OMAD stands for "One Meal A Day" - exactly what it sounds like. You eat all your daily calories in a single meal, typically within a one-hour window, then fast for the remaining 23 hours. It's the most extreme form of time-restricted eating, and it's growing in popularity.
Unlike many fad diets, OMAD has some scientific rationale behind it - it's essentially a more intense version of intermittent fasting. But that doesn't mean it's suitable for everyone, or even most people.
How OMAD Works
The concept is brutally simple:
- One eating window - Typically 1 hour, same time each day
- 23-hour fast - Only water, black coffee, or plain tea allowed
- No calorie counting required - You eat until satisfied in your window
- No food restrictions - Eat whatever you want during your meal (in theory)
Most people choose dinner as their one meal, making social eating easier. Others prefer lunch to avoid going to bed on a full stomach.
The Science Behind Extended Fasting
OMAD taps into several metabolic processes that occur during extended fasts:
Insulin sensitivity improves. Extended periods without food allow insulin levels to drop significantly, potentially improving how your body handles glucose. Research on time-restricted eating shows benefits for metabolic health, though most studies use 16:8 patterns rather than OMAD specifically.
Autophagy may increase. This cellular "cleanup" process, where your body breaks down and recycles damaged cells, appears to increase during fasting periods. However, most autophagy research comes from animal studies, and the human evidence is still emerging.
Calorie reduction happens naturally. It's genuinely difficult to eat 2,000+ calories in a single sitting. Most people on OMAD eat less overall, creating a calorie deficit without explicit restriction.
Potential Benefits
- Simplicity - no meal planning or prep
- Natural calorie reduction
- Mental clarity during fasting (some report)
- Time saved on multiple meals
- May improve insulin sensitivity
- No foods are "off limits"
Significant Drawbacks
- Hunger can be intense initially
- Difficult to meet nutritional needs
- Risk of binge eating behaviours
- Energy crashes during fasting
- Poor for athletic performance
- Socially isolating
The Nutrition Challenge
Here's OMAD's biggest practical problem: getting adequate nutrition in one meal is genuinely difficult.
An active adult needs roughly 2,000-2,500 calories, 50-100g protein, 25-30g fibre, plus adequate vitamins and minerals. Eating all this in one sitting is uncomfortable at best, impossible at worst. Many OMAD practitioners end up chronically under-eating, which causes its own problems - muscle loss, fatigue, nutrient deficiencies, and metabolic adaptation.
Protein Timing Matters
Research suggests protein is used most efficiently when spread across multiple meals - roughly 25-40g per meal for muscle protein synthesis. Consuming 100g in one sitting means much of it may be oxidised for energy rather than used for muscle maintenance and repair.
Who OMAD Might Work For
Potentially Suitable
People with sedentary lifestyles who don't have high protein needs, those who genuinely feel better eating less frequently, and people who've tried other approaches and found them unsustainable. Also those using it short-term (a few weeks) rather than as a permanent lifestyle.
Should Avoid OMAD
Athletes and highly active individuals, anyone with a history of eating disorders, pregnant or breastfeeding women, people with diabetes or blood sugar regulation issues, those on medications requiring food, and anyone under 18. The risks outweigh any potential benefits for these groups.
OMAD vs Other Fasting Protocols
If the idea of fasting appeals to you, less extreme versions may offer similar benefits with fewer downsides:
- 16:8 fasting - 8-hour eating window, much more sustainable and better researched
- 5:2 diet - Normal eating 5 days, very low calories 2 days
- Alternate day fasting - Fast every other day, eat normally between
These patterns allow for adequate nutrition while still providing fasting benefits. The research base is also stronger for 16:8 specifically.
If You're Determined to Try OMAD
Despite the caveats, some people thrive on OMAD. If you want to experiment:
- Transition gradually - Start with 16:8, then 20:4, before attempting OMAD
- Make your meal count - Prioritise protein and vegetables, not empty calories
- Consider supplements - A multivitamin and electrolytes may help during the fast
- Stay hydrated - Drink plenty of water throughout the fasting period
- Listen to your body - Persistent fatigue, irritability, or poor performance are warning signs
- Set a time limit - Try it for 2-4 weeks, not indefinitely
The Bottom Line
OMAD is an extreme form of intermittent fasting that works for some people but carries significant risks around nutrition adequacy and disordered eating patterns. The simplicity is appealing, but the execution is harder than it looks. For most people, less extreme fasting patterns like 16:8 offer similar benefits with fewer downsides. If you try OMAD, approach it as an experiment rather than a permanent lifestyle - and stop if you experience persistent negative effects.
References
- Stote, K.S., et al. (2007). A controlled trial of reduced meal frequency without caloric restriction in healthy, normal-weight, middle-aged adults. American Journal of Clinical Nutrition, 85(4), 981-988. doi:10.1093/ajcn/85.4.981
- Sutton, E.F., et al. (2018). Early Time-Restricted Feeding Improves Insulin Sensitivity, Blood Pressure, and Oxidative Stress Even without Weight Loss in Men with Prediabetes. Cell Metabolism, 27(6), 1212-1221. doi:10.1016/j.cmet.2018.04.010
- Schoenfeld, B.J., et al. (2018). How much protein can the body use in a single meal for muscle-building? Journal of the International Society of Sports Nutrition, 15, 10. doi:10.1186/s12970-018-0215-1
- de Cabo, R. & Mattson, M.P. (2019). Effects of Intermittent Fasting on Health, Aging, and Disease. New England Journal of Medicine, 381(26), 2541-2551. doi:10.1056/NEJMra1905136
