The OMAD Diet: What 14 Studies Say about Eating Once a Day

You’ve read a few posts and articles from people on the One Meal a Day diet (OMAD) who say they’ve lost weight by eating once a day.

It all sounds very simple and easy, and you’re thinking about giving it a try.

Does eating once a day get results? Can you really pig out every night and still lose weight? Or is the OMAD diet just going to mess up your metabolism and wreak havoc with your hormones?

What Is the One Meal a Day Diet?

The first time I across the idea of eating one meal a day was in a book called The Warrior Diet, which I talk more about here. The idea behind it is very simple. You eat one big meal whenever you want, although most people prefer to do so at night. That meal is where you get most of your calories for the day.

Because all your food for the day is squeezed into one meal, you don’t have to exercise much restraint. On the OMAD diet you get to eat, within reason, whatever you like. As a result, you finish that meal feeling full and satisfied.

During the rest of the day, your calorie intake is minimal. You’re allowed some light snacks, maybe a protein shake after you’ve been to the gym. But that’s about it.

The Benefits of Eating Once a Day

In theory, the benefits of eating once a day sound great.

On the OMAD diet, you don’t need to count calories, cut out entire food groups, give up your favorite meals or stick to a set of complicated diet rules. You’ll also spend a lot less time cooking and preparing food, not to mention clearing up afterwards.

Some people like eating once a day for reasons that have nothing to do with weight loss. They say that fasting gives them more energy and focus. And they don’t have to fight the urge to fall asleep at their desk in the middle of the afternoon.

What does the science say about eating once a day? Is fasting all day and then eating one big meal at night going to help you lose weight? Or is the OMAD diet just another fad that will leave you tired, irritable and hungry?

SEE ALSO: Should You Eat Carbs and Fat Together?

The One Meal a Day Diet: What the Science Says

To lose fat, the only thing you need is a calorie deficit. And eating once a day is a very simple way to create that deficit. That simplicity gives the OMAD diet a big advantage over most other diets.

The calories you normally have during the day are eliminated from your diet. This means that your calorie budget can be “spent” on one big meal, rather than multiple smaller ones.

As a result, that meal can be larger and more satisfying than the sort of thing you’d normally eat on a diet.

For example, let’s say you can lose fat on 2000 calories per day.

Diet one has you divide those calories across three meals, while the second diet involves eating the same number of calories in one big meal at night. On paper, both diets will lead to the same amount of fat being lost.

Eating most of your calories later in the day won’t put the brakes on fat loss, as was once believed [1]. Nor does eating little and often speed up your metabolism, increase fat burning, suppress your appetite or help you lose weight more quickly [2, 3, 4].

Is It Good to Eat One Meal A Day?

In and of itself, eating a one meal a day diet isn’t bad for you. It’s not going to make you fat, shut down your metabolism or wreck your hormones.

However, there are some downsides to consider.

For one, eating one meal a day can leave you feeling like crap. I can usually make it through until the early afternoon without eating. After that, I start to get hungry, tired and irritable. For me, the idea of the OMAD diet isn’t a pleasant one.

Eating once rather than three times a day has also been shown to raise levels of ghrelin – known as the hunger hormone – as well as increasing both hunger and the desire to eat [5, 6].

On an OMAD diet, some people will be so hungry by the evening that they end up eating more overall calories for the day. Rebound binging on junk food, which can happen when you’re tired and hungry, can easily leave you in a calorie surplus for the day, rather than the deficit required to lose fat.

Of course, this isn’t going to be the case for everyone. Google around, and you’ll find plenty of people who have successfully lost weight on the OMAD diet. It can be done.

SEE ALSO: The Metabolic Confusion Diet: Why You Can’t Outsmart Your Metabolism

Time-Restricted Feeding and Weight Loss

You don’t need to cut down to one meal a day to get the benefits of fasting. Studies show that simply limiting the period of time in which you consume food can have very similar benefits.

What exactly do I mean by this?

Let’s say your first meal of the day is at 7am, and your last meal of the day is at 9pm. In this case, your feeding window lasts 14 hours. With time restricted feeding, you reduce the length of the daily feeding window so that it lasts somewhere between 4-10 hours.

Tell someone to reduce the size of their feeding window, with very few rules or limits on what they can eat during that window, and they will often lose weight.

In one study, a group of overweight and obese men and women were told to eat all their food for the day within a 10-hour eating window [7]. No other dietary advice was given.

After 16 weeks, subjects reduced their daily feeding window by an average of 4 hours and 35 minutes. As well as losing a little over seven pounds (3.3 kg) in weight, they also felt more energetic and slept better.

In another study, a group of obese participants with type II diabetes were told to fast for 18-20 hours each day [8].

During the feeding window, participants could eat whatever they chose, but were encouraged to include at least 1/3 plate of protein to help fill them up.

After two weeks of fasting, the average amount of weight lost was three pounds (1.4 kg). What’s more, 6 out of 10 participants said they would continue with fasting after the study was over, in a full or modified capacity.

Dodging the Mid-Afternoon Slump

One of the other claims made about eating once a day is that it boosts your energy levels and makes you more productive. Fasting is supposed to give you an “amazing surge of energy that lasts the entire day.”

Back in the day when I used to work in an office, I would always get very tired after lunch. My energy levels would take a dive, my productivity went down the drain and I was practically falling asleep at my desk.

In fact, there were numerous occasions where I did end up nodding off.

However, not everyone is affected to the same extent. Some people say they don’t feel the dip at all. Others will nip off to their parked cars and have a quick snooze, simply because they can’t fight off the urge to close their eyes.

What’s more, getting tired in the afternoon appears to be part of your natural circadian rhythm, and can occur even when you don’t eat lunch.

From a paper published in the journal Clinics in Sports Medicine [14]:

“Although it does not occur in all individuals and all measures, the notion of a post-lunch dip in performance is a real phenomenon that can occur even when the individual has had no lunch and is unaware of time of day. This phenomenon is related to an innate human propensity for sleep during the early afternoon hours.

“The post-lunch dip has its roots in our biology, and may be linked to the size of the 12-hour harmonic in our circadian rhythms. It is certainly exacerbated by a high-carbohydrate lunch, and may be more likely to occur in extreme morning-type individuals.”

In other words, a heavy lunch may make the post-lunch dip worse. But, that doesn’t mean fasting is the only way to avoid it – a light protein-rich lunch may do the job just as well.

That said, there is some research out there to show that fasting can increase your energy levels and make you more alert.

In one of the trials I mentioned earlier, subjects who were restricted to a 10-hour feeding window did report feeling more energetic [7]. In another, fasting between 3am and 6.30pm led to an increase in levels of orexin-A, a neurotransmitter that promotes wakefulness [14].

Eating Once a Day and Muscle Growth

While eating once a day can work just fine for weight loss, it’s less than ideal when it comes to muscle growth.

Some folks need to eat a lot if they want to gain muscle. If you’re supposed to be eating upwards of 4000 calories per day, it’s difficult to squeeze all those calories into a single meal. Unless you have a huge appetite, you’re better off getting those calories from 4-6 smaller meals.

One of the other downsides with the OMAD diet, from a muscle-building point of view at least, is the low protein frequency.

There is research to suggest that spreading your protein intake evenly throughout the day does a better job at increasing muscle protein synthesis – a key driving force behind muscle growth – compared to the same amount of protein squeezed into a smaller number of larger meals [9, 10].

In other words, meal frequency might not have much of an impact on your rate of weight loss. But protein frequency may very well affect your ability to gain muscle while you drop fat.

Granted, this is still a controversial subject.

There are some studies to show that skewing your intake of protein towards dinner works just as well at preserving muscle mass during weight loss as distributing it equally across three main meals [11, 12].

However, other research shows that eating more frequently (6 versus 2 meals per day) means that you’re more likely to hold on to your muscle while you lose weight [13].

All things considered, if gaining or retaining muscle mass is a priority, I think you’re better off spreading your daily protein intake across the day, rather than getting it in one huge meal in the evening.

SEE ALSO: How Much Protein to Gain Muscle: A No-Nonsense Look at the Science

Eating Once a Day and Exercise

You’ve decided that you want to go on the OMAD diet, fast all day and eat one big meal at night. But, you still want to go to the gym 3-4 times a week.

Does eating once a day mean that exercising is out of the question? Or can you do both and still see good results?

Let’s say that you go to the gym first thing in the morning and lift weights for 30-40 minutes. If you eat one big meal at night, finishing your workout at 8am means that you’ve still got another 10 hours or so before it’s time to eat.

That’s far from ideal if building muscle is your main goal.

To maximize the muscle-building response to a bout of resistance training, it’s a good idea to eat 20-40 grams of protein an hour or so before and/or after you train. That protein can be in the form of regular food or a protein supplement. Some whey protein mixed with water will do the job just fine.

The same rule applies if you train at lunch time. If you’ve been fasting since your evening meal the night before, you won’t have eaten anything for 15 hours or so. And you still have 5 hours or so before it’s time to eat. Again, if your goal is to gain muscle as fast as humanly possible, this is far from optimal.

The post-exercise “window of opportunity” does stay open a lot longer than was once believed [14]. But you still don’t want to go for such a long period of time without eating any protein. Doing so will almost certainly slow the rate at which muscle is gained.

If you’re fasting all day and training hard in the evening, the chances are very high that your performance in that workout is not going to be all that it could be. You’ll feel (and perform) a lot better if you eat something before you train.

SEE ALSO: Fasted Weight Training: The 6 Questions Everyone’s Asking Google

The OMAD Diet: Final Thoughts

If you’re trying to lose fat, you can do so by eating once a day. When it comes to fat loss, what counts is your total calorie intake for the day, rather than when or how often those calories are eaten.

Plenty of people have lost weight by fasting all day and eating one big meal at night, and there’s no good reason why you can’t do the same.

However, the OMAD diet isn’t for everyone, and it’s certainly not for me. Simply reducing the size of your feeding window offers many of the same benefits of eating one meal a day, but without some of the downsides.