Seen people raving about intermittent fasting but not sure how it works or if it's safe? 4 top experts share their verdict

The latest wellness trend or something to try?

Intermittent fasting: A salad
(Image credit: Getty Images)

While it might be a trend you've seen doing the rounds on your social media feeds, a fun fact for you: intermittent fasting has been around since the 5th century BC. That's right - people have been fasting for religious and spiritual reasons for thousands of years, but in recent years, we've seen it glorified as a health "cure-all" and fat loss no-brainer.

Top experts including epidemiologist Tim Spector rave about the benefits of fasting on your weight, metabolism, and more, and on TikTok, the hashtag #intermittentfasting alone has over 1.6 billion views. Many users have taken to the app to praise the eating habit for helping them manage their weight, lose body fat, reduce inflammation and even change their hormone function. 

So, question: What's the scientific evidence for and against fasting, who (if anyone) is it suitable for, and what are the risks of giving it a go, if you are weighed up on the current research? All good questions, which is why we've picked the brains of four top experts to debunk the trend once and for all. 

Keep scrolling for their take. Keen to read expert-led takes on the Cambridge diet, inflammation diet and 4:3 diet, while you're here? You're in the right place. Bye, bye, fad diets.

Intermittent fasting is having a moment - 4 experts on whether it's worth trying or another fad

What is intermittent fasting?

First things first - an explainer of what intermittent fasting (IF, for short) actually is. Stereotypically, it's a type of eating which involves consuming all of your meals within a set time frame during a 24-hour period. Outside of that period, you fast - that is, drink water, sip tea, but largely give your stomach a rest.

A way of eating that doesn’t focus on the foods you eat, but rather the time you’re eating those foods, common intermittent fasting diets include the 16:8 diet - where you’ll eat within an eight-hour time frame and fast for 16 hours - or Time Restrictive Eating - where you’ll eat within an eight to 10 hour period and fast for the rest. Other intermittent fasting diets include the 5:2 method which involves fasting for two days of the week - not consecutively - and eating at any time throughout the other five days.

Do note here, though: Jane Mostowfi, a BANT registered nutritionist, emphasises that no matter what type of intermittent fasting diet you try, you should never fast long-term. “Long-term fasting can do more harm than good as calorie restriction can stress out the delicate endocrine system which looks after our hormones," she explains. 

Is intermittent fasting safe?

Now, on to the question you're all dying to know the answer to. Is intermittent fasting actually "good" for you and aside from that, safe? Let's be clear on this one: fasting is a subject that divides experts and it'll depend on what your definition of "healthy" is.

If you're looking to lose fat, some nutritionists rate this form of eating as a short-term method to support metabolism and fat loss. The science on the matter is mixed, however. There is some research (like this 2016 study) that suggests intermittent fasting supports fat loss,  while other studies (such as this 2022 paper) suggest it has no real impact and that calorie restriction is more beneficial than time restriction when it comes to fat loss and improved overall health. 

“Intermittent fasting, when used correctly for the right person in the right time frame, can work if fat loss is the goal," explains Mostowfi. "But key phrases in that sentence being, right person and right time frame."

What about fasting for general health and wellbeing? Well, this 2020 research suggests that intermittent fasting in short time windows is safe for most adults while similarly, the National Institute of Health confirms that preliminary findings indicate fasting could have a positive impact on blood sugar control, blood pressure, and inflammation. That said, there's no conclusive research proving the health benefits as of yet.

5 benefits of intermittent fasting

Mostowfi shares that she uses intermittent fasting alongside a balanced, nutrient-dense diet when clients come to her hoping to lose fat. She advises fasting as it puts the body into a state of ketosis, resulting in the body beginning to burn fat for energy instead of glucose. 

However, Mostowfi caveats that it’s not a suitable approach to fat loss for everyone. "You must make sure your blood sugars are balanced and that the meals you do eat contain all three macronutrients - protein, good fats, and at least some starch," she says. 

Benefits after a period of fasting, Mostowfi tells us, are proven with blood test work from her own clients before and after periods of fasting. These have shown reduced blood pressure, improved antioxidant activity, and improved triglycerides levels (which when high can increase the risk of heart disease), and less inflammation in the body, she explains.

The expert goes on to add that studies suggest fasting may improve insulin resistance, blood sugar, blood pressure, inflammation, short-term weight loss, and brain health. "However, longer-term studies are needed to confirm this," she adds.

5 cons of intermittent fasting

While there is some research to suggest that intermittent fasting can support fat loss, it’s important to dig deeper into these studies and what exactly it is that they're measuring, warns nutritional therapist Hannah Alderson

"There is a whole host of research (including this study that looked at the impact of fasting on adults) that supports intermittent fasting and fat loss – but if fat is the only quantified result that the study has looked into, you have to question what impact this has had on muscle mass, hormonal health, electrolyte balance and nutrient status," she explains.

What’s more, most of the research into intermittent fasting doesn't take into account the impact it can have on female hormones. “If you have a hormonal condition like polycystic ovarian syndrome, you need to be careful around long morning fasting and high-intensity exercise when in a fasted state," Alderson warns. "Both of these have the potential to drive elevated cortisol, which can then elevate your testosterone levels and insulin resistance. Plus, there is a window of vulnerability for women with endocrine disorders like PCOS and risk of disordered eating."

It's clear that more research needs to be done to make sense of the current findings on the subject. Mostowfi agrees with Alderson on the above and warns that anyone with hormone imbalances, thyroid issues, or difficulty balancing blood sugar, not to mention those who might be or are pregnant and those who have previously had an eating disorder or disordered eating should steer clear of intermittent fasting. 

"I tried intermittent fasting, but found the disordered eating habits it triggered outweighed the health benefits."

Marie Claire UK Health Editor Ally Head tried intermittent fasting in her early 20s - but she warns that there isn't enough research proving the health benefits, plus that it can become all too restrictive.

"I tried intermittent fasting for around a year or so during my third year of university. I'd read some studies about the health benefits you can see if you eat within an eight-hour window so thought I'd give it a go."

"That said, there are so many variations of intermittent fasting out there - a few of which are, by definition, fad diets - that the landscape quickly became difficult to decipher. I went from eating a nutrient-dense 2,000 calories a day but within an eight hour time slot to aiming for around a quarter of our recommended daily calorie allowance pretty quickly, showing you how quickly you can become convinced to give these clearly extreme diets a go."

"Let's be clear here - some of the iterations of the diet are dangerously low calorie and left me with little to no energy to study or workout. Case in point: A recent study in Canada found a link between diets like intermittent fasting and disordered eating in young women. You only have to search the term on TikTok or YouTube to find thousands of videos glamorising the eating method."

"Nutritionist at Renpho Lee Mitchell agrees, adding: "Focusing on restrictive eating patterns rather than balanced nutrition can contribute to an unhealthy relationship with food. Individuals might start to associate guilt or shame with eating, which can lead to a cycle of restriction and overconsumption, fostering an unhealthy approach to nourishment."

"What's more, Alderson points out that intermittent fasting diets don't take into account the food you're fuelling your body with. "There is a large focus on calorie content with intermittent fasting diets like the 5:2 diet, which can overlook nutrient status and blood sugar balance," she says. "Following the 5:2 rules you could eat 500 calories of ultra processed candy for your daily intake."

"Not to mention the fact I hadn't taken into account the blood glucose spikes my body was already experiencing as a result of my PCOS (fasting can make these insulin spikes worse for women with the condition)."

"Now, I stick to three meals and one snack most days (other days, I have three snacks - I listen to my body and respect my own hunger cues). I do believe that giving our bodies a break from eating can be great for digestion, that said, it can also trigger disordered eating patterns, and the benefits don't outweigh the potential negatives, in my opinion."

"There simply isn't a one size fits all when it comes to health, which is why I think it's so important to see a qualified professional before you consider starting any new diet or lifestyle."

The verdict on intermittent fasting?

More research is certainly needed to determine the impact of intermittent fasting on our overall health, not to mention the safety of practising this eating habit long-term. 

Most nutritionists believe that some form of time-restricted eating can be beneficial to health, that said, they confirmed that it's also something most people already do without realising it. After all, the term breakfast comes from the idea of breaking a fast after a night of sleep. "Not eating after 8pm and until 8am the next day is generally considered safe and is a fairly common pattern for many people already," Mostowfi notes.

All approaches to diet and lifestyle should be personalised and the same goes for fasting, adds Cynthia Thurlow, intermittent fasting and nutrition expert and spokesperson for Lumen. There are even stages within the menstrual cycle that are more suitable for fasting than others, so it's worth taking this into account, too. "The first three weeks of your cycle are the best times to fast if you have a 28-day cycle," Thurlow says. "Your hormones are more stable, and it's a great time to decrease insulin, reduce inflammation, and activate autophagy."

She adds: "Fasting during the five to seven days preceding your menstrual cycle may unknowingly lead to the depletion of nutrients and hormones necessary in the luteal phase."

On the other hand, some experts promote alternative methods altogether. "Instead of emphasising extreme fasting methods, I advocate for a well-rounded, individualised approach to nutrition," says Mitchell. "This involves consuming a variety of nutrient-dense foods in appropriate portions throughout the day to support optimal health and wellbeing. By adopting a sustainable and inclusive approach to eating, you can avoid the pitfalls of disordered eating and promote a healthier relationship with food and your body.”

What is the 5:2 intermittent fasting method?

The 5:2 fasting method involves eating normally at any time five days of the week and fasting for the remaining two days of the week. On the fasting days, you consume around 500 to 600 calories. 

"I am strongly against the 5:2 diet," says nutritional therapist Hannah Alderson. "It suggests a grown adult should aim to consume around half that of what you would expect a toddler to eat – not to mention, there's a large focus on calorie content which can overlook nutrient status and blood sugar balance," she explains. 

What is time restricted eating?

"Time-restrictive eating is a type of intermittent fasting that has shown to significantly increase metabolic health due to improvements in insulin sensitivity, oxidation and inflammatory levels and even the weight by just restricting the "eating window" during the day, more specifically, between no more than eight to ten hours during daytime," explains Ulrike Kuehl, Registered Dietitian at Lumen. "Intermittent fasting can improve your 'metabolic flexibility' because it helps you burn through your carb stores during the fasting period and helps use more fat for fuel."

Can your menstrual cycle impact intermittent fasting?

More research is needed, but as Cynthia Thurlow, intermittent fasting and nutrition expert and spokesperson for Lumen points out, your cycle can determine whether or not you should be fasting.

"The first three weeks of your cycle are the best times to fast if you have a 28-day cycle," she explains. "Your hormones are more stable, and it's a great time to decrease insulin, reduce inflammation, and activate autophagy." She adds: "Fasting during the five to seven days preceding your menstrual cycle may unknowingly lead to the depletion of nutrients and hormones necessary in the luteal phase"

She adds that if you are pregnant or trying to conceive, you should not try any form of intermittent fasting. 

Ciara McGinley
Contributing Health Writer

Ciara McGinley is a freelance journalist, editor and mindfulness meditation practitioner. She covers health, wellbeing and lifestyle topics for her favourite women's lifestyle publications including Marie Claire, Stylist, Red Magazine and Woman & Home. She's all about betting that mind-body connection, and takes her self-care and night-time routine very seriously... When she's not writing or teaching meditation, you'll find her trying out the latest wellness trend, or escaping London for a hiking weekend.