23 hours daily · advanced

OMAD (one meal a day)

A single meal in roughly a one-hour window, fasting the other 23. Simple to describe and hard to meet nutritional needs on. Unusually for a protocol this widely searched, it is supported by almost no protocol-specific trial evidence.

Who should not do an OMAD fast

Anyone taking insulin or a sulfonylurea, unless supervised by the prescribing clinician. A 23-hour fast is a substantial hypoglycaemia risk on these drugs.
Anyone who is pregnant or breastfeeding.
Anyone with a current or past eating disorder. A single permitted meal per day is a restrictive rule with a high relapse risk, and it can normalise binge-restrict cycling.
Children and adolescents.
Anyone underweight, or with elevated nutritional requirements, including anyone recovering from illness or surgery.
Anyone on medication requiring food, or requiring doses spread through the day.

This is not medical advice and no clinician has reviewed it. If any of the above applies to you, speak to a doctor before fasting rather than after.

A schematic timeline of a fast from 0 to 72 hours. A tone band along the top darkens as the body moves from fed to deeply fasted. Beneath it, liver glycogen is drawn as a stipple field that is largely spent within the first 24 hours. Below that, three traces on independent scales: glucose falls then levels, insulin falls steeply and stays low, ketones rise from about 12 hours and climb through the second and third day. The pattern is well described but individuals vary widely, and the horizontal scale breaks at 24 hours. — The hours of a fast. Above, the phases as a tone that darkens as the body moves from fed to deeply fasted. In the middle, liver glycogen as stipple, largely spent inside the first day. Below, three traces on independent scales: glucose, insulin, ketones. A schematic of a well-described pattern. Individuals vary widely, and the scale breaks at 24 hours.

What is actually happening

OMAD is the extreme end of time-restricted eating: a fasting interval of roughly 23 hours, repeated daily. By the time the single meal arrives, liver glycogen is substantially drawn down and the body has shifted further toward fat oxidation than a 16:8 schedule reaches. The distinguishing problem is not metabolic but nutritional. A day of energy, protein, fibre and micronutrients has to fit into one sitting, and appetite at hour 23 does not reliably scale to that. The other distinguishing feature is the size of the single meal: a large mixed meal eaten into a deeply fasted state produces a correspondingly large glucose and insulin excursion, which is the opposite of what most people adopting OMAD believe they are buying.

What the evidence shows

OMAD has almost no protocol-specific published evidence. A PubMed title search for "one meal a day" returns a single case report of one 42-year-old patient, which is close to the entire protocol-specific record for a term searched roughly 18,100 times a month in the United States.
The evidence usually offered for OMAD is evidence for time-restricted eating in general, and in the 2025 BMJ network meta-analysis of 99 randomised trials, time-restricted eating showed no weight benefit over continuous calorie restriction.
Alternate-day fasting was the only fasting schedule in that network meta-analysis to beat continuous calorie restriction, by 1.29 kg, and OMAD is not alternate-day fasting, so that result does not transfer to it.

What the evidence does not support

There is no randomised trial evidence that OMAD specifically outperforms any other eating schedule, including simply eating less across the day. Claims that it does are extrapolated from trials of different protocols.
A single case report cannot establish benefit, harm, or how often either occurs. It is cited on this page as evidence about the state of the literature, not as evidence about the protocol.
The evidence does not support OMAD as a distinct metabolic strategy. Where benefits are observed in time-restricted eating trials, the windows studied are typically six to ten hours, not one.
Nutritional adequacy on OMAD has not been well characterised in trials, so the most likely practical risk of the protocol is also the least studied.

How it is run

One meal, usually within a window of about an hour, at a consistent time of day.
Water, black coffee and plain tea for the remaining hours.
Protein and micronutrients are the binding constraint, not calories. Meeting a day of both in one meal takes deliberate planning rather than appetite.
Most people who run OMAD long term are eating at a substantial calorie deficit whether or not they intend to. That is worth knowing rather than discovering.

What breaks an OMAD fast

The rules differ by protocol. Check any food, drink or supplement against this protocol below.

See the full index

Sources

Management of prediabetes and class III obesity in a 42-year-old male through One-Meal-a-Day · J Family Med Prim Care, 2025PMID 41403545
What it does not show: A single patient with no control and no comparison. It is cited here as evidence about the literature rather than about OMAD: a case report is close to the whole of the protocol-specific published record, which is the point being made.
Intermittent fasting strategies and their effects on body weight and other cardiometabolic risk factors: systematic review and network meta-analysis of randomised clinical trials · BMJ, 2025PMID 40533200
What it does not show: Ninety-nine trials, 6,582 adults, and the great majority ran under 24 weeks. Alternate-day fasting was the only schedule to beat continuous calorie restriction, by 1.29 kg. Among the 17 trials lasting 24 weeks or more, no diet strategy beat any other. The separations appear only in short trials. It does not establish that any fasting schedule produces better long-term outcomes than eating fewer calories on an ordinary schedule.
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 Intern Med, 2020PMID 32986097
What it does not show: Twelve weeks, 116 participants, and estimated energy intake did not differ between the groups. It shows 16:8 produced no significant weight advantage over three structured meals, and that appendicular lean mass index fell significantly further in the fasting group. It does not show that a person who uses the window to eat less will fail to lose weight, only that the window on its own did not do it.
Cardiometabolic Benefits of Intermittent Fasting · Annu Rev Nutr, 2021PMID 34633860
What it does not show: A narrative review by researchers active in the field, not a systematic synthesis. Cited for mechanism and for its own account of where the human evidence is thin.

No clinician reviews this site. Sources on this page were verified against PubMed by the author, who is not a doctor or a dietitian. What that means.

Last updated 2026-08-16.

The phases of a fast covers the physiology behind every protocol on this site, hour by hour.