What eating one meal a day does to your body – and what results can you see after a month?

It is possible to exercise on the OMAD diet, “but I absolutely wouldn’t recommend any strenuous exercise like strength training, as you will put strain on your muscles and not be able to recover properly afterwards, meaning you’ll do more damage than good,” says Morgan. Gentle exercise like walking, swimming, yoga and Pilates are all safe to carry out.

While Morgan recommends eating your meal after any exercise, “to refuel and nourish,” Hope believes it would be better eaten before any exercise “to give you the energy you need”. In either case, exercising will be tricky, “as you’ll expend more energy, meaning that you really need more food than you otherwise would have,” Hope says. “This is one reason why I don’t recommend this diet – it can inhibit the other factors that make up a healthy lifestyle”.

“The amount of weight you lose on this diet depends on the weight you’re starting at, and how much you’re eating in each of your meals,” says Hope. Though there is some evidence to support fasting as a more impactful weight loss method, “you’re unlikely to lose any weight if you eat a meal that contains the amount of calories that you need to eat each day to maintain your weight”.

Losing a kilogram requires a deficit of 7,700 calories. To lose this much weight over the course of a week would mean eating 1,100 fewer calories than you need each day. The average British woman – at 11st 6lbs and 5ft 4in – could therefore expect to lose half a kilogram each week by eating one meal a day, containing no less than 1,200 calories. The average British man, meanwhile, weighing 13.5st and being 5ft 9in tall, might expect to lose around a kilogram.

Over the course of a month, this means that the average woman could lose 2kg, while the average man might lose 4kg. After three months, each might have lost 6kg and 12kg respectively.

A smaller meal would result in more rapid weight loss, but with the safe rate of weight loss being 0.5-1kg per week, according to the NHS, “it’s important not to cut your calories dramatically,” says Hope.

While fasting can prompt your body to burn fat, eating in such a restricted way can also lead to lean muscle mass loss. This might not affect someone who is overweight to begin with, but someone who eats one meal a day at a healthy weight could notice loss of muscle mass. Research into the likelihood of this while doing the OMAD diet is lacking, but one study has found that several months of intermittent fasting can cause people to lose 1-2kg of muscle.

It might not sound like much, but this kind of change in your body composition can lead to a less toned look, even after losing fat. Eating an extremely small meal once a day would likely lead to more muscle loss than this.

Some of the benefits of the OMAD diet include:

Increased fat loss
Better focus
Increased energy
More time
Improved gut health

“When you fast for between 12 and 17 hours, the body starts burning fat as fuel rather than sugar. After the 17-hour mark, your body goes into autophagy, a process in which old cells get ‘cleaned out’ the body and new cells are produced,” says Morgan. This can make OMAD more powerful than intermittent fasting, which has longer eating windows of six to eight hours and typically involves skipping breakfast and eating your last meal of the day before 8pm.

Then there are the impacts of fasting on our brains. “Lots of people report that they can think more clearly and more efficiently while fasting,” Morgan says, an experience also well-represented in scientific studies. Some research indicates that fasting improves cognitive function, boosts neuroplasticity (the brain’s ability to adapt) and even protects the brain against injury and disease.

“Fasting can also improve your gut health, as it gives your gut time to rest and repair. When your gut lining is compromised this can lead to bloating, gas, burping and irregular bowel movements,” Morgan adds. “I often recommend eating one meal a day a few times a month to clients who have gastrointestinal problems, as it can help to relieve pain and bloating and promote healing.”

There are some people who benefit more from the OMAD diet than others. “Middle-aged men might find this diet helpful, as they can often struggle the most to lose weight,” says Hope.

Some people who are put on highly restricted diets by their doctors might prefer to eat only once a day. “Patients who are prediabetic or in the early stages of diabetes might eat once a day, because doing so reduces the frequency of insulin spikes, making their blood glucose levels easier to keep under control, although these individuals should always seek personalised professional advice when changing their diet,” explains Hope.

These are some of the risks associated with the OMAD diet:

Nutrient deficiency
Loss of lean muscle mass
Fatigue
Disordered eating
Change in metabolism

“If you’re eating just once a day, it’s very difficult to consume all the nutrients you need,” says Hope. “This can lead to nutrient deficiencies and symptoms like muscle weakness, irritability, brittle nails and hair and fatigue.”

While fasting can promote rapid fat loss, it can also lead to the loss of lean muscle mass if carried out too frequently. This can have a number of detrimental effects on your health as “your muscle mass is really the metabolic organ for your whole body, and dictates a lot of your hormonal activity,” Morgan explains. Muscle mass naturally decreases with age, making this particularly troublesome for older people. A lack of muscle mass causes frailty, falls and increased risk of an earlier death.

“Fasting for long periods can also be very disruptive to women’s endocrine systems, which can increase stress and actually lead to weight gain in the long term,” says Hope. “For this reason I’d be especially cautious about recommending the OMAD diet to perimenopausal or menopausal women.”

What’s more, “your body will adapt to the plan over time and start to conserve more energy, meaning that you likely won’t lose as much weight and you might find it hard to lose weight in the future, as your metabolism won’t work in the same way,” Hope says.

One study, involving 32 people split into two groups, found that the metabolisms of those who ate 1,114 calories per day slowed over twice as much as those in the other group, who ate 1,462 calories each day – yet both experienced similar weight loss. Such an effect “is one reason why I suggest that people “cycle” on the OMAD diet, following it for only a few days a month,” says Morgan.

“If you’re going to eat one meal a day, at least a quarter of your plate should be made up of lean protein like chicken, salmon or tofu,” says Hope. “Another quarter should be complex carbohydrates such as quinoa, sweet potatoes or brown rice. A quarter should be non-starchy vegetables like broccoli or cauliflower, and the last quarter should be a healthy fat, like nut butter, avocado, hummus or cream cheese. Focusing on whole foods is crucial, as this will give you the highest possible amount of nutrients.”