Wed Oct 11

What is the OMAD diet? Is one meal a day actually good for weight loss? And is it safe?

Written by: Nick Fuller, Charles Perkins Centre Research Program Leader, University of Sydney

What do British Prime Minister Rishi Sunak ^[1] and singer Bruce Springsteen ^[2] have in common?

They’re among an ever-growing group of public figures touting the benefits of eating just one meal a day.

As a result, the one meal a day (OMAD) diet is the latest attention-grabbing weight loss trend. Advocates claim it leads to fast, long-term weight loss success and better health, including delaying the ageing process.

Like most weight-loss programs, the OMAD diet makes big and bold promises. Here’s what you need to know about eating one meal a day and what it means for weight loss.

The OMAD diet explained

Essentially, the OMAD diet is a type of intermittent fasting, where you fast for 23 hours and consume all your daily calories in one meal eaten within one hour.

The OMAD diet rules are presented as simple and easy to follow:

You can eat whatever you want, provided it fits on a standard dinner plate, with no calorie restrictions or nutritional guidelines to follow.
You can drink calorie-free drinks throughout the day (water, black tea and coffee).
You must follow a consistent meal schedule, eating your one meal around the same time each day.

Plate of chicken and veggies, next to a cup of dried fruit

The one meal a day diet significantly restricts your calorie intake. Ella Olsson/Unsplash ^[4]

Along with creating a calorie deficit, resulting in weight loss, advocates believe the OMAD diet’s extended fasting period leads to physiological changes ^[5] in the body that promote better health, including boosting your metabolism by triggering a process called ketosis, where your body burns stored fat for energy instead of glucose.

What does the evidence say?

Unfortunately, research into the OMAD diet is limited. Most studies have examined its impact on animals ^[6], and the primary study ^[7] with humans involved 11 lean, young people following the OMAD diet for a mere 11 days.

Claims about the OMAD diet typically rely on research into intermittent fasting, rather than on the OMAD diet itself. There is evidence ^[8] backing the efficacy of intermittent fasting to achieve weight loss. However, most studies ^[9] have focused on short-term results only, typically considering the results achieved across 12 weeks or less.

One longer-term study from 2022 ^[11] randomly assigned 139 patients with obesity to either a calorie-restricted diet with time-restricted eating between 8am and 4pm daily, or to a diet with daily calorie restriction alone for 12 months.

After 12 months, both groups had lost around the same weight and experienced similar changes in body fat, blood sugar, cholesterol and blood pressure. This indicates long-term weight loss achieved with intermittent fasting is not superior and on a par with that achieved by traditional dieting approaches (daily calorie restriction).

So what are the problems with the OMAD diet?

1. It can cause nutritional deficiencies and health issues.

The OMAD diet’s lack of nutritional guidance on what to eat for that one meal a day raises many red flags.

The meals we eat every day should include a source of protein balanced with wholegrain carbs, vegetables, fruits, protein and good fats to support optimum health, disease prevention and weight management ^[12].

We’re likely to miss out on key nutrients if we eat one meal a day. Kampus Production/Pexels ^[13]

Not eating a balanced diet will result in nutritional deficiencies that can result in poor immune function, fatigue and a decrease in bone density, leading to osteoporosis.

Fasting for 23 hours a day is also likely to lead to extreme feelings of hunger and uncontrollable cravings, which may mean you consistently eat foods that are not good for you when it’s time to eat.

2. It’s unlikely to be sustainable.

You might be able to stick with the OMAD diet initially, but it will wear thin over time.

Extreme diets – especially ones prescribing extended periods of fasting – aren’t enjoyable, leading to feelings of deprivation and social isolation during meal times. It’s hard enough to refuse a piece of office birthday cake at the best of times, imagine how this would feel when you haven’t eaten for 23 hours!

Restrictive eating can also lead to an unhealthy relationship with food, making it even harder to achieve and maintain a healthy weight.

3. Quick fixes don’t work.

Like other popular intermittent fasting methods, the OMAD diet appeals because it’s easy to digest, and the results appear fast.

But the OMAD diet is just another fancy way of cutting calories to achieve a quick drop on the scales.

As your weight falls, things will quickly go downhill when your body activates its defence mechanisms ^[14] to defend your weight loss. In fact, it will regain weight – a response that stems from our hunter-gatherer ancestors’ need to survive periods of deprivation when food was scarce.

The bottom line

Despite the hype, the OMAD diet is unsustainable, and it doesn’t result in better weight-loss outcomes than its predecessors. Our old habits creep back in and we find ourselves fighting a cascade of physiological changes to ensure we regain the weight we lost.

Successfully losing weight long-term comes down to:

losing weight in small manageable chunks you can sustain, specifically periods of weight loss, followed by periods of weight maintenance, and so on, until you achieve your goal weight
making gradual changes to your lifestyle to ensure you form habits that last a lifetime.

At the Boden Group, Charles Perkins Centre, we are studying the science of obesity and running clinical trials for weight loss. You can register here ^[16] to express your interest.