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Time-restricted5 studies in people cited

One Meal a Day

23 hours without food · Daily

Written by Reviewed Sep 2026

Also called: 23:1, One meal per day, Warrior-style eating

You eat all of a day's calories in one meal. In studies that kept calories the same, fat went down. But in those same studies, blood pressure, LDL cholesterol and morning blood sugar handling all got worse.

Overview

OMAD (one meal a day) is the tightest daily eating pattern in common use. It also has the fewest and oldest studies. Most of what exists fed lean, healthy people enough to keep their weight steady. That kind of study answers one question well: what happens to the body when calories stay the same but are squeezed into one sitting?

The answer is truly mixed, and both halves are in the same papers. At the same calories, one meal a day lowered body fat and the stress hormone cortisol. At the same time it clearly raised blood pressure and total, LDL and HDL cholesterol. A companion paper looked at blood sugar. Morning fasting blood sugar was higher. Blood sugar rose more, and stayed up longer, after a sugar drink test. And insulin was slower to respond.

That is not a verdict against OMAD. It is what was measured. Any write-up that only reports the fat result is telling you half a paper.

How it works

You eat once a day, and all your food goes in that meal. In the studies where people were given their food, the meal was in the evening. It was between 4pm and 8pm (16:00 to 20:00) in the fewer-meals studies. It was between 5pm and 7pm (17:00 to 19:00) in a study of lean people eating enough to keep their weight steady.

Meal timing is not a small detail here. The study where morning blood sugar handling got worse put the one meal late. The studies where insulin handling got better on a six-hour window put it early, with dinner done before 3pm (15:00). Same basic change, opposite ends of the day, opposite blood sugar results. Timing is the most obvious reason why. No study has tried one meal a day in the morning against one meal in the evening.

Eating all your food at once also makes the protein-per-meal question real, not just theory. That is covered under body make-up below.

What 23 hours actually reaches

With calories kept the same and eaten in one evening meal, here is what was measured. Morning fasting blood sugar was higher. Blood sugar rose more, and stayed up longer, during a sugar drink test. Insulin was slower to respond. The hunger hormone ghrelin was higher. Fasting insulin and the other hormones checked (leptin, adiponectin, resistin and BDNF) did not clearly change. The problem went away when people went back to normal meals, and it was not tied to changes in fat hormones. In the companion study, blood pressure clearly rose, and total, LDL and HDL cholesterol all rose. Meanwhile body fat fell and the stress hormone cortisol fell. People kept their weight within 2 kg across the six months, which is what the study was designed to do. A separate study had 11 lean people eat the same calories as one meal or three meals, for 11 days each. On one meal, they lost more weight (1.4 kg against 0.5 kg) and more fat (0.7 kg against 0.1 kg), even though calories were the same. They burned more fat during exercise. Fitness and strength did not drop. Nobody has measured autophagy or mitophagy (the cell's clean-up processes) on a 23-hour daily fast. The closest measurement in people was a 24-hour fast done three days a week. Muscle samples after the 24-hour fast showed one marker (SQSTM1) going up. But the authors concluded that clean-up markers in human muscle went down overall, and blamed weight loss. That is a different plan, and it does not point the way the popular claim assumes.

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

    Still digesting

    Burning
    Sugar from the meal you just ate. Insulin is high enough to keep fat locked away.
    • Insulin is at its after-meal peak. While it is up there, the enzyme that releases fat from fat cells stays switched off, so fat is being stored, not released.
    • The hour count on most fasting charts starts at the last bite. But the body is still digesting a mixed meal here, so the clock and what the body is doing are not yet the same thing.

    Read the biologyInsulin and insulin sensitivity during a fast

    Measured in people
    The hour-by-hour fasting studies in people start later than this. Klein 1993 took its first sample at 12 hours, in 6 healthy men. Rothman 1991 gives its first figure for new sugar making as an average across the first 22 hours. This row is the fed state those measurements are compared against. No measurement from a long fast falls inside it.

  2. 4h to 12h

    Meal digested, overnight fast

    Burning
    A mix of leftover sugar and fat. The liver keeps the brain supplied with sugar.
    Blood sugar
    5.58 mmol/L (100.5 mg/dL) at the 12 hour mark, in 6 healthy men
    Ketones
    Total ketones around 0.20 mmol/L after an overnight fast. That figure is acetoacetate and BHB added together, not BHB on its own.
    Insulin
    64.6 pmol/L (9.3 microunits/mL) at 12 hours, in the same 6 men
    • At 12 hours, glycerol is released at 2.08 and palmitic acid at 1.63 micromol/kg/min. These are two markers of fat being released. They are the starting numbers everything later in the fast is compared against.
    • Making new sugar from scratch already supplies most of the body's sugar at this point. It is not a backup waiting to be called on. This is the first place the popular "switch" story breaks down.

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

    Measured in people
    Klein 1993 measured blood sugar, insulin and fat release with tracers at 12 hours, in 6 healthy men. Féry 1983 measured total ketones in people after an overnight fast and put the starting level at 0.20 mmol/L. Rothman 1991 measured liver glycogen again and again with a special MRI scan through a 68 hour fast in healthy adults. It worked out that new sugar making supplied 64% of all the body's sugar across the first 22 hours.

  3. 12h to 18h

    Ketones have not moved yet

    Burning
    Fat use rising. More and more of the body's sugar is being made fresh, not pulled from storage.
    Ketones
    A middle change of 0% between 12 and 18 hours, measured every 6 hours in 34 adults
    • BHB, the main ketone, does not move in this window. In 34 adults sampled every 6 hours through the standard 72 hour fast doctors use for testing, the middle change from 12 to 18 hours was zero.
    • Making new sugar supplies 64% of the body's sugar across the first 22 hours. A second, unrelated method put it at about 47% by 14 hours. The body does not switch from stored sugar to fat at a set hour.
    • In 11 overweight adults, activity of the autophagy gene LC3A in blood cells was 22% higher after an 18 hour fast than after a 12 hour fast. In the same people, activity of the MTOR gene also rose, by 9%.

    Read the biologyGlycogen depletion and gluconeogenesisKetogenesis and beta-hydroxybutyrateAutophagy

    Measured in people
    Service 2005 measured BHB every 6 hours in 34 adults whose 72 hour test fasts came back clear for a rare insulin-making tumour. Rothman 1991 measured the share of new sugar making with a special MRI scan. Jamshed 2019 measured gene activity in blood cells in an 11-person, 4 day study where each person tried two eating windows: 8am to 2pm, and 8am to 8pm. That is one gene's activity in blood cells. It is not the real speed of autophagy, and it is not muscle. No study in any species has tested a 16 hour time point for autophagy.

  4. 18h to 1d

    The biggest hours for fat release

    Burning
    Fat and glycerol, with ketone making starting its climb
    Ketones
    Climbing. The middle rise from 18 to 36 hours was +333%, the steepest stretch of a 72 hour fast.
    Insulin
    70% of the whole drop seen across a 72 hour fast has already happened by 24 hours
    • The single biggest jump in fat release across a whole 72 hour fast comes between 18 and 24 hours. 60% of the total rise in fat release happens between 12 and 24 hours. The authors put that down to falling insulin, not to any change in blood sugar.
    • In a study of 30 healthy adults, growth hormone was clearly higher after a 24 hour water-only fast than after a normal eating day. Each person did both. The size of that rise is not in the published summary, so it is not given here.
    • In the same study, haemoglobin, red blood cell count and haematocrit all rose together at 24 hours. That pattern means the fluid part of the blood shrank. It does not mean new red blood cells were made.
    • In 20 volunteers, energy burned over 24 hours, measured in a sealed room, fell during a 24 hour fast. How much it fell varied from person to person.

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

    Measured in people
    Horne 2013 had 30 seemingly healthy adults do both a 24 hour water-only fast and a normal eating day, in random order. It measured growth hormone, blood counts, cholesterol, triglycerides, bicarbonate and weight. Klein 1993 measured fat release with tracers. Hollstein 2020 measured energy burned over 24 hours in a sealed room in 20 volunteers. The exact percentage drop is not in the published summary, so it is not given here. On autophagy, a 24 hour fast raised p62 (also called SQSTM1) in the thigh muscle of 50 women. That points toward less recycling in muscle, not more.

  5. 1d to 36h

    Thyroid drops before metabolic rate does

    past this window
    Burning
    Fat. The remaining sugar is built from glycerol, lactate and protein building blocks.
    Ketones
    1.308 mmol/L on average after day 1, in 13 men starting a 10 day water fast, up from 0.177 at the start. People varied a lot: the spread around that average (1.053) was nearly as big as the average itself.
    • Making new sugar supplies 82% of the body's sugar between 22 and 36 hours.
    • In 8 healthy men, the thyroid hormone T3 fell from 2.30 to 1.84 nmol/L by 30 hours. TSH, the hormone that tells the thyroid to work, fell below 1 mU/L, and its usual night-time peak disappeared. Blood cortisol did not change.
    • So the thyroid slows down within the first 30 hours, well before anyone can measure a drop in resting metabolic rate.
    • Salt loss has not turned yet. In 9 women with obesity who kept eating their usual amount of salt while fasting, salt in the urine started to rise after about 48 hours and then went above what they took in.

    Read the biologyGlycogen depletion and gluconeogenesisKetogenesis and beta-hydroxybutyrateAutophagy

    Measured in people
    Rothman 1991 measured the share of new sugar making with a special MRI scan in healthy adults. Hugues 1984 fasted 8 healthy men for 30 hours and tested their thyroid hormones again and again. Dai 2022 measured blood BHB daily in 13 men fasting in a supervised facility. Sigler 1975 collected urine in 3 hour blocks from 9 women with obesity on a hospital ward. On autophagy, the only study in people with several time points in one fast took thigh muscle samples at 2, 12, 24 and 36 hours. The markers LC3I, LC3II and p62 all fell, but only in untrained people. The authors sum it up as muscle recycling being only mildly affected by 36 hours of fasting.

What studies in people found

There are two papers from one study that fed healthy, normal-weight middle-aged adults all their meals. There is one study of lean people eating enough to keep their weight steady. And there is one study of a 4-hour window, which is the closest test of this kind.

  • Cienfuegos 2020 is the closest comparison, though it used a 4-hour window, not one meal. In adults with obesity over 8 weeks, people lost 3.2% of their weight, 2.8 kg of fat and 0.8 kg of lean mass. They ate about 550 fewer calories a day without counting.
  • Stote 2007. The same people ate all the calories they needed to keep their weight steady as three meals a day for 8 weeks and as one meal a day for 8 weeks. On one meal a day, hunger clearly rose and body make-up clearly changed, including less fat. Blood pressure and total, LDL and HDL cholesterol clearly rose. The stress hormone cortisol clearly fell. Heart rate, body temperature and most other blood tests did not change. People kept their weight within 2 kg across the six months. We could not confirm how many people started or finished, so no number is printed here.
  • Carlson 2007 looked at blood sugar in the same study. The one meal was eaten between 4pm and 8pm, and people did sugar drink tests in the morning. On one meal a day, morning fasting blood sugar was higher. Blood sugar rose more and stayed up longer during the test. Insulin was slower to respond, and the hunger hormone ghrelin was higher. Fasting insulin, leptin, adiponectin, resistin and BDNF did not clearly change. The blood sugar problem went away afterwards.
  • Meessen 2021, 11 healthy lean people who finished (13 started), living at home. The same people ate the same calories as one meal from 5pm to 7pm or as three meals, for 11 days each. On one meal, weight fell 1.4 kg against 0.5 kg, and fat fell 0.7 kg against 0.1 kg, even though calories were the same. They burned more fat during exercise. Fitness and strength did not drop. Blood sugar was lower in the second half of the day.

What this does not tell you: The two fed-every-meal papers come from one study, in healthy, normal-weight middle-aged adults, published in 2007. We could not confirm how many people took part. The study of lean people had 11 finish over 11 days. No study has tested one meal a day for weight loss in people with obesity. No study has compared a single morning meal with a single evening meal. Nothing here ran longer than six months.

Reading the research record

The most useful thing about the OMAD research is that calories were held steady. The rest of the fasting research mostly lacks that. The disagreement it raises is not between research groups. It is between results inside the same study. Body fat and cortisol went one way. Blood pressure, LDL cholesterol and morning blood sugar handling went the other way. Weight and calories stayed the same the whole time.

That does not fit either simple story. It does not fit the idea that squeezing your eating window does nothing except make you eat less, because these people were not eating less and their body make-up still changed. It also does not fit the idea that longer daily fasts are good across the board, because the same study made blood sugar handling worse and raised blood pressure.

Meal timing is the obvious link. The one meal was late, and the studies that improved insulin handling at similar fast lengths put the window early. It is a possible answer. Nobody has tested it by moving the one meal.

One more thing, for anyone who has seen the headline. A published analysis of a big US health survey (NHANES) looked at eating-window length and death. People who ate within less than 8 hours a day had 2.35 times the rate of death from heart disease, compared with people who ate across 12 to 14 hours. It found no link with death from any cause or from cancer. The eating window came from two days of people recalling what they ate. That work, and the formal reply to it, are laid out on the 5:2 and alternate-day pages. One criticism is that the short-window group was likely full of shift workers, not people fasting on purpose. That criticism applies fully to anyone reading it as a study about OMAD.

What happens to muscle

Body fat fell in both studies that fed people all their meals at the same calories. That is the result OMAD is usually sold on. Neither study gave a lean mass number we could check. Lean mass means the non-fat part of you, mostly muscle and water. The study of lean people reported that fitness and strength did not drop over 11 days, but did not report lean mass on its own.

The closest body make-up data at a similarly short window is the 4-hour group in Cienfuegos 2020. It lost 2.8 kg of fat and 0.8 kg of lean mass over 8 weeks. That was the better of the two groups in that study. The 6-hour group lost 1.5 kg of lean mass and 1.4 kg of fat. Neither group reported protein per kg of body weight, and neither lifted weights.

A common worry about one meal a day is that a single big dose of protein gets wasted. The evidence in people does not back that up. In 36 active young men, 100 g of milk protein after a full-body weight workout gave a bigger and longer muscle-building response than 25 g. It lasted more than 12 hours. Muscle building was about 20% higher over the first 4 hours and about 40% higher from 4 to 12 hours. It barely changed how fast the body broke down or burned protein. Forty grams also beat 20 g after a full-body workout in trained men, and after exercise in older men.

What the evidence does not show is that one feeding a day is as good as spread-out meals for lean mass over months. The study that tested a short window at 1.0 g of protein per kg a day found more lean mass loss than its 12-hour group, while daily muscle building was the same. Nobody has run that test at one meal a day.

Doing it

Meal timing is the untested part
The fed-every-meal studies put the meal between 4pm and 8pm, and the study of lean people between 5pm and 7pm. Morning blood sugar handling got worse with the late meal. Six-hour windows that ended before 3pm improved insulin handling. No study has moved the one meal to the morning.
Protein in one sitting
100 g of protein gave a bigger and longer muscle-building response than 25 g, lasting more than 12 hours, and very little extra protein was burned off. The per-meal limit people worry about is not backed up. Total daily protein is still the number that tracked lean mass across the short-window studies.
Check blood pressure and cholesterol
At the same calories, one meal a day clearly raised blood pressure and total, LDL and HDL cholesterol. That was in healthy, normal-weight adults over 8-week stretches.
Electrolytes and fluid
No published plan checks salts and minerals for a daily fast under 24 hours. What is known is that falling insulin makes you pass more salt early in a fast. Feeling faint or light-headed is the most common medium-level problem reported in supervised long fasts. A 23-hour daily fast sits just under anything anyone has studied.
Expect a blood sugar test to look worse
On one meal a day, with weight held steady, morning fasting blood sugar was higher and the sugar drink test response lasted longer. It went away afterwards. A single fasting blood sugar test taken while you are doing this cannot be read the same way as one taken on three meals a day.

Breaking the fast

The one meal is the fast being broken. So the question is what goes in it, not how to ease back into food. Here is what has been measured. After a full-body weight workout, 100 g of protein gave a bigger and longer muscle-building response than 25 g, with no real rise in protein being burned off. 40 g beat 20 g in trained young men after a full-body workout, and in older men after exercise. In young men at rest, 15 g of essential amino acids (protein building blocks) gave the same muscle-building response in one serving or split into four. Either way it took at least 90 minutes to kick in. Refeeding syndrome is not a worry at a daily 23-hour fast. That risk belongs to fasts that last days, not hours, and to people who meet the NICE risk list going in.

Sources

  1. Study in people2007
    A controlled trial of reduced meal frequency without caloric restriction in healthy, normal-weight, middle-aged adults

    American Journal of Clinical Nutrition

    Healthy, normal-weight middle-aged adults ate their usual calories as three meals a day for 8 weeks and as one meal a day for 8 weeks. On one meal a day they were much hungrier and lost some fat, but blood pressure and cholesterol went up and cortisol, a stress hormone, went down. Weight stayed within 2 kg. We could not check how many people started and finished.

    Randomised crossover, two 8-week periods, all weight-maintenance calories as three meals or one meal per day. On one meal a day: significantly increased hunger, reduced fat mass, significantly increased blood pressure and total, LDL and HDL cholesterol, and significantly decreased cortisol. Weight maintained within 2 kg across the six-month period. Heart rate, body temperature and most other blood variables unaffected. The enrolled and completed counts could not be verified from the accessible record.

  2. Study in people2007
    Impact of reduced meal frequency without caloric restriction on glucose regulation in healthy, normal-weight middle-aged men and women

    Metabolism

    Healthy, normal-weight middle-aged adults ate their usual calories as three meals or as one meal between 16:00 and 20:00, 8 weeks each, with an 11-week break between. On one meal, morning blood sugar was higher, blood sugar rose more and stayed up longer after a sugar drink, insulin was slow, and the hunger hormone ghrelin was higher. Fasting insulin and several other hormones did not change. It went back to normal afterwards.

    Randomised crossover, two 8-week periods with an 11-week off-diet interval, weight-maintenance calories as three meals or one meal eaten between 16:00 and 20:00, with morning oral glucose tolerance tests. On one meal a day: higher morning fasting plasma glucose, greater and more sustained glucose elevations during the tolerance test, delayed insulin response, elevated ghrelin. Fasting insulin, leptin, adiponectin, resistin and BDNF unaffected. The impairment was reversible.

  3. Study in people2021
    Differential Effects of One Meal per Day in the Evening on Metabolic Health and Physical Performance in Lean Individuals

    Frontiers in Physiology

    13 healthy lean adults started and 11 finished. Each spent 11 days eating the same calories as one meal from 17:00 to 19:00 and 11 days as three meals. On one meal they lost 1.4 kg against 0.5 kg, and 0.7 kg of fat against 0.1 kg, even with matched calories. They burned more fat during exercise, fitness and strength held up, and blood sugar was lower later in the day.

    Randomised crossover, eucaloric feeding as a single meal eaten 17:00 to 19:00 versus three meals per day, 11 days per period, free-living healthy lean individuals. 13 included, 11 completed. Body mass -1.4 kg versus -0.5 kg (P = 0.03) and fat mass -0.7 kg versus -0.1 kg (P = 0.049) despite matched calories. Exercise fatty acid oxidation increased (P < 0.001). Aerobic capacity and strength not impaired (P > 0.05). Plasma glucose lower during the second half of the day.

  4. Study in people2020
    Effects of 4- and 6-h Time-Restricted Feeding on Weight and Cardiometabolic Health: A Randomized Controlled Trial in Adults with Obesity

    Cell Metabolism

    The closest fair test to eating one meal a day: 58 adults with obesity over 8 weeks. Eating only from 15:00 to 19:00 cut 3.2% of body weight, 2.8 kg of fat and 0.8 kg of lean mass, the same weight loss as a 6-hour window but more of it from fat. People in both windows ate about 550 fewer calories a day without counting. A marker of cell damage fell 37% in the 4-hour group.

    The nearest randomised comparator to a single daily meal. 58 randomised, 8 weeks. The 4-hour arm (15:00 to 19:00) lost 3.2% of body weight, 2.8 kg of fat mass and 0.8 kg of lean mass, matching the 6-hour arm on weight while splitting it more favourably. Energy intake fell about 550 kcal/d in both without calorie counting. 8-isoprostane fell 13 pg/mL, a 37% reduction, in the 4-hour arm.

  5. 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 lifted weights for 60 minutes, 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, given 0 g, 25 g or 100 g of intrinsically labelled milk protein after 60 minutes of whole-body resistance exercise, with a quadruple isotope tracer approach. 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. Protein ingestion had negligible effect on whole-body protein breakdown or amino acid oxidation rates.

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.

  • Hunger. It clearly went up in the fed-every-meal study, and it is the most steadily measured feeling at this window.
  • Trouble physically eating a whole day of food in one sitting.
  • A simple rule, with no food decisions during the day.
  • Hitting a protein target, not total calories, being the real limit.

Sources: These are self-reports with no comparison group, except for hunger. Hunger was measured in the fed-every-meal study and clearly went up on one meal a day. The hunger hormone ghrelin was also higher in the companion paper. 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 23-hour daily fast comes closer than any shorter window on this site to the conditions that cause ketoacidosis (a dangerous acid build-up) while blood sugar looks normal. Your blood sugar reading will not warn you.
  • Anyone on insulin, a sulfonylurea or a meglitinide. In the research on intermittent fasting, people who cut their long-acting insulin by 50% on fasting days still often had low blood sugar. Cutting the dose alone does not keep you safe.
  • Anyone with type 1 diabetes outside a set program with glucose monitoring.
  • Pregnancy and breastfeeding.
  • Anyone with a past eating disorder, or who now has episodes of eating they cannot control. Holding back all day and then eating one big meal 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.
  • Older adults at risk of muscle loss with age. They need more protein per meal to switch on muscle building, and one meal gives one chance to get there.
  • Anyone with high blood pressure or high LDL cholesterol who is not keeping an eye on both. Both clearly rose at the same calories in the fed-every-meal study.

Questions

Can the body use a whole day of protein in one meal?
Going by the evidence in people, mostly yes. 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 extra protein was burned off. What has not been shown is that one feeding a day is as good as spread-out meals for lean mass over months. No study has tested that at one meal a day.
Why did blood pressure and cholesterol go up?
That is what was measured. People were fed all their meals, with calories kept at the level to hold their weight, and weight stayed within 2 kg. On one meal a day, blood pressure and total, LDL and HDL cholesterol all clearly rose. In that same study, body fat fell and the stress hormone cortisol fell. The study did not work out why. Both sides belong in any summary of it.
Does one meal a day worsen blood sugar?
In the study that measured it, the meal was eaten between 4pm and 8pm and calories were kept the same. Morning fasting blood sugar was higher. Blood sugar rose more and stayed up longer after a sugar drink test. Insulin was slower to respond. The problem went away afterwards. Nobody has tested whether a single morning meal works the same way.