Everything You Need to Know About the OMAD Diet

Medically reviewed by Karina Tolentino, RD, CHWC

The one-meal-a-day (OMAD) diet is a form of intermittent fasting (IF) that involves eating only a single meal a day. The diet has become a weight loss trend, but eating only one meal a day (and fasting in general) may not be ideal for people with certain health conditions.

This article will cover everything you need to know before starting the OMAD diet, including how to follow the diet, possible health benefits, and risks.

How to Follow the OMAD Diet

The OMAD diet is an extreme version of intermittent fasting. You restrict your eating window to one hour per day while you fast (go without food) the remaining 23 hours of the day. For this reason, it is sometimes referred to as a 23-to-1 IF diet. Eating for an hour within the same four-hour time frame each day is recommended.

During your one-hour eating window, you are allowed to eat whatever you want. There are no restrictions on the types of foods or beverages consumed. Additionally, there is no restriction on the number of calories you should eat. However, eating as many calories as you would normally eat in a full day within your one-hour eating window is encouraged.

During your 23-hour fasting window, you are not allowed to eat anything. However, some no- or very low-calorie beverages are allowed, such as water, black coffee, and unsweetened tea.

Pros to the OMAD Diet

There is not much research specifically on the health effects of consuming only one meal per day. More research is needed to determine how eating only one meal a day might affect health, especially in the long term.

Nevertheless, there is some research on IF in general that may provide an understanding of the possible benefits of OMAD.

May Help You Burn Fat

The body’s preferred source of energy is glucose. Carbohydrates in food and beverages are broken down into glucose. In order to use glucose as energy, there has to be glucose (or carbohydrates) coming into the body.

During a 23-hour fasting window, all the readily available glucose is used up. This leads your body to turn to a different source of energy—stored fat.

A small 11-day study compared a 22-to-2 intermittent fasting diet (one meal per day in a two-hour time frame) to a regular diet of three meals per day with snacks. The researchers found that the single-meal-per-day group experienced increased fat breakdown (for energy) compared to the regular diet group.

The one-meal-a-day group also saw slightly greater weight and fat loss compared to the three-meals-per-day group.

An older but longer study from 2007 looked at the effects of consuming one meal per day for two separate eight-week periods over six months compared to three meals per day (control group). At the conclusion of the study, participants eating one meal per day had significant reductions in fat mass and body composition compared to the control group.

Improves Metabolism

Some research has shown that changing eating patterns through intermittent fasting may result in improved metabolism (the processes that create and use energy in the body).

The improved metabolism may be a result of IF inhibiting multiple pathways that promote growth and instead stimulating reactions that break down and destroy old, damaged cells, proteins, and other substances. This, in turn, improves energy production at the cellular level, as well as overall cell metabolism.

Helps You Feel More Alert

Intermittent fasting during the day has been shown to increase the amount of orexin-A, a chemical that makes you feel more alert.

Though this hasn’t been studied specifically in OMAD, choosing to consume your one meal in the evening (but not too close to bedtime) would mimic this daytime fasting. After eating your meal, orexin-A levels would decrease and may help you sleep better at night.

Can OMAD Lead to Weight Loss?

Though OMAD does not restrict the number of calories or types of food you eat, it might help with weight loss in some people. A review of six studies on IF concluded that IF was comparable (not superior) to calorie-restricted diets for short-term weight loss in adults who were overweight or had obesity.

A 2017 study with over 50,000 participants researching meal timing and frequency concluded that an 18- to 19-hour overnight fasting window and a five- to six-hour eating window with two meals (preferably breakfast and lunch) and no snacks may be beneficial for weight management.

Cons to the OMAD Diet

Difficult to Sustain

OMAD may be difficult to maintain over time. It isn’t convenient to skip several meals a day, especially during social engagements or when traveling. Due to the restrictive nature of the OMAD diet, it tends to have a high dropout rate.

Can Make You Hungrier

When you are eating one time per day and fasting the rest, your body may produce more ghrelin—a hormone that stimulates hunger.

A study involving people with type 2 diabetes compared eating six small meals daily to two large meals daily (breakfast and lunch). The group eating two meals per day had higher fasting ghrelin levels than the group eating six small meals daily.

If you are hungry throughout the day, you may be unable to concentrate, feel irritable, or develop a headache.

Not Effective at Calorie Reduction

The OMAD diet is not calorie-restricted. You are allowed to eat as much as you want during your one-hour eating window. Going so long without food each day, you might be very hungry by the time your eating window comes.

This may lead you to binge-eat whatever foods are readily available and then overeat more than your body needs in a day. Those readily available foods are often less healthy options, filled with many calories, added sugar, saturated fat, and/or sodium.

Risk of Not Getting Enough Nutrients Each Day

With such a short time frame in which to eat, you may not be able to fit in the variety of foods and nutrients your body needs each day. It can be hard enough getting a variety of foods in each day with three meals and snacks, let alone one meal.

A multivitamin and omega-3 supplement can help. However, it’s preferred to get your nutrients through foods.

Risks and Safety Concerns

The OMAD diet is not for everyone. There may be potential health risks for people with cardiovascular (heart) disease or diabetes. It also might not be a good idea for people with digestive conditions, those under age 18, people with a history of eating disorders, or who are pregnant or nursing to follow the OMAD diet.

Mixed Results on Blood Pressure and Cholesterol

The 2007 study looked at the effects of consuming one meal per compared to three meals per day in healthy individuals.

While the study showed some positive effects on health, such as decreased fat mass, it also showed some negative health effects. In particular, significant increases in blood pressure and in total cholesterol, low-density lipoprotein (LDL, considered bad) cholesterol, and high-density lipoprotein (HDL, considered good) cholesterol levels were seen.

However, further studies have shown conflicting results with respect to IF.

One study looked at blood pressure and blood pressure variability in people with newly diagnosed hypertension (high blood pressure) or prehypertension (the stage before high blood pressure) before and during IF. The study concluded that IF either significantly decreased or had no significant change in blood pressure at different times of the day.

However, the researchers saw an increase in blood pressure variability during IF, especially in people who woke early to eat before sunrise. Higher blood pressure variability may increase the risk for coronary artery disease, stroke, and cardiovascular death.

Another study in men with prediabetes (the stage before type 2 diabetes) found that IF improved blood pressure, oxidative stress (not having enough antioxidants in the body to get rid of unstable molecules called free radicals), and insulin sensitivity (when cells effectively use glucose to reduce blood sugar). A 2022 review of studies concluded that IF may improve both blood pressure and cholesterol levels.

If you are at risk for or have high blood pressure or high cholesterol, talk with a healthcare provider before starting a new diet, including IF or the OMAD diet.

May Cause Blood Sugar to Spike and/or Crash

Eating an entire day's worth of food all within one meal in an hour can cause blood sugar (glucose) levels to rise significantly after eating. Following that blood sugar spike with fasting for 23 hours can then lead blood sugar levels to drop and crash or go too low

Low blood sugar levels (hypoglycemia) appear to be the most severe acute (immediate, short-term) side effects of IF.  One study investigated IF in people with diabetes during the month of Ramadan, the ninth month of the Islamic calendar in which followers fast. Researchers identified hypoglycemia events during periods of IF, particularly in people who also used blood glucose–lowering medications, such as insulin.

An older (2007) randomized controlled trial found that people who ate one meal per day had higher morning fasting blood sugar levels,  as well as greater and more sustained rises in blood glucose levels compared with people who ate three meals per day. People who ate one meal per day also experienced a delayed insulin response during an oral glucose tolerance test.

However, despite these risks, some studies show IF being beneficial in regard to blood glucose levels. A randomized controlled trial found that IF resulted in significant reductions in blood glucose.

Another study in men participating in IF during Ramadan found that IF had no significant change in fasting blood glucose levels but did improve insulin sensitivity. A review and meta-analysis of studies concluded that IF may improve blood sugar levels in people with impaired blood sugar metabolism.

Always talk with a healthcare provider before starting a new diet, especially if you are at risk for or have a chronic condition such as diabetes.

Summary

OMAD (one meal a day) is a severe form of intermittent fasting (IF) where you fast for 23 hours a day and are allowed to eat whatever and however much you want during a one-hour time frame. There is not a lot of quality research on the OMAD diet specifically. However, there is much research on the health effects of IF in general.

IF may help you burn fat, be more alert, improve metabolism, and aid in weight loss or weight management. Results are mixed on whether IF is beneficial for blood pressure or cholesterol.

IF may pose a risk to people with diabetes, especially those who take insulin or other glucose-lowering medications. People with other health conditions may be at risk when trying IF and OMAD as well, including those with digestive conditions, a history of eating disorders, or who are pregnant or nursing.