Fasting works – but which diet should you choose?

From extreme fasts to time-restricted eating, the benefits of controlled calorie consumption extend beyond weight loss. Maria Lally asks the experts how to find the one that best suits you

Back in 2009, long before most people had heard of fasting for health, I visited the Vivamayr Clinic in Austria. I went along to interview the owner, Dr Harald Stossier, about a new book he was publishing about his clinic’s detox, which involved vegetable broth, Epsom salts in warm water, the odd bit of chewy spelt bread and not a lot else.

I came back half a stone lighter, energised and evangelical about the benefits of fasting. While I was there, Dr Stossier explained that most of us in the Western world, even normal-weight adults, eat far too much and that giving your organs a break from endlessly processing three meals a day (plus snacks) acts as a kind of cellular spring clean.

Ten years on, fasting has exploded in popularity, and the benefits have been found to extend far beyond weight loss. Studies suggest it can reverse type 2 diabetes, slow the onset of Alzheimer’s and help reduce the risk of heart disease and certain cancers.

Advocates also enthuse that fasting disrupts negative eating habits, such as snacking, and resets our relationship with food.

“Studies are increasingly showing that our bodies are used to shutting down for periods of time and not having food,” says Tara Kelly, a research associate dietitian who studies the effects of fasting at Newcastle University.

“We don’t know all the answers yet, but we do know that our modern habit of consuming and snacking all day long isn’t how our bodies are designed to eat. It’s not so much what we eat but

we eat that has the most impact on our health.”

The most recent study, published this week by a team of researchers at the University of Graz in Austria, found that restricting your calories every other day – also known as “calorie cycling” – lowers cholesterol levels, improves heart health and extends life expectancy in otherwise healthy adults.

Experts agree that fasting is not suitable for everyone and should be avoided by certain groups, such as children and teenagers, pregnant or breastfeeding women, the over 70s, and those with any history of eating disorders – and you should seek medical advice before beginning any kind of extreme diet regimen.

However, such is the evidence for this dietary method that a flurry of different approaches have now emerged, from alternate day fasting – eating normally one day and severely restricting (or even cutting out) calories the next – to the 5:2 diet, popularised by Dr

Michael Mosley, which involves only eating 600 calories for two days of every week.

Then there’s time-restricted eating, where you only eat meals within a certain time window – a

12-hour fast is the most basic, where you eat your last mouthful at 9pm and nothing until 9am the next day, but the so-called 16:8 diet is also

attracting attention – here you only eat during an eight-hour window in the day. Others advocate an extreme fast – going down to just 500 or 600 calories a day, for about five days – once or twice a year. So, which one is right for you?

Fasting for… weight loss

Professor Valter Longo, a gerontologist at the University of Southern California and a leading fasting expert, has developed the Prolon diet (it stands for pro-longevity and not, as many believe, Prof Longo), which is a five-day diet, with calories limited to 500 per day, that mimics the effects of fasting.

Fans include Benedict Cumberbatch – and

Telegraph writer Victoria Lambert, who was the first person in the UK to try it when it launched in 2016.

“I initially did it to lose weight and, at 500 calories a day, it was the most effective weight loss method I’ve ever tried, but the effects went far beyond that,” she says.

“I began sleeping fantastically, my sugar cravings disappeared and, after I completed it, my entire approach to eating changed. I no longer tied myself to a three-meals-a-day routine, and instead listened to my body’s hunger signals.

“Professor Longo told me that if you’re otherwise healthy, like I was, you can do it a maximum of two to three times a year as a reset for your metabolism. If you’re overweight, however, he advises doing three fasts one after the other, with a month’s break in between.”

Research by Prof Longo on 100 people found those who did his five-day diet every month for three months lost weight and body fat and also saw improvements in blood pressure, cholesterol and the hormone insulin-like growth factor-1 (IGF-1), which is linked to ageing and disease.

For gentler, more gradual weight loss, time-restricted eating may be more suitable.

Fasting for… brain health

Studies have suggested that fasting can delay the onset of Alzheimer’s and other neurological conditions, like Parkinson’s. Animal trials show that intermittent fasting – such as time-restricted eating – promotes the growth of new brain cells, though this is yet to be shown in humans.

“Fasting is a challenge to your brain, and we think that your brain reacts by activating adaptive stress responses that help it cope with disease,” says Mark Mattson, a professor of neuroscience at the Johns Hopkins School of Medicine. “From an evolutionary perspective, it makes sense your brain should be functioning well

when you haven’t been able to obtain food for a while.”

Some believe that cutting down calories dramatically for periods of time boosts our cognitive performance.

Silicon Valley types have leapt on the bandwagon, with high-profile fans like Twitter CEO Jack Dorsey endorsing its benefits. Earlier this year he tweeted that he does “a 22-hour fast daily (dinner only), and recently did a three-day water fast”.

However, while dietitian Helen Bond says that intermittent fasting, or time-restricted eating, has some good observational studies linking it to a reduced risk of Alzheimer’s and improved cognitive ability, a 22:2 fast is extreme.

“A 12-hour overnight fast, or even a 16:8 one would be better. It’s important to remember that somebody like Jack Dorsey would have a team of experts ensuring his one meal a day is nutritionally balanced. Most of us wouldn’t know how to get everything we need, nutritionally, in just one meal.

Rather than boosting cognitive function, it would more than likely cause low blood sugar levels, grogginess and a lack of concentration.”

Fasting for… diabetics

The evidence for fasting is strongest with type 2 diabetics. Researchers at Newcastle University put 11 people with type 2 diabetes on a 600-calorie a day diet for eight weeks, after which time all were disease-free. Three months on, seven were disease-free. The team believe weight loss from fasting reduces fat inside the liver and pancreas, allowing the organs to return to normal function.

“We saw most people in the study going under the threshold for diagnosis,” says Kelly. “The idea is to then transfer patients back on to a balanced diet, with exercise as well. The most interesting thing was that the study participants didn’t feel as hungry or as miserable as they expected, and quickly got used to fasting.”

Dr Mosley, who credits fasting for reversing his type 2 diabetes, recommends kick-starting your weight loss with an 800-caloriea-day fast, for between two and 12 weeks and then, once you’re nearing your target, switching to a 5:2 diet.

Animal studies also show fasting may prevent the development of

diabetes, too, by reducing the build up of fat around internal organs.

A study from the University of Alabama found that pre-diabetic patients who did early time-restrictive eating helped ward off diabetes. Scientists at the university found that

those who ate between 7am and 3pm for five weeks had improved insulin sensitivity, compared to those who ate the same number of calories spread out over the day.

Diabetics should always consult their GP before beginning a fast.

‘The participants didn’t feel as hungry or as miserable as they expected’