Does intermittent fasting (IF) really help you lose weight? And is it good for you? There are competing views on the diet method, which involves switching between fasting and eating on a regular schedule. One recent study that looked at the 16:8-style diet (more on that below) revealed that it raised the risk of heart attacks and strokes. In another study, fasting for 15 to 16 hours each day for a month led to a reduction in blood pressure in patients whose readings had been dangerously high.
But now the latest study on IF suggests that it’s more effective than calorie restriction for weight loss, and also has other health benefits. People on a 4:3 intermittent fasting regime, where calorie intake is restricted for three days in a week, lost a little more weight over a 12-month period than those on a daily calorie restriction diet.
Victoria Catenacci, an associate professor of medicine at the University of Colorado Denver School of Medicine and the lead author of the study, said: “Participants in the fasting group also achieved more favourable cardio-metabolic outcomes, including systolic blood pressure, total and low-density lipoprotein cholesterol levels, and fasting glucose level.”
• Why fasting could beat calorie-counting for weight loss
In the research, published in the Annals of Internal Medicine, participants who were intermittently fasting lost 7.6 per cent of their body weight, while those who restricted calorie intake daily lost 5 per cent by the end of the 12 months.
The results don’t surprise me. Everyone I know who has successfully lost weight has done so through intermittent fasting. This might be via time-restricted eating (TRE), where calories are consumed within a small number of hours each day, such as the 16:8, in which all your meals are consumed within an eight-hour window, or through low-calorie fasting days, where one or two small meals are consumed on a set number of days every week, as per the 5:2.
There are endless variations on the fasting theme. In 2022, researchers from the University of Alabama put their slant on the 16:8 by reporting in the medical journal JAMA that obese people lost significantly more weight if they ate all meals between 7am and 3pm. But whatever the fasting remit, the basic premise is that by skipping some meals every week you consume fewer calories overall.
“All of these diets are popular because they offer a straightforward way to reduce energy intake, and that often leads to weight loss,” says the dietician Rhiannon Lambert, the author of Science of Nutrition.
“Everyone I know who has successfully lost weight has done so through IF”, says Peta Bee
GETTY IMAGES
Undoubtedly, much of IF’s appeal is its simplicity — that, and knowing that deprivation is temporary and you can relieve yourself of dietary starvation once the fasting phase is over. Men seem to love it as much as women do, not just because of the accelerated and sustained weight loss but because of evidence that it can improve health and longevity.
Fasting even for a few hours triggers a biological response to food scarcity called metabolic switching, that means once cells have used up their stores of sugar they begin burning body fat instead. A break from the constant calorie consumption of endless snacking also gives the metabolism and the energy centre of the body’s cells a chance to reset.
“Intermittent fasting can bring many metabolic benefits when it is done properly, which means following a healthy balanced diet with plenty of vegetables, wholegrains and all the nutrients you need when you do eat,” says Dr Linia Patel, a dietician and spokeswoman for the British Dietetic Association. “There’s evidence that it helps with insulin control, the reduction of body fat and with lowering cholesterol levels.”
It won’t work for everyone. “If you are someone prone to disordered eating or if your basic diet is not balanced, then IF is definitely not for you,” Patel says. “Studies tend to show it works best for men and post-menopausal women and that it can be less effective for younger women who need regular energy intake for their reproductive health.”
On a scale of IF that ranges from a few hours a day without food to severe daily restriction, Patel says that the less extreme it is, the more likely it is that you’ll adhere to it. “It will only be effective if the fasting periods work for you and there are opportunities for nutritious meals at other times,” she says. “Severe deprivation is bad news.” So which fasting form is for you?
16:8 diet
Rules
Consume all your daily calories within an 8-hour window, leaving 16 hours without food and with only water or calorie-free drinks.
Who does it?
Hugh Jackman is a fan and Jennifer Aniston has said: “I do intermittent fasting, so no food in the morning. I noticed a big difference in going without solid food for 16 hours.” Nadia Sawalha skips breakfast as “we do the 16:8 intermittent fast and only eat in an 8-hour window”.
How it works
The 16:8 is among the more widely studied versions of intermittent fasting and there is evidence that it works, not least because it is very doable. In theory you can select an 8-hour slot that suits you, but while most people find it easiest to skip breakfast, the research from Alabama suggests an eating window of 7am-3pm is optimal. Researchers at the University of Illinois showed how 16:8 dieters consumed 350 fewer calories a day than even study participants following other forms of fasting diet and they lost 3 per cent more body weight over 12 weeks.
Hugh Jackman is one of the 16:8 diet’s celebrity fans
ANDREW SIMS FOR THE TIMES
Downsides
Sticking to 7am-3pm won’t suit everyone, Patel says. “Much depends on your chronobiology. If you are not a morning person and don’t need breakfast, another window would be better.” Others have shown it to be most effective if you finish your last meal before 7pm, meaning your first meal is at 11am; leave it any later and you’re likely to be eating when your metabolism is sluggish.
5:2 diet
Rules
On two days a week eat only about 500 calories. On the other five days eat what you like (within reason).
Who does it?
Benedict Cumberbatch, Gisele Bündchen, Jennifer Lopez, Jimmy Kimmel and Jennifer Metcalfe are all rumoured to have tried it.
The actor Benedict Cumberbatch is said to have tried the 5:2 diet
GETTY IMAGES
How it works
Dr Michael Mosley is widely credited with firing up the intermittent fasting trend after a 2012 Horizon documentary in which he investigated the health benefits of alternate-day fasting and the 5:2 approach, followed by a book called The Fast Diet that outlined the approach. There’s plenty of scientific evidence supporting its metabolic and weight-loss benefits and it remains among the most popular methods of fasting.
Downsides
Twice-weekly fasting days can be tough and take some getting used to. “A lot of men seem to prefer this approach,” Patel says.
• Ten years of fasting diets: the legacy of Michael Mosley
Eat Stop Eat diet
Rules
Fast for 24 hours once a week, consuming only liquids, and eat normally the rest of the time. Also known as the 6:1.
Who does it?
Chris Martin says he fasts one day a week and Kourtney Kardashian does a 24-hour fast every week.
How it works
Martin drinks only water for his 24-hour fast while Kardashian takes water and bone broth. “You need to be pretty determined to eat nothing for 24 hours, but it works for some people,” Patel says.
Downsides
Fatigue could be an issue on a one-day fast, Patel says. Liquids including soup could be an option for added energy.
Fast 800 diet
Rules
Eat no more than 800 calories a day for up to 12 weeks.
Who does it?
Saira Khan, the former Loose Women presenter, is among those who follow the plan, which was championed by Mosley.
How it works
The latest addition to Mosley’s fasting portfolio, the Fast 800 promises a short-term approach to “flip the metabolic switch” for rapid fat burning. Expect to eat a healthy Mediterranean-style diet rich in protein, high-fibre carbs and healthy fats — or if it’s a headache working out calorie counts you can purchase the Fast 800 nutritionally balanced shakes and soups (from £22.99; thefast800.com) to see you through. It has been shown to be beneficial in controlling blood sugar and reducing the risk of type 2 diabetes.
Downsides
It’s not designed to be sustainable long term, and it is suggested you move on to the 5:2 diet after 10-12 weeks.
Fasting Mimicking diet (FMD)
Rules
Eat about 750 calories for 5 days every 4-6 weeks.
Who does it?
Jennifer Aniston.
How it works
The brainchild of Professor Valter Longo and researchers at the University of Southern California, who carried out extensive research into periodic fasting. Dubbed the cheat’s fast, FMD promises all the benefits of IF but with just five days a month of caloric restriction. A commercial diet called ProLon — it stands for PROmote Health and LONgevity — is backed by the scientists. The appeal of FMD is that it is deliciously short term. See it through the five days and you are effectively off-duty for weeks, yet you’re likely still to see your weight dropping, with participants in Longo’s trials shedding an average of 5.7lb in 1-3 months. It has also been shown to bring health benefits including improvements in a range of metabolic markers linked with ageing and diseases, such as reduced blood sugar levels, cholesterol and blood pressure. “For a normal-weight person with no health issues, the benefits of a single five-day fast can extend to several months,” Longo says.
Downsides
You can DIY, but if you go full-on ProLon be prepared to pay. A delivery of daily supplies of soups, energy bars and snacks coming in at about 750 daily calories for a 5-day fast costs £199 (prolon.co.uk).
OMAD diet
Rules
OMAD stands for One Meal a Day Diet.
Who does it?
Elizabeth Hurley is fan, once claiming “the only meal I have is dinner”, as are Jack Dorsey, the former head of Twitter, who has said he eats once a day, and the Iceman, Wim Hof.
Elizabeth Hurley has said she only eats one meal a day
REX
How it works
In theory your one meal a day should be filling and nutritious, although fans of this approach tend not to be foodies. Even when he eats his only meal of the day after 6pm, Hof sticks to vegetable soup or pasta, while Dorsey says he usually has a meal of protein and vegetables between 6.30pm and 9pm.
Downsides
Only for the iron-willed.
TRE diet
Rules
Time Restricted Eating involves consuming all your meals within a predetermined window of your choosing.
Who does it?
Justin Theroux reportedly eats all his meals within a 12-hour window, an approach sometimes adopted by Scarlett Johansson. Chris Hemsworth and his wife Elsa Pataky follow a 15:9 eating plan.
Scarlett Johansson has also followed an intermittent fasting plan
GETTY IMAGES
How it works
Strictly speaking, the 16:8 falls under the TRE umbrella but has attracted enough of a reputation to be considered a diet in its own right. But TRE can be adjusted to suit you and if you can’t manage 16 hours without food each day, then even a 9, 10 or 12-hour daily fast will provide a level of calorie deficit and give your digestive system a break. “It’s great to experiment and find what you can stick to long term,” Patel says.
Downsides
Potential to cheat when you set your own rules — decide on your eating window and stick to it.
Warrior Diet
Rules
Although based on the principles of IF, the original Warrior Diet allows you to eat very small amounts of fresh fruits, vegetables and nuts, yoghurt and cottage cheese for 20 hours and then eat as much as you like for 4 hours in an evening meal that includes some protein-rich foods like fish, beans, quinoa or nuts and seeds. It has evolved into the more extreme 20:4 diet followed by a hardcore brigade who eat nothing for 20 hours of the day.
Who does it?
There are a huge number of posts with the hashtag warrior diet on Instagram.
How it works
An early incarnation of the intermittent-fasting trend, this has been around since the early 2000s and was developed by Ori Hofmekler. A former member of the Israeli special forces, in his book of the same name he outlined the protocol of “underfeeding” for 20 hours followed by a 4-hour “overfeeding” window. He believes the approach mirrors the eating patterns of ancient warriors, boosting the body’s survival instincts and helping to shed pounds.
Downsides
Pretty extreme deprivation. “Fasting for this long every day is unlikely to provide the opportunity for a healthy nutrient intake long term,” Patel says.