over a normal day. Stratton 2020 set and checked the same calorie cut and the same protein in both groups, and the difference vanished. In Moro's study, people ate their usual way, and the window group ended up eating about 175 fewer calories a day. A review that pooled eating window plus weight training studies found people ate 174.88 fewer calories a day, almost exactly the same. The reading that fits all of this is that the window works as a hidden way to eat less. That is an interpretation, not a proven finding.

On lean mass, the studies truly split. The possible reasons can be ranked but not settled.

Length: every study that found no difference ran 4 to 8 weeks, and the only 12-month study found limb muscle size falling.

Protein: the studies that kept lean mass used 1.6 to 2.2 g per kg a day, and the one that lost more used 1.0.

Calories: when people ate 10% more than needed, the short window did not hurt lean mass at all. It gained more than the comparison group.

Measuring method: the studies using detailed four-part methods and ultrasound found no difference between groups. The two that found losses measured limb size with tape and skinfolds. Still, the direction of the 12-month result is hard to blame on the method, because the group that lost fat is the group whose measured limbs got smaller.

Counting method: in the same 2019 study, the fat result shows either no difference or a window advantage, depending on whether you count everyone or only people who followed the plan closely.

One popular protein result needs correcting, not repeating. A study of intermittent fasting with "protein pacing" reported more weight loss and more belly fat loss than calorie cutting. It is often summed up as showing protein pacing keeps muscle. The paper's own numbers say both groups lost about 1.5 kg of lean mass. What went up was lean mass as a share of body weight, 5.7% against 3.0%, because the protein pacing group lost more fat. That is a percentage effect. It is not keeping muscle.

The study was paid for by Isagenix International. Its products were the study foods in the protein pacing group, while the other group ate plain home-cooked food. Company employees are co-authors. The two groups differed in protein, food form and fibre all at once. As of this review, no other research group has repeated it.

What happens to muscle

This is the question this plan exists to answer, and the numbers are specific.

At 1.6 to 2.2 g of protein per kg of body weight a day with supervised weight training, four studies found lean mass and strength the same on a short window as with normal eating. They ran 4 to 8 weeks, and two used detailed four-part methods and ultrasound. When people ate 10% more than needed at 2.2 g per kg a day, the window group gained more lean mass than the comparison group: 2.67 against 1.82 kg.

At 1.0 g per kg a day over 10 days, lean mass fell 1.0 kg, against 0.2 kg in the 12-hour group. Daily muscle building was 1.28 against 1.26 %/day. The process was unchanged, but the result was different.

Across calorie cutting in general, the share of weight lost as lean mass was lower, and fat loss higher, at 1.6 and 2.4 g of protein per kg a day than at 0.8. The muscle-building response to a high-protein meal held up at the two higher amounts and was blunted at the standard daily recommendation (RDA). Note that 1.6 and 2.4 did equally well in that study. The benefit came from getting above the RDA, not from pushing as high as possible. In athletes eating 60% of their usual calories, lean mass loss was 1.6 kg at about 1.0 g per kg a day and 0.3 kg at about 2.3.

How much lean mass you lose also depends on how much fat you start with. Total protein lost per kg of weight lost is lower the more body fat you carry. It is roughly 20 g of nitrogen per kg in people without obesity, and roughly 10 g per kg in people carrying 50 kg or more of fat.

Using standard textbook math (6.25 g of protein per g of nitrogen, and lean tissue being about 20% protein), that means about 60% of weight lost comes from lean tissue in a lean person, and roughly 30% in someone with severe obesity. That math is textbook, not measured. It is why a lean lifter and a person with obesity should not use the same fasting plan.

One warning about reading body scans. The "lean" part on a body scan is mostly water. Stored sugar in muscle and liver (glycogen) holds about three times its own weight in water. In a 10-day fasting study, 58% of the lean loss on the scan was water and stored sugar, not tissue. A scan at the end of a fast makes lean loss look bigger than it is. A scan after eating again makes lean regain look bigger than it is.

Doing it

Protein target
1.6 to 2.2 g of protein per kg of body weight a day is the range every muscle-keeping study used. Above a total of about 1.62 g per kg a day, extra protein gave no more lean mass across 49 studies and 1863 people who were not losing or gaining weight. With a deep calorie cut and hard training, 2.4 g per kg a day gave a 1.2 kg lean mass gain, where 1.2 g per kg a day gave 0.1 kg.
Check what the protein number is counted per
The original high-protein crash diet plans (the protein sparing modified fast) counted grams per kg of ideal body weight, not lean body mass. When the amount was tested head to head on a 500-calorie diet, 1.5 g per kg of ideal body weight held protein balance level, while 0.8 g per kg lost 2 g of nitrogen a day. Counting per kg of lean body mass is a later habit in clinics.
Lift weights, and think about training after eating
Every study where people kept or gained lean mass while eating less had them lift weights. In the one study where the window group trained on an empty stomach, they did 374 fewer total reps and gained 4.0 kg less on their best squat, with calories and protein matched. What is rarely said is that in most other studies the window group trained inside its eating window.
Spreading protein matters most if you eat too little
Short lab studies often find that spreading protein matters. Longer body make-up studies mostly do not. The best-run study that found no difference used two tracer methods in 24 older adults and found 2.16 against 2.23 %/day. If you already hit your daily total, the evidence supports having at least one meal big enough to max out muscle building.
Slow the rate of loss
For the same total weight lost, top athletes losing 0.7% of body weight a week gained 2.1% lean mass, and those losing 1.4% a week lost 0.2%. The slower group also got more training time, which the authors point out muddies the result.
Age narrows the margin
Muscle building after a meal is about 16% lower in older men. After exercise, 40 g of protein beat 20 g in men around 71, while 20 g was enough in young men. One study that pooled and re-checked older data put the top useful amount per meal at 0.40 g per kg of body weight in older men and 0.24 in younger men. That difference was not clearly real, and only became clear when counted per kg of lean body mass. No short-window study has been run in older adults looking mainly at lean mass.

Breaking the fast

The strongest single measurement behind this plan comes from eating again after a long fast, not from a daily window. 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. The same pattern showed up in the research on high-protein crash diets. There, the body started building protein again on a protein-only, low-calorie refeed after lean mass had been lost. For a daily window, what matters is the amount, not the order. After a full-body weight workout, 100 g of protein gave a bigger and longer muscle-building response than 25 g. It lasted more than 12 hours, and very little of the extra was burned off. So a big first meal is not wasted. If you eat 150 g of protein across three meals in an 8-hour window, the evidence in people says you are not throwing the extra away.

Sources

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

    Obesity (Silver Spring)

    18 healthy 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, and total weight loss was similar, 1.6 kg against 1.1 kg. But the short window group lost more lean mass, 1.0 kg against 0.2 kg, and their blood sugar across the day fell.

    18 healthy males, 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 with repeated muscle sampling. Synthesis did not differ (1.28 versus 1.26 %/day, p = 0.82). 24-hour glucose area under the curve fell (p = 0.001). Total mass fell similarly (-1.6 versus -1.1 kg, p = 0.22) but lean mass loss was greater in the compressed arm (-1.0 versus -0.2 kg, p = 0.01).

  2. Study in people2019
    Time-restricted feeding plus resistance training in active females: a randomized trial

    American Journal of Clinical Nutrition

    40 women who already lifted weights trained under supervision for 8 weeks, eating in a window of about 7.5 hours or over 13.2 hours, all at 1.6 g of protein per kg a day. Lean mass rose 2% to 3% in every group, with no difference between them. Muscle thickness and performance improved in all groups. A fat loss edge for the window showed up only when counting the people who stuck to the plan.

    40 resistance-trained women randomised, 8 weeks of supervised training, achieved windows 7.5 and 7.6 hours against 13.2 in control, protein 1.6 g/kg/day in all groups (p = 0.58), body composition by a modified four-compartment model plus ultrasound. Fat-free mass rose 2% to 3% in all groups with no between-group difference; muscle thickness and performance improved in all groups. The fat mass difference was significant per protocol and not in the intention-to-treat analysis.

  3. Study in people2020
    Four Weeks of Time-Restricted Feeding Combined with Resistance Training Does Not Differentially Influence Measures of Body Composition, Muscle Performance, Resting Energy Expenditure, and Blood Biomarkers

    Nutrients

    26 people trained three times a week for 4 weeks, all on the same 25% calorie cut and 1.8 g of protein per kg a day, with or without an eating window. Both groups hit those targets. Body make-up and strength changed the same way in both, and lean mass held equally. The stress hormone cortisol rose only in the normal diet group.

    26 analysed, 4 weeks, supervised training three times a week, both arms prescribed an identical 25% deficit and 1.8 g/kg/day protein and achieving it (1.83 versus 1.83 g/kg/day, p = 0.956; energy 1946 versus 1939 kcal/day, p = 0.950). Body composition by a four-compartment model plus ultrasound. Significant time effects and no group by time interaction on any body composition or strength variable; fat-free mass maintained equally. Cortisol rose significantly only in the normal diet group.

  4. 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 an eating window, and trained inside the window. Fat fell more in the window group, while lean mass, limb size and top strength held in both. In the window group only, testosterone fell from 21.26 to 16.86 nmol/L and IGF-1, a growth signal, fell from 216.94 to 188.90 ng/mL.

    34 resistance-trained men, 8 weeks, energy and macronutrients matched, protein 1.93 versus 1.89 g/kg/day, supervised training performed inside the feeding window. Fat mass fell in the compressed arm (interaction P = 0.0448) with fat-free mass, limb area and maximal strength maintained in both. Testosterone fell from 21.26 to 16.86 nmol/L and IGF-1 from 216.94 to 188.90 ng/mL in the compressed arm only.

  5. 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 men at 12 months, 20 measured, with protein matched at 1.79 to 1.93 g per kg a day. In the window group arm muscle size fell 4.31% and thigh size 2.90%, while the comparison group gained 11.87% and 6.94%. The window group ate 6.4% less without being told to, and strength rose the same in both groups. A widely quoted drop in belly fat was not a real difference, and limbs were measured with tape, not scans.

    The same cohort at 12 months, 20 analysed, protein matched throughout at 1.79 to 1.93 g/kg/day in both arms. Arm muscle cross-sectional area -4.31% (p = 0.003) and thigh -2.90% (p = 0.03) in the compressed arm while the control arm gained 11.87% and 6.94%; fat-free mass interaction p = 0.002. Energy intake in the compressed arm fell 6.4% spontaneously (p < 0.0001). Strength rose equally in both arms. The widely quoted 18.8% visceral fat reduction was not significant (p = 0.518). Limb area by anthropometry, not imaging.

  6. Study in people2025
    Hypercaloric 16:8 time-restricted eating during 8 weeks of resistance exercise in well-trained men and women

    Journal of the International Society of Sports Nutrition

    17 well-trained men and women lifted weights four days a week for 8 weeks, all eating 10% more calories than they needed and 2.2 g of protein per kg a day, with or without an eating window. The window group gained 2.67 kg of lean mass against 1.82 kg, and the other group added 1.4 kg more fat. But squat strength gains were 4.0 kg lower in the window group, and they lifted less in total. This is the only study here where the window group trained while fasted.

    17 well-trained participants, 8 weeks of supervised progressively overloaded training four days a week, both arms in a 10% caloric surplus at 2.2 g/kg/day protein, body composition by a three-compartment model. Fat-free mass rose 2.67 kg in the compressed arm against 1.82 kg (p = 0.04) and the fed arm added 1.4 kg more fat mass (p = 0.04). Squat one-rep max gains were 4.0 kg lower in the compressed arm (p = 0.05), alongside significantly lower total training volume (6960 versus 7334 repetitions). The only trial in the set where the compressed arm trained fasted.

  7. Study in people2016
    Higher compared with lower dietary protein during an energy deficit combined with intense exercise promotes greater lean mass gain and fat mass loss: a randomized trial

    American Journal of Clinical Nutrition

    40 young men ate about 40% less than they needed for 4 weeks while lifting weights and doing hard interval training six days a week. Half ate 2.4 g of protein per kg a day and half ate 1.2 g. The higher protein group gained 1.2 kg of lean mass against 0.1 kg, and lost 4.8 kg of fat against 3.5 kg. Performance improved the same in both.

    40 young men, 20 per group, 4 weeks at roughly 40% below energy requirements with resistance plus high-intensity interval training six days a week, 1.2 versus 2.4 g/kg/day, four-compartment model. Lean body mass +1.2 kg on higher protein versus +0.1 kg (p < 0.05); fat mass -4.8 versus -3.5 kg (p < 0.05). Exercise performance improved similarly in both. Lean mass increased during a 40% deficit rather than merely being spared.

  8. Study in people2010
    Increased protein intake reduces lean body mass loss during weight loss in athletes

    Medicine and Science in Sports and Exercise

    20 athletes who lifted weights ate 60% of their usual calories for 2 weeks, with about 1.0 g or about 2.3 g of protein per kg a day. The higher protein group lost less weight, 1.5 kg against 3.0 kg, and much less lean mass, 0.3 kg against 1.6 kg. Fat loss and performance did not differ. The higher protein group did report more tiredness and feeling worse than normal.

    20 resistance-trained athletes, 2 weeks at 60% of habitual energy intake, 15% of energy as protein (about 1.0 g/kg) versus 35% (about 2.3 g/kg). Total mass loss -3.0 versus -1.5 kg (p = 0.036); lean body mass loss -1.6 versus -0.3 kg (p = 0.006). Fat loss and performance did not differ. The abstract reports fatigue ratings and worse-than-normal scores were higher in the high-protein group.

  9. Study in people2023
    The anabolic response to protein ingestion during recovery from exercise has no upper limit in magnitude and duration in vivo in humans

    Cell Reports Medicine

    36 active young men did a 60-minute full-body weight workout, then had 0 g, 25 g or 100 g of milk protein, 12 men per group. 100 g gave a bigger and longer muscle-building response than 25 g, lasting more than 12 hours. Muscle building was about 20% higher over the first 4 hours and about 40% higher from 4 to 12 hours. Very little of the extra protein was broken down or burned off.

    36 recreationally active young men, 12 per group, 0 g, 25 g or 100 g of intrinsically labelled milk protein after 60 minutes of whole-body resistance exercise. 100 g produced a greater and more prolonged response than 25 g, lasting beyond 12 hours, with myofibrillar synthesis roughly 20% higher over 0 to 4 hours and roughly 40% higher over 4 to 12 hours, and negligible effect on whole-body protein breakdown or amino acid oxidation.

  10. Study in people1984
    Metabolic effects of very low calorie weight reduction diets

    Journal of Clinical Investigation

    17 healthy women with obesity spent 5 or 8 weeks on 500-calorie diets. One diet gave 1.5 g of protein per kg of ideal body weight, the other 0.8 g plus some carbs. After 3 weeks, the higher protein diet held body protein steady, while the lower one lost 2 g of nitrogen a day, a sign of protein loss. Protein was set by ideal body weight, not actual lean mass.

    17 healthy obese women randomised for 5 or 8 weeks to two isocaloric 500 kcal diets, one providing 1.5 g protein per kg of ideal body weight and the other 0.8 g/kg plus 0.7 g carbohydrate/kg, with leucine and alanine tracer infusions. After 3 weeks of adaptation, nitrogen balance was zero on the 1.5 g/kg diet and -2 g of nitrogen per day on the 0.8 g/kg diet. Note the denominator: ideal body weight, not lean body mass.

  11. Study in people1976
    Nitrogen metabolism and insulin requirements in obese diabetic adults on a protein-sparing modified fast

    Diabetes

    This is where the protein-sparing modified fast started. 7 adults with obesity and diabetes, taking 30 to 100 units of insulin a day, ate 1.2 to 1.4 g of protein per kg of ideal body weight. In the 3 studied most closely, body protein held steady over time. All 7 stopped insulin within 0 to 19 days, 6.5 days on average.

    The origin of the protein sparing modified fast prescription, in 7 obese adults with adult-onset diabetes taking 30 to 100 units of insulin per day. Protein was given at 1.2 to 1.4 g per kg of ideal body weight, with nitrogen balance maintained chronically at 1.3 g/kg of ideal body weight in the 3 patients who had extensive balance studies. Insulin was discontinued in all 7 patients after 0 to 19 days, mean 6.5 days.

  12. Study in people1990
    Protein metabolic effects of a prolonged fast and hypocaloric refeeding

    American Journal of Physiology

    10 women with moderate obesity fasted for 3 weeks, then spent one week refeeding on 80 g of carbs or 80 g of protein a day. By the end of the fast, the body was losing 5.9 g of nitrogen a day, a sign of protein loss, and protein turnover markers had fallen 31%. Protein refeeding turned that into a gain of 1.7 g a day. Carbs only cut the loss to 3.1 g a day.

    10 moderately obese women, a 3-week fast followed by random allocation to one week of refeeding with 80 g of carbohydrate or 80 g of protein. After the fast, plasma leucine flux and urinary 3-methylhistidine excretion had both fallen 31% and nitrogen balance was -5.9 g/day. Protein refeeding made nitrogen balance positive at +1.7 g/day (P < 0.05); carbohydrate refeeding improved it only to -3.1 g/day while leucine flux fell a further 18%.

  13. Study in people2011
    Effect of two different weight-loss rates on body composition and strength and power-related performance in elite athletes

    International Journal of Sport Nutrition and Exercise Metabolism

    24 elite athletes lost weight slowly, aiming for 0.7% of body weight a week, or fast, aiming for 1.4%, while lifting weights four times a week. Both lost about the same weight, 5.6% and 5.5%. The slow group lost more fat, 31% against 21%, and gained 2.1% lean mass, while the fast group lost 0.2%. The slow group also trained more in total, which the authors say muddies the result.

    24 elite athletes randomised to a slow reduction targeting 0.7% of body weight per week (n = 13) or a fast reduction targeting 1.4% (n = 11), with four resistance sessions a week in both and DXA. Body weight fell 5.6% and 5.5%, matched. Fat mass -31% versus -21%. Lean body mass +2.1% in the slow group (p < 0.001) versus -0.2% in the fast group (between-group p < 0.01). The slow group also accumulated more total training exposure, which the authors name as a confound.

  14. Study in people2023
    Intermittent fasting and protein pacing are superior to caloric restriction for weight and visceral fat loss

    Obesity (Silver Spring)

    41 adults started and 39 were counted over 8 weeks. The group on protein pacing plus fasting lost more weight, 8.2 kg against 5.0 kg, and more belly fat, 33% against 15.8%, than the calorie-cutting group. Lean mass actually fell about 1.5 kg in both groups, and only its share of body weight rose. Isagenix, whose products were the study foods, paid for the study and its staff were authors, and nobody else has repeated it.

    41 randomised, 39 analysed, 8 weeks. The protein pacing arm lost more weight (-8.2 versus -5.0 kg, p = 0.009) and visceral fat (-33% versus -15.8%, p = 0.030). On lean mass the paper's own words are that absolute fat-free mass decreased in both groups by about 1.5 kg; what rose was the proportion of fat-free mass to body weight (5.7% versus 3.0%, p = 0.030). Sponsored by Isagenix International, whose products were the study foods in the protein pacing arm while the comparator ate unbranded home-cooked food, with sponsor employees as co-authors. No independent replication was identified.

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.

  • Getting enough protein inside the window is the real limit, not hunger.
  • Training inside the eating window rather than before it.
  • Bigger protein servings than usual, which the protein-amount research supports.
  • Tiredness at very high protein while cutting calories. This was also reported inside a study.
  • Feeling lower on energy when training on an empty stomach, reported next to a measured drop in training volume.

Sources: These are self-reports with no comparison group, with two exceptions. The tiredness item was reported in the summary of the two-week athlete study, as higher in the high-protein group. The energy item was reported next to a measured 374-rep drop in training volume in the study where people trained on an empty stomach. 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 who cannot reliably get 1.6 g of protein per kg of body weight a day inside their window. That is exactly what the 1.0 g per kg study measured: lean mass fell more, even though muscle building did not change.
  • Anyone taking an SGLT2 inhibitor, unless a prescriber stops it first.
  • Anyone on insulin, a sulfonylurea or a meglitinide, unless a prescriber changes the dose for a shorter eating window.
  • Pregnancy and breastfeeding.
  • Anyone with a past eating disorder, or who now has episodes of eating they cannot control. Adding a protein target on top of a window rule gives you a second thing to judge yourself by.
  • Children and teens.
  • Anyone with a BMI under 18.5, or who meets the UK NICE risk list for refeeding problems.
  • Anyone with severe kidney disease. Both major protein guidelines for older adults name it as the exception when they advise 1.0 to 1.2 g of protein per kg a day for people over 65, and at least 1.2 g per kg a day for those who exercise.
  • Adults over 65 who want to follow the published studies, because there are none. No short-window study has been run in that age group looking mainly at lean mass, and the per-meal protein research suggests there is less room for error.
  • Anyone already lean and cutting calories hard. One review of athletes who lift and cut calories puts their protein needs at 2.3 to 3.1 g per kg of fat-free mass, which is hard to fit into a short window.

Questions

Does a compressed eating window cost muscle?
Not with enough protein, in studies of 4 to 8 weeks. Four studies at 1.6 to 2.2 g of protein per kg of body weight a day, with supervised weight training, found lean mass and strength the same as normal eating. When people ate 10% more than needed, the window group gained more. The study at 1.0 g per kg a day lost 1.0 kg of lean mass, against 0.2 kg on its 12-hour window, even though daily muscle building was the same. The way people lose muscle is low protein plus a short window plus time.
How much protein, exactly?
1.6 to 2.2 g per kg of body weight a day is the range that kept lean mass in every short-window study. Above about 1.62 g per kg a day, extra protein added no more lean mass across 49 studies of people not losing or gaining weight. With a 40% calorie cut and hard training, 2.4 g per kg a day gave a 1.2 kg lean mass gain, where 1.2 g per kg a day gave 0.1 kg. In a separate study, 1.6 and 2.4 g per kg a day did equally well. So the benefit came from getting above the standard daily recommendation (RDA), not from pushing as high as possible.
Does it matter how the protein is spread across the window?
Less than the short lab studies suggest. Tracer studies often find that spreading protein changes muscle building. Longer body make-up studies mostly do not. The strongest study that found no difference used two tracer methods in 24 adults aged 65 to 80, and found 2.16 against 2.23 %/day. One review of the field says you cannot separate spreading from amount. Adults already hitting their total do better having at least one meal big enough to max out muscle building.
Does the fasting growth hormone rise protect muscle?
It helps, but not the way the claim suggests. Growth hormone goes up three to five times within two to five days of fasting, while IGF-1 goes down. Blocking it during a 40-hour fast raised protein waste in the urine by about 50%. But when fat release was blocked, adding growth hormone did nothing to protein breakdown. The authors concluded it mainly saves protein by releasing fat to burn. In people fully adapted to long starvation, it did not cut total protein loss: urea fell 50% while ammonia rose 50%.
What has not been tested?
A study comparing a short window at 1.0 against 1.8 g of protein per kg a day, with the same calories, weight training, and detailed body scans, running at least six months. Nothing like it exists. Every study in this section analysed between 17 and 26 people, so "no difference" results are weak evidence, not proof the approaches are the same.