5:2 dieters risk undoing their good work by overeating and under-exercising on non-fasting days
People on the 5:2 intermittent fasting diet may be over eating and reducing their physical activity in preparation for their restricted diet days, a new study suggests.
Nutrition and exercise scientists at Nottingham Trent University, who carried out the work, argue that doing this makes the diet about half as effective as it should be for producing a calorie deficit.
The 5:2 diet involves two days of heavily-restricted calories and unrestricted food intake on the other five days.
As part of the study – which also involved Loughborough University – male participants undertook two separate three-day trials, which differed only by the diets they were given on the second day.
Both trials involved participants being allowed to eat and drink freely on day one after being told that on day two they would receive either a low-calorie diet of approximately 700kcal or a normal diet of approximately 2,700kcal. On the third day they could eat an unrestricted breakfast.
Food intake was monitored on the first and third day, and participants also wore activity monitors to measure the amount of calories they burned through physical activity across each day.
The researchers found that participants increased their food intake – without reporting feeling hungrier – both the day before and the morning after their calorie intakes were restricted. They also reduced their physical activity before and during the day their intake was restricted.
The researchers, writing in the journal Appetite, say that this change in participants’ behaviour was sufficient for half of the 2,000 calories ‘saved’ from the restricted eating to be replaced. They say that the change in eating behaviour and activity from participants was largely not a conscious decision.
Few studies have looked at the role of exercise within intermittent fasting diets, and the team argues that their work shows that it should be considered more.
Mindful eating and incorporation of some exercise should take place alongside period of severe calorie restriction to help improve the effectiveness of the 5:2 diet, they say.
“Our findings reveal a passive overconsumption of food and reduced physical activity among intermittent fasters,” said lead researcher Dr Dave Clayton, a lecturer in nutrition and exercise physiology in Nottingham Trent University’s School of Science and Technology.
He said: “Suffering through a day of severe calorie restriction, where you consume only 500-700 kcal for the day, is only about half as effective as it might have been because of this. We got a snapshot as part of a three-day study, but if this behavioural pattern continued then people may not get the returns they expect from this type of diet.
“There are also potential health implications in relation to acute overeating and reduced physical activity.
“The assumption with intermittent fasting is that you fast – or severely restrict – your intake for a few days and then do what you want the rest of the time. Our study suggests intermittent fasters won’t be very effective with this mind-set.
“Another key point is the change in physical activity, particularly because participants’ reduction was mainly from already ‘low intensity’ activity, which are very subtle movements that most people may not even consider.”
Notes for editors
NTU was named University of the Year 2019 in the Guardian University Awards. The award was based on performance and improvement in the Guardian University Guide, retention of students from low-participation areas and attainment of BME students. NTU was also the Times Higher Education University of the Year 2017, and The Times and Sunday Times Modern University of the Year 2018. These awards recognise NTU for its high levels of student satisfaction, its quality of teaching, its engagement with employers, and its overall student experience. The university has been rated Gold in the Government’s Teaching Excellence Framework – the highest ranking available.
It is one of the largest UK universities. With nearly 32,000 students and more than 4,000 staff located across four campuses, the University contributes £900m to the UK economy every year. With an international student population of more than 3,000 from around 100 countries, the University prides itself on its global outlook. The university is passionate about creating opportunities and its extensive outreach programme is designed to enable NTU to be a vehicle for social mobility. NTU is among the UK’s top five recruiters of students from disadvantaged backgrounds and was awarded University of the Year in the UK Social Mobility Awards 2019.
A total of 82% of its graduates go on to graduate entry employment or graduate entry education or training within six months of leaving. Student satisfaction is high: NTU achieved an 87% satisfaction score in the 2019 National Student Survey.
- Subject area: Sciences including sport sciences
- Category: Press office; Research; School of Science and Technology