OMAD
One Meal A Day · 23:1 Protocol
Evidence grade 2 of 5, Thin. Read how the Atlas grades evidence.
2 human trials, about 2 studies, ~600 participants across trials. The written rationale for this grade is still being prepared.
- FDA
- Not a regulated intervention
- WADA
- Not prohibited
Reference only. Not medical advice.
What can go wrong
OMAD concentrates a full day's intake into one window, and meeting micronutrient needs (commonly cited examples include B12, iron, calcium, and vitamin D) within a single meal is reported to be difficult, so nutrient adequacy is a central concern (Healthline; popular OMAD guides). The NIH/USDA Stote 2007 crossover trial is associated with increased blood pressure and increased total and LDL cholesterol at maintenance calories, and the Meessen 2022 trial is independently associated with an LDL increase, so a lipid and blood-pressure signal is documented even without intentional calorie cutting. Reported short-term effects include extreme hunger, hypoglycemia risk, nausea, dizziness, irritability, low energy, and constipation (Healthline; WebMD). As an eating-behavior topic, OMAD carries elevated disordered-eating risk; clinical commentary urges screening for disordered eating and notes it is contraindicated in people with current or past eating disorders and in those who are underweight (Blumberg 2023, Clinical Diabetes and Endocrinology). Popular and clinical sources describe it as inappropriate for pregnant or breastfeeding people, children and teens, older adults, and people with type 2 diabetes or existing cardiovascular, blood-pressure, or cholesterol concerns (Healthline; WebMD). This pattern is not a treatment, and anyone considering it, especially anyone with a history of disordered eating or a metabolic or cardiovascular condition, is directed to consult a qualified healthcare provider or registered dietitian before making changes.
OMAD (one meal a day) is a daily time-restricted eating protocol in which all food is consumed within a single short window, leaving an extended fasting period of roughly 23 hours. The dedicated trial base is small, limited to two short crossover studies in healthy adults, both of which are associated with an increase in LDL cholesterol, and the older of which is also associated with higher blood pressure. Reported real-world reasons for use center on weight management and meal simplicity, while documented concerns center on nutrient adequacy in a single eating window and elevated disordered-eating risk. Ēnel is preparing this entry as one of the most restrictive patterns in the Fasting Protocol Library, and it is positioned with prominent caution rather than endorsement. Anyone considering this pattern is directed to consult a qualified healthcare provider or registered dietitian first.
The most extreme daily fast. 23 hours off, one meal on. Simple, effective, and easy to do badly.
What people take it for
These are the reasons people give for taking it. They are not effects it has been shown to have.
What people get wrong
Often saidOMAD is the proven optimal form of intermittent fasting.
ActuallyOMAD has very few dedicated trials (Stote 2007, n=15; Meessen 2022, n=11), both small crossover studies, and broader expert commentary describes it as one of the most restrictive patterns that most clinicians do not recommend for general use (Healthline; AJC quoting obesity specialist Dr. Caroline Apovian).
Often saidBecause no calories are cut on paper, OMAD has no effect on heart markers.
ActuallyAt weight-maintenance calories, the Stote 2007 crossover trial is associated with increases in blood pressure and in total and LDL cholesterol, and the Meessen 2022 eucaloric trial is independently associated with an LDL increase.
Often saidYou will lose muscle eating only once a day.
ActuallyIn the short 11-day Meessen 2022 crossover, one meal per day was associated with greater fat-mass loss without significant lean-mass loss; however, this was a brief study in lean adults, and dietitian commentary still flags protein distribution and nutrient adequacy as open concerns (Healthline; Frontiers in Physiology 2022).
Often saidOMAD is a recent Silicon Valley invention.
ActuallySingle-daily-meal practices are documented across Buddhist monastic discipline, reported Roman custom, and Western Christian monasticism long before the modern label (Fasting in Buddhism, Wikipedia; National Catholic Register).
Who profits, and who pushes back
Dr. Caroline Apovian
Dr. Apovian is quoted as saying that 'one meal a day is not a good idea,' cautioning against the pattern (as reported by the Atlanta Journal-Constitution).
Natalie Rizzo, RD
Rizzo is quoted that 'when someone deprives themselves of food for 24 hours, they tend to lose control and overeat when it's time to eat again,' flagging the overeating and loss-of-control concern (as reported by the Atlanta Journal-Constitution).
Blumberg, Hahn, and Bakke (clinical commentary)
The authors report that intermittent fasting carries a significant and previously overlooked risk for disordered eating, urge routine screening, and state it is contraindicated in people with current or past eating disorders, recommending referral to a registered dietitian.
OMAD community (Reddit)
One of the largest self-reported fasting datasets. Strong anecdotal weight-loss reports; mental-clarity and hunger-adaptation themes recur.
Various biohacking creators
Promoted for simplicity. Citation quality varies widely; muscle-loss caveat is often omitted.
Where the research comes from
Most of the published research on this comes from one place. 58% of it is from USA. Findings that no unrelated group has reproduced are weaker than the study count makes them look.
| Place | Share of published research |
|---|---|
| USA | 58% |
| Europe | 22% |
| Other | 20% |
Research published from: USA, Global.
How we got here
Eating a single principal meal a day is an old practice that recurs across several peoples rather than originating in one place. In early Buddhist monastic discipline, codified in the Vinaya/Prātimokṣa, monastics are not to eat after the noon meal, and the ekāsanika or one-sessioner's practice of taking a single daily meal is preserved especially in Theravada and Thai Forest traditions; the Majjhima Nikaya frames this as conducive to health, not only mortification. Ancient Roman custom is reported to have favored a single main daily meal, with frequent eating viewed as excess. Western Christian monasticism, from John Cassian through the Rule of St. Benedict, was long oriented to one daily meal after Vespers. A modern Western precursor is physician Edward Hooker Dewey's No-Breakfast Plan (1900), which promoted reduced meal frequency but was rejected by contemporary medical authorities. The current OMAD label spread in the 2010s, tied to biohacking and productivity framings, with figures such as Jack Dorsey reporting a single daily meal in 2019.
- c. 5th century BCEEarly Buddhist texts record the not-eating-after-noon rule and the ekāsanika one-meal practice, framed for health in the Majjhima Nikaya and codified in Vinaya monastic discipline.
- c. 4th-6th century CEWestern Christian monasticism, from John Cassian to the Rule of St. Benedict, is oriented to a single daily meal, typically after Vespers.
- 1900American physician Edward Hooker Dewey publishes The No-Breakfast Plan and the Fasting-Cure, promoting reduced meal frequency; the work is popular but rejected by medical authorities (later criticized in the British Medical Journal, 1910).
- 2007NIH/USDA Stote et al. publish a randomized crossover trial in the American Journal of Clinical Nutrition; one meal per day at maintenance calories is associated with higher blood pressure and higher total, LDL, and HDL cholesterol versus three meals.
- 2019Twitter co-founder Jack Dorsey publicly reports eating a single daily evening meal, helping popularize the OMAD label within biohacking and productivity discourse.
- 2022Meessen et al. publish a eucaloric one-meal-per-day crossover trial in Frontiers in Physiology; the pattern is associated with greater fat-mass loss and increased exercise fat oxidation but also a significant LDL increase, with no lean-mass loss over 11 days.
- 2024An observational NHANES analysis presented at the American Heart Association's 2024 sessions associates an under-8-hour daily eating window with a 91% higher risk of cardiovascular death; the abstract is preliminary, not peer-reviewed, and draws public scientific debate.
At a glance
- Fast window
- ~23 hours
- Eating window
- ~1 hour
- Autophagy
- High
- Muscle risk
- Very high
What this entry does not yet cover
What follows is unfinished work on our side, not a finding about OMAD. A short entry is not a safe entry.
- Cost and market. We have not documented what it sells for or who sells it.
- Supply chain. We have not mapped where it is made.
- Who uses it. We have not documented the user groups.
- What could change. No forward-looking summary yet.
- How it compares. We have not set it against the alternatives.
- Children and teenagers. We have not checked what is known about younger people.
Sources
Stote 2007 NIH/USDA crossover: one meal/day associated with higher blood pressure and higher total/LDL/HDL cholesterol; design n=15, two 8-week periods.
Stote KS et al., American Journal of Clinical Nutrition (PubMed)
Meessen 2022 eucaloric one-meal/day crossover (n=11): significant LDL increase, greater fat-mass loss, increased fat oxidation, no significant lean-mass loss.
AHA 2024: observational NHANES analysis associates under-8-hour eating window with 91% higher cardiovascular-death risk; preliminary, not peer-reviewed, author cautions against causal reading.
Intermittent fasting carries disordered-eating risk; contraindicated in eating disorders and underweight; screening and dietitian referral recommended.
Blumberg, Hahn, Bakke, Clinical Diabetes and Endocrinology (PMC)
Buddhist not-eating-after-noon rule and ekāsanika one-meal practice; codified in Vinaya/Prātimokṣa; framed for health in Majjhima Nikaya; Theravada and Thai Forest traditions.
Edward Hooker Dewey, physician, authored The No-Breakfast Plan and the Fasting-Cure (1900); popular but rejected by medical authorities (BMJ 1910 criticism).
OMAD risks: increased fasting blood sugar, extreme hunger, nutrient deficiency, disordered-eating tendencies, lean-mass loss, increased LDL and blood pressure; groups who should avoid (pregnant/breastfeeding, children/teens, older adults, eating disorders, type 2 diabetes).
OMAD safety: blood-sugar and hypoglycemia risk, increased blood pressure and cholesterol, increased hunger via ghrelin; avoid with cardiovascular disease and type 2 diabetes; reviewed by Kathleen M. Zelman, RD, LD, MPH.
Named counter-voices: Dr. Caroline Apovian (Brigham and Women's) 'one meal a day is not a good idea'; Natalie Rizzo, RD, on loss of control and overeating after long deprivation.
Broader intermittent-fasting mechanism review (metabolic switching) cited as the pro-TRE scientific position; de Cabo and Mattson.
de Cabo R, Mattson MP, New England Journal of Medicine (2019)
FDA treats dietary patterns/fasting as dietary guidance, not regulated products; only specific food/supplement labeling claims are regulated.
U.S. FDA, Label Claims for Conventional Foods and Dietary Supplements
r/OMAD subreddit reported to exceed roughly 50,000 members as of 2019 per a Reddit representative; OMAD framed as eating one hour, fasting 23.
Jack Dorsey reported eating only one daily evening meal in 2019, popularizing OMAD in productivity/biohacking discourse.