It's known by lots of names but the 5:2 diet is hailed by many as the easiest way to lose weight and keep it off.

Based on the idea of intermittent fasting, the 5:2 diet has remained consistently popular since it was first introduced in the United Kingdom during the 2010s.

What is the 5:2 diet?

The 5:2 diet (or fast diet as it is less commonly known) is based on the concept of intermittent fasting and is, according to nutritionist Rick Hay, simple to follow.

"It's just five days of the week where you eat normally — but when I say normally that doesn't mean pizza and coke," the author of The Anti Ageing Food & Fitness Plan told news.com.au.

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"Then the other two days, which don't have to be in a row, you are restricted to five, six, seven hundred calories, depending on whose regimen you're following."

READ MORE: • Live Well Road Test: Does intermittent fasting actually work?

Dr Michael Mosley first introduced the idea of eating a restrictive diet two days of the week on a 2012 episode of BBC science show Horizon, touting it as a "surprisingly easy" way to incorporate intermittent fasting into your life.

The Fast Diet by Dr Michael Mosley. Photo / Supplied
The Fast Diet by Dr Michael Mosley. Photo / Supplied

Expanding on his work on intermittent fasting presented on Horizon, Dr Mosley published the book "The Fast Diet" in January 2013, according to BBC Good Food.

Just a month later journalist Kate Harrison published "The 5:2 Diet Book" and since then the diet has been one of the most popular choices for people trying to lost weight.

In both Dr Mosley and Harrison's versions, a person eats normally for five days and then restricts their calorie intake for two days, either 600 for men or 500 for women.

On calorie-restriction days most people either eat two meals instead of three or shorten the time window in which they eat, hence the link to intermittent fasting.

5:2 diet benefits

One of the biggest benefits of the 5:2 diet is that it is easy to follow and is suitable for most adults to do, Hay said.

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"I think this is a really good one for women and men in their 40s who have maybe tried a lot of diets and haven't tried this one … that's my go-to category, but really it is a pretty across-the-board diet, so I would be happy for anyone over 18 to try it," he said.

"If you're not really into dieting and counting calories seven days a week this is a good one to do because you only have to count them twice an week … it's a good one for men too. Men don't like to bother much about diets, they just want it to be easy."

Hay said the 5:2 diet or some other form of intermittent fasting was also a good choice for people looking to lose weight or maybe sustain weight loss over a long term period.

Dr Michael Mosley came up with the diet in 2012. Photo / Supplied
Dr Michael Mosley came up with the diet in 2012. Photo / Supplied

"I think it's very important to mix it up, however. The 5:2 would probably be one of the easiest to sustain long term if you continue to get resolve," he said.

"It's a good way once you've lost the weight to keep it off. There's even the alternate-day fasting where people are doing four, three, one (days of restrictive calories) and all that, and they seem to be getting results as well. It is one of my favourites because it's pretty simple."

When it comes to health benefits, Mr Hay said there were studies that suggested intermittent fasting such as what was done in the 5:2 diet was beneficial to helping reduce insulin levels and improve cardiovascular and cholesterol health.

However, a 2017 study published in the Journal of the American Medical Association found that intermittent fasting diets such as the 5:2 didn't help people lose more weight compared with those who restricted calories daily.

The 5:2 diet has spawned many meal plans and recipe books since its inception. Photo / Supplied
The 5:2 diet has spawned many meal plans and recipe books since its inception. Photo / Supplied

5:2 diet downsides

Following the 5:2 diet requires a lot of planning for fasting days, and those in high-stressed jobs should be wary of not having fasting fall on busy days, Mr Hay warned.

"If your work is very fast paced, be careful on those two days because if you aren't eating properly wholesome wholegrains and really healthy food on those two days to nourish your brain, I've found people can get very spacey, very dizzy, very ditzy on it, so I would have that caution in there as well," he said.

People also should be conscious of not just counting calories and need to still make sure they are eating "colourful" meals with lots of vegetables.

"It's variety and not just focus on empty calories because you can have a croissant for breakfast and that's 120 calories or 150, or your choice might have been a green smoothie with plant-based protein powder in it," Mr Hay said. "I think people need to chose those calories wisely on the two days."

As with most diets that involve calorie restrictions it probably isn't suitable for pregnant or breastfeeding women as well as children.

"I think you've got to be careful if you're pregnant, it wouldn't be something I was doing if you're breastfeeding," Mr Hay said.

The 5:2 Diet Book by Kate Harrison. Photo / Supplied
The 5:2 Diet Book by Kate Harrison. Photo / Supplied

"I wouldn't be doing it with teenagers and children unless you were monitoring them and probably as well if you're already underweight that's probably not the one for you."

Most importantly, Hay said to eat what you want on the five unrestricted days within reason.