The Science Behind OMAD and How Intermittent Fasting Works

Could you limit yourself to eating one meal a day over an extended period of time? It might sound crazy, but that’s the idea behind a diet that’s growing in popularity. It's known as OMAD, or “one meal a day,” diet. 

Proponents claim this extreme form of intermittent fasting offers benefits like weight loss and improved cognitive function. With this in mind, here’s a breakdown of how the OMAD diet works — and what science says about its weight loss potential.

OMAD stands for "One Meal a Day." It's a diet that involves a 23:1 fasting-to-eating schedule. So, someone undertaking the diet eats one large meal during a one-hour feeding window. Then they fast for 23 hours without consuming any additional calories. OMAD makes other intermittent fasting approaches look pretty wimpy in comparison.

While the fast may be brutal, many people still find a lot to like about the one-meal-a-day diet. You can eat or drink pretty much whatever you want (even pizza, burgers and beer), and as much as you want, as long as you do so during your scheduled mealtime.

The goal of intermittent fasting for weight loss is to speed up the use of stored fat for energy by restricting food intake. Many popular intermittent fasting plans rely on time-restricted eating, which involves limiting food consumption to a certain time window. 

According to Krista Varady, a nutrition researcher at the University of Illinois at Chicago, fasting forces our bodies to rely on our sugar and fat stores first for fuel. But she says intermittent fasting’s success largely relies on something that’s pretty boring and familiar for many who have tried to lose weight: calorie restriction. “I just think it’s a way to fool the body into eating less, I don’t think there’s anything magical about it,” Varady says.

Read More: 4 Science-Backed Diets to Improve Your Health

Although 23:1 fasting hasn’t been studied in humans per se, one paper published on PubMed examined a one-meal-per-day eating regime’s impact on weight and other health measures. The researchers wanted to know what would happen if people simply ate a day’s worth of calories during a time-restricted window.  

In the study, normal-weight, middle-aged men were put on a three-meal-per-day plan for eight weeks. Then they were switched to an intermittent fasting diet with a four-hour eating window for another eight weeks (with a bit of a break between the stints).

Yes, it is possible to lose weight by eating one meal a day if it results in a reduction of your overall calorie consumption. Most participants lost around 10 pounds over the course of the study. It showed that eating one meal per day promoted modest fat loss that didn’t occur with eating the standard three square meals per day, but not for the reason you might think. 

Many men simply found it difficult to consume such a large meal during their eating window — which unintentionally created a calorie deficit. Eating this way also elevated blood pressure and cholesterol among some participants.

“They were almost force-feeding them. Eating 2,000 calories, or even 1,500 calories, in one sitting is kind of tough for people,” Varady says.  Varady is in the process of publishing a new study that looks at a time-restricted eating pattern involving obese men and women. For two months, participants ate between 3 p.m. and 7 p.m. and fasted the rest of the time. 

“Time-restricted eating is amazing in that you just have to watch the clock. You just stop eating when you’re supposed to stop eating, and people just naturally calorie-restrict,” Varady says.

Intermittent fasting is generally considered healthy for many individuals, but like any dietary approach, its suitability and benefits can vary from person to person. It's important to check with your doctor before starting especially if you have underlying health conditions. When done safely, the study showed that adhering to a feeding window not only contributed to weight loss, but also reductions in blood pressure, insulin, insulin resistance, and oxidative stress.

No, a common misconception around intermittent fasting is that it promotes autophagy, or “self-eating,” on a cellular level. The idea behind autophagy is that, when you fast, your body can spend time cleaning up damaged cells because it isn’t busy dealing with a constant influx of food. But this process has never been observed in humans, and scientists don’t even have a way of measuring it in people, Varady says.

Read More: What Factors Matter Most For Weight Loss?

While eating one meal a day may help you lose weight, the OMAD diet raises concerns about nutritional balance. People may want to be extra cautious when it comes to following OMAD's more extreme 23:1 feeding schedule. Having only one hour to eat every 24 hours might make nutritional and caloric deficiencies more likely. (And, by the way, adults generally aren’t supposed to regularly eat less than 1,200 calories per day.)

“If they’re not counting calories, they’re probably only eating around 1,000 calories. If it’s that low, it would be hard to get all your nutrients in. I’d almost recommend taking a supplement just to be careful,” Varady says.

Fasting, as you may have heard, can be hellish. But Varady says most people will adjust within 10 days after starting — and that the same is probably true for OMAD. “Some people find fasting difficult for a number of reasons. They report that they find it hard to concentrate, they might feel weak or irritable, they feel hungry, they get constipated and they miss out on social interactions,” wrote Amy Hutchison, an obesity researcher at the University of Adelaide in Australia, in an email to Discover.

Another caveat of OMAD fasting is that it might be unrealistic to follow long-term, Hutchinson says. Not many of us can live the rest of our lives eating one meal a day. And that’s OK. Successful weight loss really just boils down to caloric restriction. OMAD and other forms of time-restricted eating are easier for some people to stick to. But part of its allure might also be that it’s different from what has been popular in the past.

“We were so obsessed with fasting carbs for so long, and I think people were getting sick of the macronutrient-counting and buying whole other sets of groceries for their pantry,” Varady says. Fasting “is a much simpler diet that just involves meal timing, and you don’t have to buy a bunch of fancy foods for it.”

Read More:  Forget Dieting. Here's What Really Works to Lose Weight

There are several types of intermittent fasting. In addition to the time-restricted fasting of the OMAD diet, there is 5:2 fasting, overnight fasting, alternate day fasting, and Eat Stop Eat. 

This method involves eating normally for five days of the week and significantly reducing calorie intake on the other two days. On the fasting days, it's common to limit intake to about 500 to 600 calories.

Overnight fasting typically involves a fasting period of 12 to 16 hours, which includes the time spent sleeping. For instance, if you finish dinner by 8 p.m. and don't eat until 8 a.m. the next morning, you've completed a 12-hour overnight fast.

There’s also alternate day fasting, which gives people the freedom to eat whatever they want on one day, followed by a fast or calorie restriction the next day. Other forms of intermittent fasting might incorporate only one or two days per week of no eating or significant calorie restriction.

Eat Stop Eat is a form of intermittent fasting that involves one or two 24-hour fasting periods each week. During the fasting period, no food is consumed, but non-caloric beverages like water, tea, and black coffee are allowed.

Read More: Calorie-Restricted Diets May Not Live Up to the Hype

Intermittent fasting is generally considered safe for most healthy adults, but it's not suitable for everyone. The safety and benefits can vary greatly depending on individual health circumstances, lifestyle, and specific needs. According to Johns Hopkins Medicine, the following people should avoid intermittent fasting:

• Children and teens (Under 18 Years)

• Pregnant or breastfeeding women

• Individuals with Type 1 diabetes on insulin

• People with a history of eating disorders

Read More: Therapy on a Plate: How Your Diet Can Benefit Your Mental Health

While eating one meal a day can be an effective approach to weight loss and has been associated with improvements in certain health markers, it also raises significant concerns about nutritional balance and long-term sustainability. The decision to follow this diet should be made carefully, considering personal health circumstances and ideally under the guidance of a healthcare professional.

The results of intermittent fasting vary by goal: Improvements in gut health can be seen in a few days. Weight loss and metabolic benefits may take about a month, and enhancing longevity and overall health generally requires long-term commitment.

The length of intermittent fasting depends on the method used. OMAD consists of daily 23-hour fasts with a one-hour eating window. 5:2 involves normal eating for five days and calorie reduction on two non-consecutive days. Overnight Fasting typically ranges from 12 to 16 hours daily, often overnight, and Eat Stop Eat features one or two 24-hour fasts per week with normal eating on other days.

Intermittent fasting can vary but the general consensus is to stick to calorie-free drinks like water, black coffee, and herbal teas. Zero-calorie beverages are also an option, but artificial sweeteners are best avoided. Small amounts of bone broth and diluted apple cider vinegar can be consumed, mainly to manage hunger pangs and provide electrolytes. 

Our writers at Discovermagazine.com use peer-reviewed studies and high-quality sources for our articles, and our editors review for scientific accuracy and editorial standards. Review the sources used below for this article:

• University of California Davis Health. Intermittent fasting: Benefits, how it works, and is it right for you?

• The American journal of clinical nutrition. A controlled trial of reduced meal frequency without caloric restriction in healthy, normal-weight, middle-aged adults

• Nutrition researcher at the University of Illinois at Chicago. Krista Varady

• Johns Hopkins Medicine. Intermittent Fasting: What is it, and how does it work?

• Obesity researcher at the University of Adelaide in Australia. Amy Hutchison

• Healthline. Pros and Cons of 5 Intermittent Fasting Methods

• Dr. Ruscio, Adjunct Professor at the University of Bridgeport. How Long Does Intermittent Fasting Take to Work?