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Intermittent fasting is becoming an increasingly popular way to lose weight. What's its appeal and is it safe to fast?

Whether it's eating in a 5:2 or 16:8 pattern, losing weight is as much about when you eat as about what you eat these days.

Recently, Twitter CEO Jack Dorsey hit the headlines when he claimed to eat just one meal a day. Social media was abuzz, with many critics calling it an extreme diet. But perhaps he was just following the latest trend.

According to a survey by the International Food Information Council Foundation intermittent fasting (IF) was last year's most popular diet.

Intermittent fasting can take many different forms but it always involves periods of time where you are taking in very little food, at regular intervals.

Fasting patterns

The 16:8 diet involves fasting for 16 hours per day and eating within an eight-hour window. This is usually done by only eating from midday till 8pm.

The 5:2 diet involves eating only 25% of a normal calorie intake on two non-consecutive days per week.

A 24 hours fast involves consuming no calories on one day of the week or month.

Image copyright Getty Images Image caption Restricting calories usually helps in weight loss

Fans of intermittent fasting claim it's a good way to lose weight, and a summary of studies into the method, published in the Annual Review of Nutrition in 2017, found that 11 out of 16 trials reported some weight loss.

''The reason for that is simple mathematics, as you are restricting calories,'' says Dr Linia Patel, a registered dietician and spokeswoman for the British Dietician Association.

However, the study found that some patterns of fasting were not advisable: alternate-day fasting led to intense hunger and was therefore considered impractical.

Enforced fasting periods

Fasting has been practised throughout history for religious, cultural and spiritual reasons, but as a way of eating it goes back much further.

Dr Patel explains that if we look at our ancestors, they had periods of enforced fasting. Hunter-gatherers only had food when they killed or collected it, so fasting was part of their everyday reality.

But nowadays we are constantly exposed to plentiful foods. We also lead far less active lives. Together, these factors have caused an obesity epidemic.

'People love rules'

Simply telling people to eat a healthy, balanced diet doesn't seem to work, according to Dr Patel.

"People love rules," she says. "I think bringing in a little discipline and having periods where we are not eating is a positive thing."

Unlike most diets, IF doesn't exclude specific food groups, such as fat, sugars or carbohydrates. All you need is a clock. Perhaps that is part of its appeal.

For Shanae Dennis, a 26-year-old journalist, it was the simplicity that appealed.

"I started intermittent fasting because I really wanted a quick, simple and easy diet. Being told to just eat between 12 and 8pm was really easy to follow so I was interested."

"In the beginning I thought I could really eat whatever I want, but soon I realised that for it to work I had to eat healthily," says Shanae.

Image copyright Getty Images Image caption Eat plenty of fruit and vegetables to get enough dietary fibre, vitamins and minerals.

'Feast and famine'

This is a common misconception, says Dr Patel.

"In my clinical practice I find that a lot of people are not doing intermittent fasting properly," she says. "For example they might keep the calories below 500 on fasting days but then they go and feast on the other days.

''That's dangerous from two points of view: you are not going to get the calorie deficit and lose weight. You need to eat sensibly."

So it's not a feast and famine thing. That's not going to work.

In order for it to be a safe, effective and healthy way of eating, the food consumed during "eating windows" needs to be of high nutritional value.

Scientists emphasise that those on IF diets should include essential fats from oily fish, nuts and seeds, lean sources of protein, whole-grains and starchy carbohydrates and plenty of fruit and vegetables to supply enough dietary fibre, vitamins and minerals.

Can you keep it up?

Calorie-controlled diets all have a similar impact on weight loss, but the diet that is really successful is one that people can sustain in the long-term.

One of the difficulties with sticking to a diet is craving forbidden foods, and with IF that is perhaps less of a problem.

"When I am on a no-carbohydrate or low carbohydrate diet all I see is pasta and rice everywhere, but now I don't crave certain food groups because nothing is restricted," says Shanae.

She has been following the 16:8 diet for four months now and finds it easier to keep up than the 5:2 diet, during which she gets very hungry.

"The hardest thing is to not reach for the breakfast muffin," says Shanae.

Image copyright Imahn Robertson Image caption Imahn likes the discipline that comes with intermittent fasting

Imahn Robertson, 26, has been following an IF diet for over four years.

"When I first started I did struggle, the body naturally craves foods at all times. But after I got into a routine it was okay," says Imahn.

"There is temptation, don't get me wrong. If I am tempted I will occasionally break my fast and adjust my fasting period the next day."

However, Imahn admits that her diet has affected her social life.

"I only eat between 12 and 8pm - if someone schedules a dinner for nine o'clock, I will go to the dinner but just drink water.

"What makes it difficult is that people are very judgemental," she says. "I find it easier to do intermittent fasting when I am single, not when I am in a relationship."

Sugar rush

Freelance cameraman Colum O'Dwyer, 27, has been practising IF for a couple of years now. He eats two large meals a day, between 12 and 8pm.

"I like the discipline that comes with it," says Colum. "It gives me a structure which is sorely needed when you manage your own time.

"I feel I'm always at risk of procrastinating by having overly elaborate breakfasts or lunches, so going from three meals a day to two frees up time and allows for greater productivity."

For Colum it's also important to minimise sugar, to avoid a sugar rush - and subsequent crash.

Image copyright Getty Images Image caption Certain foods can have an impact on insulin levels

Dr Patel says that mood swings - caused by a change in insulin levels - can be a consequence of fasting, depending on what you are used to eating.

"If you have a diet that is very high in refined carbohydrates and you snack a lot, of course fasting is going to mean that you will be irritable," says Dr Patel.

'Not for everyone'

IF can be unsuitable for people with certain health conditions.

People with advanced diabetes, history of eating disorders such as anorexia and bulimia, those suffering from chronic diseases and pregnant or breastfeeding women should not attempt intermittent fasting. Fasting should not be attempted unless they are under the close supervision of a physician.

People who have stomach ulcers, should not fast either.

This kind of dieting has gained popularity in the last 10 years or so, but it has not been around for long enough to prove that it's any better or worse than other diets, says Dr Patel.

"We don't have massive trials but the evidence in this area is increasing. It's not black and white yet but what we are seeing from the existing human studies that we have is that there are positive attributes when done properly."