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People diet for many reasons, including to lose weight, gain muscle or improve sporting performance. The requirements of these diets can be worlds apart, but there’s one thing they often share: ‘cheat days’.

A cheat day is a scheduled break in a diet. The concept emerged around the same time as ‘clean eating’, and is based on the idea that a dieter can ‘cheat’ for one day a week as long as they eat to their diet plan for the remaining six days. Social media has increased awareness of these days off, notably among those dieting to gain muscle.

Instagram heavy-weight and former wrestler Dwayne ‘the Rock’ Johnson is well known for his cheat day posts. Once a week, he appears to eat whatever he wants – usually a lot of it – and he is not alone.

What is a cheat day?

published in the International Journal of Eating Disorders analysed a sample of 600 Instagram images from 1.6 million tagged with #cheatmeal. More than half of these contained “very large quantities of calorie-dense food”, providing an insight into the types of meals considered a cheat. Hamburgers, chips, pizzas and ice cream were commonly featured. The study also found #cheatmeal is often featured alongside muscular bodies.

Two main motivations for #cheatmeals are identified. First is the theory that cheat days boost your metabolism, causing you to burn more calories. When you restrict your calorie intake, your body eventually adapts and resets your metabolism to your new lower intake, but the argument goes that cheat days reduce or prevent this. Second is the theory that cheat days help you stick to your diet. Your levels of leptin (the hormone responsible for suppressing feelings of hunger) fall when you diet, and this can make it harder to resist eating. The theory goes that cheat days help keep your leptin levels up. Both these ideas suggest that cheat meals should be planned into a diet rather than being a spur-of-the-moment thing.

Will a cheat day boost your metabolism?

So could cheat days be good for your metablism? “There is no rigorous scientific research to support this”, says nutritionist Fiona Hunter.

“From a physiological perspective, the argument one needs a cheat day is faulty”, says personal trainer Scott Laidler. “Someone who is following a meal plan or getting in shape for the first time shouldn’t be scheduling in huge meals.”

Big cheat meals may not be a big deal for the Rock, as he is an ex-wrestler who has built up a diet and exercise routine over decades and is already in the shape he wants to be in. But that does not mean they’re not a big deal for the rest of us.

Can a cheat day help you stick to your diet?

A cheat meal can be more of a psychological tool than a physical one. “Put simply, it’s something to look forward to”, says Laidler.

Can cheat days actually keep your levels of leptin up and therefore suppress your feelings of hunger? “Whether leptin has a role to play in controlling body weight is still a subject of debate among scientists, and the evidence that leptin actually increases after a cheat day is inconclusive”, says Hunter.

Wanting a cheat day can also be a “reflection of a very restrictive and unenjoyable diet”, says Emma Randall, a mindful eating consultant. The more you restrict some foods, the more you’ll think about them. “Will-power is a bit like a phone battery; it tends to run out by the end of the day”, she says. Randall suggests a moderate weight-loss diet that doesn’t leave you craving cheat days is likely to be more beneficial for physical and mental health than a very restrictive diet with cheat days.

To cheat or to treat?

Sports and eating disorder dietitian Renee McGregor says some cheat day enthusiasts undo the progress they have made by tucking into a cheat meal, while others feel anxious about their cheat meal and analyse it to “get it right”. But she adds that referring to any foods as a ‘cheat’ may cause anxiety, because of its negative connotations.

One version of the ‘cheat day’ is the ‘treat day’, which sounds more positive, but it still alludes to some foods being good and others bad. “No one food is bad, it is how much and how often you eat it that counts”, says Randall.

“Some foods are more nutritionally dense than others”, says McGregor, but food isn’t just good or bad, and needs to be “repackaged” to add other elements, such as enjoyment.

Should you try a cheat day?

A suggests the most effective strategy for long-term weight-loss and heart health is a healthy dietary pattern that’s compatible with what you like to eat and your lifestyle. It’s about finding what works for you. Hunter agrees: “When I’m asked, ‘What’s the best type of diet to help you lose weight’, my reply is always the same: ‘the one you can stick to’. Cheat days can have a psychological effect, and it can be positive or negative – for some people it can be easier to stick with a restricted diet if you have a cheat day to look forward.”

Laidler has seen how different plans can work for different people. “There are two types of people when it comes to dieting”, he says, “those who need to let off steam in exchange for the discipline, and those who feel a sense of achievement from adhering to a plan”. He says letting off steam through a cheat day can work, as long as your cheat diet is sensible and doesn’t include lots of high-calorie or unhealthy foods, such as those high in saturated fat, salt or sugar.

However, many people go over the top on their cheat days, which some suggest could increase the risk of eating disorders.

So what if you’re trying to build muscle rather than lose weight? “You could argue cheat days are less likely to have a negative impact for body builders, because calories aren’t such an issue, but there is no evidence that they are suited to any type of diet”, says Hunter.