Fasting for 36 hours: the experts’ view on Rishi Sunak’s weekly habit

The prospect of getting through Monday while missing out on breakfast, lunch and dinner is enough to make most people weep. But not Rishi Sunak, who does this every week while also running the country – and experts say it might bring benefits.

A source close to the prime minister told the Sunday Times that Sunak undertook a 36-hour fast at the start of every week, consuming only water, tea or black coffee from 5pm on a Sunday to 5am on Tuesday.

While the practice might sound like the type of extreme regime championed by Silicon Valley’s tech-bros, Adam Collins, associate professor of nutrition at the University of Surrey, said Sunak’s approach was, essentially, a more stringent version of the popular 5:2 diet, in which people restrict themselves to 500-600 calories twice a week.

“When you add [the two days] together, it’s pretty much 36 hours,” he said.

However, James Betts, professor of metabolic physiology at the University of Bath, stressed a restricted calorie diet did not put the body into a fasted state – unlike Sunak’s 36-hour approach.

Such abstinence means the body runs through its normal energy stores.

“In essence, what you’re doing is shifting the body from using fundamental carbohydrates for fuel towards using fat as a fuel over that 36-hour period,” Collins said.

Collins said his own research, comparing people who fasted for 36 hours with those who followed a calorie-restricted diet for the same period and those who ate normally, found evidence of a dose-response. In other words, the more stringent the fast, the greater the shift from burning carbs to using fat – although Collins added that this comes with greater temporary glucose intolerance.

A fast such as the one Sunak undertakes, he noted, could bring “metabolic flexibility” – the ability for the body to switch between fuels. That, Collins said, can lead to “metabolic resilience”, in other words a better ability to cope with the pressures of modern diet and lifestyle – be it periods of overeating, inactivity or stress.

There is also evidence fasting can trigger autophagy – a process Collins likens to spring cleaning. “Your cells start to break down cellular components and then recycle those components for other things,” he said, adding that might be help to explain some of the apparent benefits of fasting for ageing and DNA repair.

Related: Is fasting a free health fix – or is it just a fad?

However, Collins noted such effects, as well as claims that fasting can lead to a longer life, were largely based on research involving animals such as rodents, who are often subjected to the equivalent of an even longer fast.

“Whether you can get those effects with just a 36-hour fast once a week, [I’m] not sure,” he said.

Betts agreed. “There [are] a lot of proposed benefits to[running on fats]. But a lot of the research hasn’t really [been borne out in] human beings. So we don’t see dramatic health benefits, certainly in the short term,” he said.

Betts added that while fasting could help with weight loss, it could also have less palatable effects, such as losing muscle. “And you can tend to be a little physically inactive during the fast as well because you just don’t have the energy levels for that,” he said.

Related: Fasting facts: is the 5:2 diet too good to be true?

The jury is out on cognitive effects, too.

While Collins noted that shifting to burning fat produces ketones, substances similar to nail polish remover, that have been linked to increased mental acuity and a suppression of appetite, he said it was unclear whether Sunak would see such benefits.

“Even [after] 36 hours of fasting, the levels of ketones are not going to be super high,” he said.

Betts said his own research, in which people do not eat from Monday to Friday, suggested it could take several days for cognitive benefits to be felt.

“What you’re waiting for is for ketones to kick in,” he said.“Thirty-six hours [of fasting] is prolonged, but you’re just about to get to the bit where you might start to feel more alert and energised.”

Collins added that Sunak’s regime was not for everyone, noting those with disordered eating or who were pregnant should not attempt it, while people with diabetes should also take care. However, he said the temporary glucose intolerance might benefit people with pre-diabetes by increasing their insulin sensitivity.

Naveed Sattar, a professor of metabolic medicine at the University of Glasgow, also offered a word of warning.

“If people do decide to take [fasting] up, they must try not to compensate by binge-eating the rest of the week,” he said.