It’s been something of a breakthrough year for intermittent fasting. Chances are you’ve had a workmate drag out the tea room chat with talk of “5:2” this or “the 16:8″ that, while it was also the most searched diet term of the year, according to Google’s annual report.
Some people think of intermittent fasting as an effective part-time diet. You restrict your calories to certain windows during the day or week. But for every convert who finds that an easier way of dieting, there’s an equal number of people who slam it for being nothing more than the latest fad.
Fasting, which bumped previously hyped high-fat keto and low-carb carnivore diets off Google’s top 10 list, has surged in popularity due to its association with helping people lose weight in a hurry.
But beyond burning fat, research suggests periodic fasting can deliver transformative health benefits too, such as reducing the risk of diseases including diabetes and heart disease and boosting the production of a protein that strengthens connections in the brain that can serve as an antidepressant to boost mood.
On the flipside, intermittent fasting has been associated with a range of debilitating side effects such as fatigue, dizziness and muscle aches and is even deemed downright dangerous for those who are pregnant or at risk of eating disorders.
So what exactly is intermittent fasting? Does it really work? And, crucially, could it work for you?
What Is Intermittent Fasting?
There are several schools of intermittent fasting, ranging from the entry-level to the extreme. Some divide the typical day or week up into windows when you can or cannot eat. Others require you to track and limit your calorie intake for a set time period.
Here, with the help of a registered GP, naturopath nutritionist and personal trainer, we break down the hotly-debated merits of each one.
The 16:8 diet
Basically, no food for 16 hours, then all you can eat for the remaining eight. With the 16:8 diet you can drink unsweetened beverages such as tea, coffee or water in the first 16 hours but, just to stress the point, no food. Hugh Jackman used this while preparing for Wolverine.
“The 16:8 may promote weight loss and improved blood sugar control,” says consultant pharmacist James O’Loan of online UK pharmacy Doctor 4 U. “And due to this 16-hour beverage window, you’re also less likely to feel hungry compared with other fasting diets.”
However, because you can eat any type of food and as many calories as you like in those eight hours, it can be hard to follow a balanced, nutritious diet. Binge eating can get all too easy, as well.
The 5:2 diet
The most popular intermittent fasting protocol. Twice a week you cut your daily intake to just 500 calories, while eating normally for the remaining five days.
“The 5:2 diet is a simple way to reduce calories,” says O’Loan. “It may help with weight loss, improve cognitive function, control cholesterol and blood sugar levels.”
It’s the most flexible of all the options but because you can spread your intake of those 500 calories across the day you might not enjoy the full benefits of giving your digestive system a complete rest. “It could even leave you feeling more hungry,” adds O’Loan. “And it can cause disruption to sleep, irritation, dehydration and bad breath.”
The eat-stop-eat diet follows a similar pattern as the 5:2 diet, where you eat normally for five days and fast for two, but it requires you to completely fast for 24 hours once or twice a week.
“This is less complicated than the other fasting diets, which can make it a more manageable option for many to follow,” says O’Loan. However, he warns, it can take some getting used to. “In the first few weeks you can become irritable and experience headaches and mood changes.”
Fasting for a full 24 hours is also a no-no for diabetics, pregnant women, and those who have suffered from eating disorders in the past.
OMAD (one meal a day)
As the name suggests, the idea here is to limit yourself to one highly nutritious main meal a day. “It cuts the need to calorie count and can help boost productivity and concentration at work as you won’t have that sluggish feeling typically experienced after a big canteen lunch,” says O’Loan.
But the list of negative side-effects are long. “Hunger, weakness, shaking and fatigue is common and this diet isn’t recommended long term as it’s unlikely you’ll consume enough vital nutrients from one solitary meal a day.”
The Benefits Of Intermittent Fasting
Most of the health benefits associated with intermittent fasting are the result of giving the digestive tract a well-deserved break.
“It gives your emunctory organs (primarily the liver and kidneys responsible for detoxification of waste products) time off so they can cleanse your body and repair damaged tissue,” explains Dr Laure Hyvernat, naturopath and founder of The Natural Consultation.
“Our modern lifestyles leave us bombarded by toxins from processed food, stress, alcohol, medications and pollutants,” she adds.
“But scientific research is unanimous on the health benefits of fasting for boosting immune function, reducing inflammation at the cellular level which is the number one cause of cancer, repairing age-related dysfunction, promoting regeneration in the body by removing damaged cells and tissues and stimulating repair.”
This digestive break, says Dr Hyvernat, has also been shown to boost general digestion, hormonal detoxification, mental clarity, mood, skin health, insulin response and metabolism, while reducing symptoms of joint pain, stress and chronic fatigue.
Another happy side-effect that can be triggered by intermittent fasting is weight loss. It’s typically the number one reason people turn to fasting in the first place.
The Cons of Intermittent Fasting
As with most things, and especially true with diets, there are always negative side-effects as well as positive ones.
“For most people, intermittent fasting is generally safe and beneficial,” says fellow Doctor 4 U GP Dr Diana Gall. “However, for those who are already fit and lean prior to fasting, there is a risk of hormonal imbalances which can cause problems with sleep.”
If this is the case, Dr Gall recommends careful monitoring by your GP. “If you have a health condition or you are taking medication such as blood pressure tablets, intermittent fasting can be dangerous,” she adds.
Diabetics can experience hypoglycaemia (low blood sugar) as a result of fasting and it’s not recommended for people with high calorie needs such as those underweight, under 18 or pregnant.
“There is also an argument that fasting for a long period of time encourages overindulgence when it’s time to eat,” adds Dr Gall. This can lead to binge eating and overconsumption when it is time to break your fast.
Not only can this undermine weight loss efforts, it can reinforce harmful habits in people who have previously had eating disorders or those at an increased risk of developing them.
How To Work Intermittent Fasting Into Your Routine
Rather than following a restrictive intermittent fasting diet year round, Dr Hyvernat prefers to prescribe a personalised three-day intensive “reset plan” done at a maximum of two to four times a year.
This way you can reboot your digestive system when it’s suitable for your schedule and means you aren’t locked into a long-term, inflexible plan.
While Dr Hyvernat urges caution and medical supervision before diving head first into a restrictive intermittent fast, others, such as the outspoken online PT James Smith, believe the whole trend should be taken with a big pinch of salt.
“Outside the spontaneous reduction of calories some experience, I don’t see the upside to intermittent fasting beyond quite simply eating less,” says Smith of jamessmithacademy.com and author of the upcoming Not A Diet Book.
“If someone wishes to improve their health they should look to manage their nutrition, sleep, relationships, stress and even favour how much vitamin D they get from sunlight,” he says. “Pushing a meal back isn’t magical in any way. Most improvements to health come from the fat loss that can occur, not the fasting itself.”
Smith argues the recommended seven to nine hours of sleep a night is more than enough “time off” for the body’s crucial regenerative processes to occur without the need for further fasts during the day.
“It kills me that some people out there sleep deprived and stressed from following a shit diet think that skipping breakfast is the holy grail to a better life,” he says. “Try living in Australia where breakfast is the best meal of the day.”
Is Intermittent Fasting Right For Me?
Whether you agree with Smith or insist intermittent fasting has its place, it ultimately comes down to your personal experience and perspective.
Generally, says Dr Gall, if you are fit and healthy then intermittent fasting in moderation can be safe. “Humans have always fasted to some extent – intermittent fasting is more of an eating pattern rather than a diet,” she says.
“If you notice health benefits from following a particular fast then there’s no reason why it can’t be part of your long term lifestyle.” Just remember to monitor your personal success with each diet, rather than blindly following the lead of a friend or celebrity.
“What works for them might not work for you,” says Dr Hyvernat. “Why? Simply because you don’t have the same symptoms, needs, metabolism, genes, microbiome or lifestyle. We are all infinitely unique, so our diet should be too.”
A Case For The 16:8 Diet
Alexander Cortes, online PT
“I first began using intermittent fasting 10 years ago when it was largely unknown. I would fast for 24 hours one day a week and found it to be very effective to maintain leanness. I discovered the 16:8 diet a few years later.
“Essentially it’s skipping breakfast and eating only lunch and dinner. I never found fasting challenging at all. The human body is evolved to go very long periods of time with minimal-to-no food and restricting eating is simplest way to lose body fat and maintain body composition. The challenges were the cognitive dissonance it caused in other people.
“Many people falsely believe something ‘bad’ will happen to their body if they don’t eat for long periods of time, or their metabolism will begin storing fat (despite not eating), or they won’t have enough energy and blood sugar levels will drop dangerously. All of these ideas are wrong and run counter to our own biology.
“Fasting improves insulin sensitivity, controls blood sugar, lowers appetite, and attenuates hunger so you understand the difference between eating out of habit versus eating because you are actually hungry. Any discomfort will be psychological more than physical.
“I still use intermittent fasting weekly, and have done so for over a decade. I modify it depending on my goals and my training. I may do 16:8 for maintenance, but use 24 hour fasts once or twice weekly to cut body fat or a 20:4 fast once or twice a week to minimise fat gain.”
A Case Against The 16:8 Diet
Lee Stephens, fitness coach at London’s Third Space
“I tried the 16:8 diet when I was looking to reduce body fat but found it incredibly tough. I found the timings very hard to fit in with my schedule. I was doing a lot of personal training and would usually coach two to three clients by 11am, so having absolutely zero fuel to get me through those sessions was hard.
“If you’re not as active during the early hours of the day, then I can see why the 16:8 intermittent fast could work for you. But for those who are active pretty much all day, I think it’s vital some fuel is consumed early in the morning to set you up and keep you focused. The biggest challenge I faced was a lack of energy in the morning around 8-9am and being relatively hungry just before bed.
“As I’m very active and try to hit 3,000 to 3,500 calories a day, I found it very hard to fit in enough meals in the eight hour window and would just end up force feeding myself. I would never restrictively diet again. I always feel like I need something to eat in the morning before coaching and enjoy the freedom to eat when it suits me.
“If I was to reduce body fat again, I would track calories and stay in a slight 10-15 per cent daily deficit. Now I’m more educated on nutrition, it really isn’t hard to reduce body fat when you know your macros (protein, carbs and fats) and rough total daily energy expenditure. I found the 16:8 diet far too restrictive and don’t think it will fit in with most people’s lifestyle.”