Is a 36-hour fast really good for you? Experts lay out the benefits and the downfalls

What makes someone powerful? It might be sitting in a spacious office with a view, or having the word “director” next to your name on a business card. Perhaps it’s owning a particular sports car, or carrying a certain designer handbag. Or you could scratch all that and simply starve yourself for 36 hours. At least, that’s what Rishi Sunak and many other powerful people today would tell you.

It might sound counterintuitive – get stronger by eating less? – but fasting has been a trend among the rich and famous for some time. Jack Dorsey, former CEO of Twitter, has previously spoken about eating just one meal a day. Meanwhile, Coldplay’s Chris Martin also recently opened up about his limited diet. “I actually don’t have dinner any more,” he said on a podcast. “I stop eating at 4pm and I learned that from having lunch with Bruce Springsteen.”

Then there’s the prime minister, who is said to live on just water and black coffee from 5pm on a Sunday to 5am on a Tuesday. “It’s true, he doesn’t eat anything at all on a Monday,” a source told The Sunday Times at the weekend. “He is incredibly disciplined.” Other famous fans of fasting range from Jennifer Aniston and Kourtney Kardashian to Cameron Diaz and Elon Musk, who claimed to lose 20 pounds by following an intermittent fasting diet.

But how helpful is fasting? In addition to weight loss, benefits are said to range from blood sugar control and heart health to curbing inflammation. Sunak has previously said that he fasts at the start of every week as part of a “balanced lifestyle”.

There are several different types of fasting diets. The most popular, intermittent fasting, involves limiting the period of time across which you eat each day. So, you may try eating within an eight-hour period, say from 11am to 7pm. There’s also the 20:4 fasting diet, whereby you fast for 20 hours and eat within a four-hour window, or the alternate day fasting routine, whereby you eat for one day and then fast the following day. Or, like Sunak, you could try prolonged periods of fasting, which means abstaining from food for at least 24 hours.

“Current human studies suggest that intermittent fasting may reduce the risk for cardiovascular disease through the improvement of weight control, hypertension and insulin sensitivity,” explains leading nutritionist and bestselling author, Rhiannon Lambert.

Refraining from food for a long period of time also may trigger the body into autophagy, “where the body removes damaged cells and regenerates new, healthy cells”, says Sebnem Unluisler, Genetic Engineer at the London Regenerative Institute. “This process is believed to have potential benefits for overall cellular health.”

“While there is some data to support that intermittent fasting could be an effective weight loss tool,” she adds, “other research concluded that there are actually no significant differences in weight loss between people following their normal dietary patterns and those choosing to do intermittent fasting. So, although there is some evidence that it can be healthy, people in my field would always recommend that people eat a balanced diet and stay active to stay healthy.”

In some cases, extended periods of fasting such as Sunak’s may actually cause more harm than good, with Lambert explaining that this practice could lead to nutritional deficiencies. Generally speaking, fasting for more than 36 hours should always be done under the guidance of a medical professional. “A well-balanced diet is crucial for providing the body with vitamins, minerals, and other nutrients necessary for its proper functioning,” she says. “If fasting periods are not properly planned with the help of a healthcare professional, individuals may miss out on key nutrients like vitamins and minerals. This can result in nutritional imbalances, leading to fatigue, weakened immune function, and impaired overall health.”

While there is extensive research on intermittent fasting, very few studies have been published on 36-hour fasts, specifically. “Studies on rodents do show that intermittent fasting can improve metabolic profiles and may reduce the risk of obesity and conditions related to it,” says dietitian Fareeha Jay. “However, data from human studies is limited regarding its positive impact. As a result, the best fasting schedule cannot be determined due to a lack of data.”

Anyone with a history of disordered eating should steer clear of fasting of any kind. Imposing these sorts of restrictions on the way you eat, and refraining from food for long periods of time, may only serve to trigger old issues around your relationship with food, particularly if you find yourself unable to stick to the regime. “Those who’ve had eating disorders are at a high risk of causing further damage to themselves and should not follow any type of fasting regimen,” says Jay.

Despite what some people may claim, fasting can also slow your metabolism as your body adjusts to conserving its energy. For some, this could even lead to weight gain. “During periods of reduced calorie intake, the body may enter a state of energy conservation to preserve resources,” says Lambert. “This can result in a decrease in the number of calories burned at rest (basal metabolic rate). While this adaptive response is a survival mechanism, it can make weight maintenance or loss more challenging in the long run.”

There can also be side effects due to the lack of regular energy supplies. These can range from physical symptoms, such as fatigue, to psychological ones. “A prolonged state of fasting can affect blood sugar levels, contributing to fluctuations that may result in mood changes,” explains Lambert. “The body’s response to hunger and the deprivation of nutrients during the fasting period can also impact neurotransmitters and hormones, potentially leading to irritability, restlessness, and difficulty concentrating.”

For obvious reasons, any kind of fasting diet can also be hard to maintain, particularly alongside any kind of social schedule, which may lead to other consequences down the line. “Not being able to continue it can cause feelings of guilt and disappointment,” adds Jay. “We need to honour our hunger. Ignoring hunger cues during fasting can impact certain chemical messengers in our body related to our appetite, such as the ghrelin and the neuropeptide Y. This can lead to sensations such as feeling constantly hungry and craving carbohydrates specifically.” Again, if this leads you to break your fast unexpectedly, or overeat in the windows when you aren’t fasting, it could potentially cause unexpected weight gain.

In short, until more research is done, the jury is out on fasting. As for how long you should fast for, that depends. “Some fasting protocols are known to go on [intermittently] for seven days, but this isn’t necessary for most people,” says nutritionist at BANT, Jo Woodhurst. “It’s about finding what works for you, your goals, your lifestyle and health picture. What matters most, if you’re going to try it, is the duration of fasting that works for you.”

Of course, there are some people who should avoid fasting diets, namely pregnant women, children, and those who have a history of disordered eating. “Additionally, it is important to know that both men and women need to approach intermittent fasting differently, as woman must take a more gradual approach to fasting in order to reduce the potential risk of negative side effects such as their bone health, reproductive health and just general wellbeing,” adds Lambert. “And remember, every single one of us is unique, and what might work for one and help us feel good, may not work for others.”