You probably first came across it with a pale-looking colleague slumped over their office desk. Or with The Fast Diet author Michael Mosely speaking effusively about it on television. Fasting, they’d have told you, is a great way to lose weight. It makes sense: eat fewer calories a couple of days a week, and don’t overeat on the others, and you’ll slim down. What’s less clear is the assumption that fasting from time to time can bring other benefits such as avoiding disease, keeping your brain sharp and even letting you live longer. With all this for the price of just a sprinkle of willpower though, surely it’s all too good to be true?
The answer is not straightforward. Perhaps unsurprisingly, the evidence is strongest with type 2 diabetes – a disease often caused by overeating. The disease means that a person can no longer control their blood sugar levels. Once diagnosed they are left staring down the barrel of a lifetime on medication, unless, think researchers at Newcastle University, they begin to fast.
They’ve tested an extreme low-calorie diet – a hunger-panging 600 calories a day for eight weeks – in 11 people with type 2 diabetes: all were disease-free by the end of the fast; seven were still disease-free three months on. Later studies suggest that the sooner people fast, the better their chances of reversing their disease. Roy Taylor, who leads the group, thinks that fasting is beneficial because it gets rid of dangerous fat in and around your organs, including two that are important in sugar control – the pancreas and the liver.
When an otherwise healthy person’s blood sugars get too high, their pancreas makes a hormone called insulin that tells the liver to remove the sugar and store it safely. “If you have fat around these organs it clogs up the way they work and your body can’t control its blood sugars,” says Taylor. After about 12 hours of fasting, he says, the body uses up all the glycogen in the liver, its go-to source of energy, and starts to dip into its fat deposits. “The first type of fat to go is that dangerous fat around the organs, freeing them up to do their job properly.” He stresses that people with diabetes should not fast without consulting their doctor – a combination of insulin drugs and fasting can be lethal.
Taylor and his colleagues are now testing their fasting diet in around 300 people with type 2 diabetes. The results of that study will give a better idea of how beneficial the diet can be. The question is how much of the effect is down to fasting and how much is down to just the weight loss? “It’s almost certain that other forms of dieting will do the same,” says Taylor. “But this low-calorie diet is one that I was confident would let people lose the roughly two and a half stone, or a sixth of their body weight, that we were looking for.”
There is, though, reason to believe that fasting might have benefits over and above weight loss. It’s down to what happens to all living organisms when they don’t have food – they begin to eat themselves. Gruesome, maybe, but it’s beneficial: it lets the body recycle energy and do some housekeeping – the first cells to go are the faulty ones.
Valter Longo is a scientist at the University of Southern California who believes that, because of this process, periodic fasting can help people stay healthy. Faulty immune cells, for instance, could be pruned back so that when a person starts to feed again, new cells are spawned from only the strongest and the fittest.
In experiments in mouse models of multiple sclerosis, a disease in which rogue immune cells erroneously attack a person’s nerve cells, he’s seen that periodic, low-calorie fasting can slow down the destruction of cells and even lead to some regeneration. His preliminary work in people with the disease suggests it could improve their quality of life.
The potential reaches further. Fasting-mimicking diets can help people with cancer undergoing radiation chemotherapy, presumably by promoting the growth of healthy cells and restricting the growth of cancerous ones. Restricting the amount a mouse eats by about 30-40% can extend its lifespan by a third.
This year Longo showed that a fasting-mimicking diet could help mice with diabetes regain blood sugar control, not only those with type 2 but also those with type 1 diabetes, caused not by overeating but by a faulty immune system. The benefits, he says, were down to a reprogramming of beta cells, a type of cell in the pancreas that makes insulin. He also starved cells taken from people with type 1 diabetes and saw a similar reprogramming.
“These results are surprising and completely new territory,” warns Gordon Weir, a diabetes researcher at Harvard Medical School. “I’d be cautious about assuming that fasting will help people with type 1 diabetes until the mouse studies are replicated in other laboratories and it has gone on to be shown to work in human beings, not just in human cells.”
Longo, too, is wary of giving false hope but is bullish about the potential of fasting. “In research over 25 years we’ve seen it in E coli bacteria, in yeast, in human cells, and in mice,” he says. “The foundations are so deep that it’s as old as life itself, but we have to respect the complexity – a yeast is a yeast, a mouse is a mouse, and a person is a person.”
The difficulty in transferring a theory from mouse to man is that people live much longer than mice. At middle age we are much farther from when our stem cells, the type of cells that make other cells, are most active, so our ability to generate new cells might not be as strong.
“We don’t have conclusive data that any of this works in humans,” Longo says, “but we do have some promising data.” He’s referring to a study of 100 generally healthy people given a fasting-mimicking diet low in calories, sugars and protein but high in unsaturated fats. Despite only a minor reduction in weight loss, he says, risk factors for ageing, diabetes, cancer and cardiovascular diseases such as heart attack and stroke were all improved. He’s planning a bigger trial in 250 people to confirm these findings and to figure out which benefits are the result purely of the act of fasting and not just the result of weight loss.
Other tests will take a little longer. Whether fasting will ever make us live longer, given the time needed to prove it, will be for only Dracula and Dorian Gray to know. What could be more compelling is the idea that fasting can keep us in better mental shape.
When the body metabolises its fat deposits during fasting, says Mark Mattson, a neuroscientist at the US National Institute on Ageing and Johns Hopkins University in Baltimore, it produces acids called ketones, a source of food for brain cells. Ketones also trigger the production of a chemical called brain-derived neurotrophic factor, which encourages the brain to make new connections.
It’s not an entirely new concept; in fact, the ancient Romans stumbled across it. Roman doctors found that by locking epileptics in a room with no food for a few days they could cure them of their disease. “They thought they were causing demons to go away but really these people’s ketones were increasing and suppressing their seizures,” says Mattson. Today, ketogenic diets that increase ketones by mimicking fasting are increasingly prescribed to people with epilepsy to help them control their seizures.
Mattson’s experiments in mice suggest that fasting could slow the onset of brain diseases such Alzheimer’s and Parkinson’s disease. “We’ve also got evidence in mice that fasting reduces anxiety and depression,” he says.
So far so good, but mouse does not equal man. The way you test anxiety or depression in a mouse is by chucking it into a beaker of water or dangling it by its tail. While we can all empathise with how that mouse might feel, the relevance of these studies to us with our more complicated lives and more complicated brains remains to be seen. Still, these are the same tests drug companies use to find promising antidepressants, so there might be something in it.
That fasting might have a beneficial effect on our brain makes sense from an evolutionary perspective. If our caveman ancestors hadn’t eaten for a few days it would make sense for them to do something about it. “This ketone signal tells the brain ‘hey, brain, you better figure out how to get some food because if you don’t there’s going to be a problem soon’,” says Mattson. “Now we’re eating three meals a day plus snacks so we’re never going to raise our ketones. If we fast from time to time, maybe we can take advantage of this evolutionary adaptation to help us in modern life.”
Like most people, if I’m going to skip a sandwich to help my inner caveman, I want him to be as pumped up and raring to go as Rocky at the end of a training montage. The problem is that nobody knows exactly how you’d do that.
“Simply too few studies have been done to know the long-term effects in people,” says Susan Jebb, a nutrition scientist at the University of Oxford. “There’s clearly something about not putting food in your system that’s beneficial, especially for diabetes, but how close to fasting do we need to get? Is it the 5:2 diet or is it long periods of a low-calorie intake? Do we need to eat only 600 calories or can we get away with 1,200?”
One reason for the paucity of studies is the lack of money to be made. With no drugs to sell, drug companies are not testing it. Nobody is suggesting they are sitting on data or getting skinny professors whacked, it’s just that it’s not their responsibility. “Pharmaceutical companies are there to make useful drugs and to turn a profit,” says Taylor. It’s as simple as that.
In lieu of evidence that periodic fasting is beneficial, we should consider the potential harms – which are few for overweight people. People with medical conditions, especially diabetes, should consult their doctor first. People should not do water-only fasts, which cause your body to start breaking down its own proteins. Messaging needs to be careful not to condone eating disorders such as anorexia.
With so much unknown about the relationship between fasting and health, Jebb urges that we don’t lose sight of the basics. “We know that if you’re overweight, losing weight will reduce your risk of disease,” she says. “For many people an intermittent fasting diet will help them lose weight, for others eating a few less biscuits every day will be better. The trick is to find the diet that works for you and go for it.”
Fast habit, free
A no-nonsense stopwatch app. Tell it how many hours you want to fast for then press a button to start. It tracks your fasting over time and, helpfully, lets you edit your record in case you “forget” to log a fast.
Zero – Fasting Tracker, free
Zero has two predefined fasting plans: 5:2 and another one based on work done by US researchers that suggests fasting has added benefits if done at night. It uses your phone’s location to remind you when the sun will set. You can download your data to a spreadsheet and geek out over long-term performance analyses.
5:2 Diet TrackMyFast, 99p
Despite having 5:2 its title, this app has other plans including alternate day fasts and the frankly weirdly named Johnson Up Day Down Day Diet. The usual weight and fasting tracking functions are supplemented with recipe ideas, which you can contribute to and share with other users.
5:2 Diet Complete Meal Planner, £1.99
This app is just a collection of recipes within different calorie brackets. Useful, but it’s tough to justify the price given that lots of recipes are available for free online. Warning: the recipes look incredible but when you make them they come out tiny.
MyFitness Pal Calorie Counter, free
Not a fasting tracker per se but contains a massive database of foods – more than 4m can be scanned by barcode – to help you manage your calorie intake.
