For most of our evolutionary history, intermittent fasting wasn’t voluntary. With food scarce and storage difficult, going for hours, or even days, without eating was a common occurrence. Fast-forward to today, though, and most people reading this will never be far from a quick snack.
That stark contrast in eating habits has left people wondering if those food-free periods were beneficial. And if they were, whether we should reintroduce them.
The quandary we face, however, is that we generally live a lot longer than our hunter-gatherer ancestors did, so intermittent fasting might have downsides for us that they never experienced.
For the sake of clarity, intermittent fasting typically refers to a diet plan in which you restrict your eating to certain times or on certain days: for instance, only eating during an eight-hour window within a 24-hour period (often referred to as 16:8 eating) or restricting the calories you consume on two days out of every seven (otherwise known as the 5:2 diet).
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The pros
Firstly, yes: fasting can probably help you lose weight, especially fat. “When you eat fewer times in a day, you generally eat less overall,” says Dr Jason Fung, author of The Complete Guide to Fasting. “Fasting builds structure around the eating day. It’s also simple, flexible and intuitive.”
Often, when we try conventional calorie-restricted diets various things happen that conspire to keep us the same size: we get hungry all the time, we move around slightly less and we use less energy.
Fasting seems to mitigate a few of these problems: in one recent small, but controlled, study, volunteers who restricted eating to a six-hour daily window managed to expend the same amount of energy as normal, while seeing a reduction in their levels of the hunger hormone ghrelin alongside a decreased desire to eat.
There’s also evidence that fasting helps your body burn fat. “When you eat constantly, you’re constantly dumping insulin into your blood, which cues the body to store fat,” says gut-health expert Dr Daryl Gioffre. “Fasting helps you create ketones, a chemical that your liver produces when it breaks down fats, which your body can then use for energy.”
Intermittent fasting also reduces the time our bodies spend in a digestive state, with raised blood sugar and blood fat. As such, it seems to be linked to a reduction in the risk of type 2 diabetes and might improve vascular health – though some of these benefits might come from weight reduction and calorie restriction.
“There’s overwhelming evidence that intermittent fasting is beneficial for cardiometabolic health,” says Brady Holmer, a researcher at Examine.com who’s conducting PhD research on cardiovascular physiology.
“It’s hard to say what the longer-term effects are because most studies on the topic are, at most, 6-12 months long. But the research I’ve read indicates intermittent fasting improves risk factors for long-term cardiovascular health like weight, body fat percentage, blood lipids and blood pressure. If intermittent fasting is good for these outcomes up to around a year, I find it hard to believe that practising it for longer would lead to adverse health outcomes.”
Finally, there’s some evidence to suggest that fasting can protect against other age-related issues: for instance, by improving autophagy, our body’s way of cleaning up damaged or dysfunctional cellular material.
This, along with some of its other effects, is leading some experts to speculate that fasting might improve brain health in old age, or reduce cancer risk, although more research is needed.
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The cons
So are there any downsides? Recently, research presented to the American Heart Association grabbed attention with claims that restricting eating to a period of fewer than eight hours per day was linked with a 91 per cent higher risk of death due to cardiovascular disease (with the researchers suggesting that one cause might be a reduction in muscle mass).
It’s important to note, though, that this wasn’t a peer-reviewed study published in a scientific journal, but a scientific abstract.
Firstly, the data came from an observational study based on dietary recall over two 24-hour periods – meaning that the people being studied weren’t necessarily doing deliberate fasting, but just remembered to eat in an eight-hour window for a couple of days.
There was also an issue with sample size: “Though the total size of the study is over 20,000 people, the group studied was around two per cent of the total, with only around 30 cardiovascular events,” says nutritionist Drew Price.
“This is a problem because it means a greater chance of your sample not being representative of the general population. In fact, it looks like the eight-hour group had a high proportion of smokers, who are more likely than the general population to die of cardiovascular disease.”
What about the researchers’ worries concerning muscle mass? Body composition can be tougher to study than other outcomes over the long term, but short-term studies are actually in fasting’s favour.
“With standard calorie-restricted diets, about three-quarters of weight lost is fat mass and the other quarter is fat-free mass [muscle]. However, some data shows that intermittent fasting might be better in terms of this fat loss ratio,” says Price.
The biggest issue with fasting might be the same one that hinders other types of diet: certain people might not find it a helpful way to eat.
Some reviews cite how people can struggle with low protein intake when fasting intermittently. People with a history of disordered eating or who frequently overeat when they’re hungry, should seek professional advice before fasting (as should anyone with health issues that require regular meals).
So what if you’re considering trying intermittent fasting?
“My perspective as both a nutritionist and former strength and conditioning coach is that the research hints at doing the same things as you would on a standard calorie-restricted diet,” says Price.
“Make sure you’re getting enough protein to mitigate any issues with muscle loss. Also, a small amount of resistance training goes a long way – one or two 20-30-minute sessions a week can really help.”
If you’re concerned about the lack of long-term evidence on fasting, it’s worth considering that you can approach it in the same way you would a regular diet: something to do occasionally, rather than forever. And, as with any diet, what you eat is still important and needs careful consideration – especially when you’re ready to break your fast.
About our experts
Dr Jason Fung is a nephrologist, intermittent fasting expert and the author of three best-selling books The Diabetes Code, The Obesity Code and The Complete Guide to Fasting.
Dr Daryl Gioffre is a gut-health expert and founder of Alkamind. His work has been published a number of times as books Get Off Your Acid, Get Off Your Sugar: Burn the Fat, Crush Your Cravings, and Go From Stress Eating to Strength Eating.
Brady Holmer is a researcher for examine.com with a BS in exercise science and MSc in Human Performance. He is conducting PhD research on cardiovascular physiology. You can find his published work on his Physiologically Speaking Substack.
Drew Price is a nutritionist with clinics in the City of London and Harley Street and has worked with Chelsea Football Club, Leeds Rhinos and other individual top-level athletes. His book, The DODO Diet or Day On, Day Off Diet is available to purchase now. He has also been published in Men's Health and Men's Fitness.
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