The pros and cons for the 5:2 diet

Eating well for lifeRestricting calories just two days a week: could this be the key to weight loss?

Words: Beth Anderson

The advent of any restrictive diet inevitably sparks debate. Atkins, Cabbage Soup, Lemon Detox, South Beach – these food regimens and others like them have had loyal followers and detractors alike.

Such is the case with the Fast Diet™: a weight-loss program that rose to prominence in the UK in 2012. At the time, the BBC broadcast a documentary, Eat, Fast and Live Longer, in which doctor and journalist Michael Mosley examined the science behind intermittent fasting. He co-authored with Mimi Spencer the first of several books on the subject, The Fast Diet, in January 2013 – topping The New York Times bestseller list a short time later.

The premise of the Fast Diet – often referred to as the 5:2 diet – is simple: participants eat ‘normally’ five days a week, then reduce their calorie intake by a quarter for the other two days (500 calories for women, 600 for men). The diet’s official website suggests that, in addition to weight loss, the program can offer a range of health benefits, including improvements to blood pressure, cholesterol levels and insulin sensitivity1. However, like any diet, there are two sides to the story.

Here, we look at some of the arguments for and against the Fast Diet.

The pros

According to the Australian Institute of Health and Welfare, obesity in Australia is getting worse, with three in five adults now considered overweight or obese2.

Kate DiPrima, spokesperson for the Dietitians Association of Australia, believes portion distortion is a major contributing factor. “There is a disparity between how much food we need and how much we’re actually eating,” she says. “In that sense, one of the few positives of the 5:2 diet is that it shows people that you can survive 24 hours on 500 calories, perhaps proving that we are all overeating.”

But is such a dramatic reduction the answer? Can people function normally – at home, at work, in the gym – on the fasting days? Obesity researcher Joseph Proietto, Professor of Medicine at the University of Melbourne and head of Austin Health’s Weight Control Clinic, says yes. He explains that intermittent fasting encourages your body to use stored energy (glycogen and fat) as its primary fuel, which can reduce your risk of chronic disease.

“The two days that you fast should not be consecutive,” he says, “so people reducing their calories shouldn’t have any trouble with performance throughout the week.” Professor Proietto adds that rapid weight loss can be motivating. “In a study we published in The Lancet Diabetes & Endocrinology3, we showed that more people reached their target weight if weight loss was fast,” he says.

Adding to these positives, DiPrima says there may be some people suited to the Fast Diet, providing it’s well balanced. “I’ve got some clients who, when faced with choices, overeat,” she says. “These kinds of people could benefit from a restrictive plan – diets like this give them structure.”

Professor Proietto agrees. “I don’t mind how people choose to reduce their energy intake,” he says. “The 5:2 diet may work for some, but not everyone.”

The cons

Despite the potential benefits, DiPrima believes the Fast Diet should be approached with caution. “I’m not a big fan of any temporary diet,” she says. “Anything that has severe swings from side to side, people are going to struggle to stick to. They’ll go back to their old eating habits in no time.”

DiPrima also stresses that the concept of eating ‘normally’ for five days of the week is open to interpretation. “Sticking to just a set amount of calories, you’re not automatically getting the right nutrient balance,” she says. “You could choose to blow the lot on a Tim Tam! And it worries me that you might end up bingeing on the regular days.”

Adding to these risks, Professor Proietto explains that, following diet-induced weight loss, people get progressively hungrier due to the body defending weight. “It does this by altering hunger-controlling hormones in the blood in a direction to make you more hungry,” he says. “In a study we published in 2011 in The New England Journal of Medicine4, we found that many of the nine hunger-removing hormones circulating in our blood decrease, while the hunger-causing hormone increases after weight loss.”

He says these changes can be long-lasting – reverting only once lost weight is regained – and that strategies to counteract this change may be needed to prevent relapse.

DiPrima adds that any dietary regimen has to be adopted into your lifestyle long term. “Get a professional to write out a meal plan,” she suggests. “If it’s too strict, they can help you adapt it to something that’s going to work for you. If it’s not monitored or managed, you can run into health problems.”

Take control of your eating

When it comes to adopting healthy eating habits, Kate DiPrima, from the Dietitians Association of Australia, recommends brushing up on the Australian Dietary Guidelines found at

“Compare the number of recommended serves on the site with what you’re actually eating,” she advises. “If you’re meeting these guidelines but still having trouble with your weight, I suggest reducing your serving sizes by about a quarter.”

Stay hydrated

Whichever diet you follow, it’s important to drink plenty of fluids. The Dietitians Association of Australia suggests 1.5–2 litres for adults and 1–1.5 litres for children is a good daily guide, with water the preferred option5.



  3. The Lancet Diabetes & Endocrinology, ‘The effect of rate of weight loss on long-term weight management: a randomised controlled trial’, October 2014
  4. The New England Journal of Medicine, ‘Long-term persistence of hormonal adaptations to weight loss’, October 2011



Information provided in this article is not medical advice and you should consult with your healthcare practitioner. Australian Unity accepts no responsibility for the accuracy of any of the opinions, advice, representations or information contained in this publication. Readers should rely on their own advice and enquiries in making decisions affecting their own health, wellbeing or interest.