A beginner's guide to intermittent fasting for weight loss and longevity
From 12:12 to 16:8 and every method in between
Lifestyle
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New research has found intermittent fasting can make weight loss easier. But what’s the best kind? And how do you do it without starving? To find out more, we tapped the experts to get the low down on time-restricted eating.
It’s rare to meet someone over 40 these days who hasn’t tried fasting. While super-fit celebs and high-flyers may see it as a way to stay in optimum health, boost brain power, performance and stay youthful, the rest of us (let’s be honest) largely see it as an effective way to lose and maintain weight.
If you haven’t yet tried the 5:2 (consuming 500-600 calories two days a week and eating normally the rest of the time) or the 16:8 (consuming all your daily meals within an eight-hour window), you’ll probably know someone who is evangelical about the kilos they’ve lost – and kept off – through one of these wildly popular approaches.
In a world of faddy diet trends, what sets fasting apart is growing scientific evidence that it works. Recently, researchers at the University of Illinois Chicago reported how the classic midlife 16:8 fast is as effective as traditional but cumbersome calorie-counting diets for shedding centimetres.
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The proof? Researchers tracked 90 adults who were assigned to one of three groups: an eight-hour time-restricted fast in which they ate between noon and 8pm only, a traditional diet that cut daily calories by 25 per cent and a control group who didn’t change their eating habits. Those in the fasting and calorie-control diets lost weight – about 4.5kg over a year.
And while this and other studies show that it’s not necessarily more effective than other diets when it comes to losing weight – and keeping it off – fasting appeals because of the additional benefits. Namely, the health-related ones. Studies have shown that fasting can actually lower cholesterol and reduce the risk of conditions like type 2 diabetes and heart disease.
The other bonus? Time-restricted eating can also slip relatively seamlessly into our lives, according to Dr Duane Mellor, a dietitian and senior teaching fellow at UK’s Aston University medical school. “Fasting is not a magic bullet, but it often works for people because they can choose the fasting rules that suit them,” he explains. From our 50s onwards, the body’s requirement for calories declines – another reason why fasting is easier to adopt as we get older. “Age, hormonal changes and a natural decline in muscle mass all affect our energy requirement,” says Mellor. “We need 100 to 300 fewer calories per day, and intermittent fasting is one way of achieving that deficit.”
Beyond weight loss, there are other reasons fasting is so appealing to midlifers. Studies show it produces health gains ranging from improved cognitive function, gut health, sleep and blood sugar control, to a slowing of the ageing process by helping the body to flush out damaged cells and regenerate healthy ones – a process called autophagy – potentially boosting longevity.
In a paper published last year, Valter Longo, a professor of gerontology and biological sciences, and director of the Longevity Institute at the University of Southern California, says that fasting helps to control and even reduce levels of insulin, C-reactive protein, insulin-like growth factor 1 and cholesterol, all of which are markers for serious disease.
“We aren’t designed to eat for an entire 24-hour cycle, and your body and digestive system need a break,” says Mellor. “An intermittent fast is one way of doing that.”
Not that fasting is easy – many find that the 16:8 hunger pangs in the first week are, well, unbearable, although they pass by week three – nor is weight loss guaranteed. “Much depends on how you choose to break each fast,” says Mellor. “If you eat lots of ultra-processed and refined foods, you will slow any potential effects, whereas consuming plenty of pulses, wholegrains, vegetables, nuts and olive oil will contribute to weight loss and health gains.”
As a committed 16:8er even before it became fashionable – I rarely eat breakfast and usually have my first meal of the day at noon – I would say it’s without doubt helped me maintain a healthy weight for decades. And the beauty of fasting: there’s a form that will work for you, and your lifestyle. Here’s exactly what to try…
12:12: The beginner's guide
New to intermittent fasting? Then you’ll need to ease yourself in gradually. The best and most realistic way to do it is to set a daily ‘eating window’ that’s manageable, and stick to it. Twelve hours is a good starting point. Once you can comfortably manage eating and fasting for 12 hours a day, start to extend the fasting segment by an hour or two to eat within 10 hours and fast for 14 (14:10).
It’s an approach used by actors Scarlett Johansson and Justin Theroux, who reportedly aim to eat all meals within a 12-hour window, while Chris Hemsworth and his wife, Elsa Pataky, follow the slightly stricter 15:9 approach. The real beauty of so-called time-restricted eating, or TRE, is that it can be worked around your lifestyle and body clock. If you’re someone who feels you need to eat breakfast, you can schedule the window to start and finish earlier than if you’re someone who doesn’t wake up hungry.
16:8: The middle fast
Hugh Jackman, Jennifer Aniston and Beyoncé are among those who swear by this approach, which has more evidence than other fasting methods in terms of studies showing its effectiveness for weight loss, midlife weight maintenance and health benefits. Its appeal is that it’s totally doable for most of us – the rules are simply to consume your daily calories within an eight-hour window, leaving 16 hours without food (although you can have water, black coffee and calorie-free drinks).
The kicker: you can select a fasting window to suit you. A study in the journal Nutrition and Healthy Aging suggested that an eating window of 7am to 3pm is optimal, although shifting later is fine if you wouldn’t normally have breakfast. Finishing your last meal no later than 7pm is best for avoiding a sluggish metabolism. “More important than timing of the eating window itself is how it fits into your lifestyle,” says Mellor. “It’s got to be something that works well for you.”
Ultimately, he adds, the 16:8 works like any other intermittent fast in that it means fewer calories consumed without actually having to count them. In one study, 16:8 dieters ate 350 fewer calories a day than study participants following other forms of fasting diet, and lost three per cent more body weight after 12 weeks. Yep, impressive results.
5:2 The weight-loss fast
This approach – eating about 500 calories two days a week and what you like (within reason) on the other five – soared in popularity after a 2012 documentary by Dr Michael Mosley, Eat, Fast and Live Longer, in which he investigated its benefits. A series of bestselling books followed, and celebs such as Jennifer Lopez and Benedict Cumberbatch became fans.
Despite a lack of specific studies on the benefits of 5:2, there’s plenty of science showing metabolic and weight loss benefits for intermittent fasting in general. One review of 40 studies showed that it can help people to lose up to 5kg in 10 weeks. Twice-weekly fasts, however, are not for everyone.
A 2021 study that compared its effects with standard advice on calorie counting and healthy eating in 300 overweight adults, showed that three quarters of people managed to stick to the 5:2 rigidly for six weeks but struggled after that, with only one third following it six months later.
6:1 The reluctant fast
If you struggle with the whole concept of fasting, doing it for one day a week and consuming only very low- (or no-) calorie liquids such as tea, coffee and broth for that 24-hour period, but eating normally the rest of the week, is probably your best bet. Mosley even suggests this as a long-term way to maintain the weight loss and health benefits after a 5:2 stint. But don’t just take our word for it.
Researchers have shown a regular 24-hour fast can reduce the risk of coronary artery disease and metabolic syndrome. Last year, researchers comparing the 5:2 with the 6:1 found that although the twice-weekly fasters lost more weight, both fasts significantly reduced weight, waist circumference, body fat and hunger levels.
Kourtney Kardashian was reportedly a fan of the 6:1 fast – prior to falling pregnant, of course. Yes, you’ll probably feel tired and hungry, but you can breakfast like a king the next day.
FMD (fast mimicking diet): The longevity fast
If fasting is not something that you can see yourself doing on a daily (or even weekly) basis, this cheat’s FMD fast – which involves eating about 750 calories a day for five consecutive days every few months – is an option. It attracts those who like the fact that for a few days of effort you’re effectively off fasting duty for weeks.
It’s been shown to deliver health benefits including improvements in a range of metabolic markers linked with ageing, such as reduced blood sugar levels, plus lower cholesterol and blood pressure, and to help those who partake lose as much as 2.6kg after the first three-month cycle.
Devised by Longo, it promises all the health gains of other forms of fasting but without too-torturous restriction. In his most recent published paper in the scientific journal, Cell, Longo described how eating within a daily window of 11 to 12 hours, and adding his five-day FMD every three to four months, may help to reduce insulin resistance and dial down blood pressure, both major health and lifestyle wins.
Omad (one meal a day): The iron-willed fast
What Dr Mindy Pelz, a nutrition expert from the US and bestselling author of Fast Like a Girl, calls the ‘dinner-to-dinner’ fast – fasting from one evening meal until the next day’s evening meal – is called OMAD (one meal a day) by Dr Jason Fung, the bestselling author of The Obesity Code. The appeal for wellness seekers? A study last year showed that people who switched from three daily meals to one meal a day lost weight and improved overall metabolic health.
It’s not for the faint-hearted, but it has an army of (largely male) fans. Among them is Chris Martin, lead singer of Coldplay. “I actually don’t have dinner any more,” said Martin. “I stop eating at four, and I learnt that from having lunch with Bruce Springsteen.” Singer Robbie Williams has his first meal of the day at 5pm, and Jack Dorsey, the former chief executive of Twitter, takes it to another extreme, reportedly eating one meal a day Sunday to Friday but nothing at all on Saturdays. Black coffee and water are permitted during the fast, and the idea is that the single daily meal is hearty and nutritious.
36-Hour fast: The belly-fat fast
Losing mid-section weight is notoriously difficult. According to Pelz, a 36-hour fast once a month is the best way to target belly fat, and that 36-hour fasts are where to start to see fat loss and anti-ageing effects. The reason? This length of fast will force your body to burn stored glucose – or stored sugars – changing the body’s metabolism, so that it begins to burn fatty acids from stored fat for energy, with research showing that abdominal fat, in particular, is the area targeted.
Several studies have shown that extended fasting does reduce belly fat, with the most compelling research published in the journal Cell Metabolism. In that trial, 60 participants either fasted for 36 hours then ate as much as they liked within a 12-hour window repeatedly, or ate normally for a month.
The verdict? Results showed that the 36-hour fasters had shed weight overall, but primarily from their mid-section. They also had lower levels of inflammatory markers linked to ageing and disease. Mellor says that the more extreme extended fasts are “not suitable for everyone” and warns that they can morph into disordered eating. Good to know before jumping on the bandwagon of what’s rapidly becoming one of the world’s most buzzed-about health approaches.
Originally published as A beginner's guide to intermittent fasting for weight loss and longevity
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