It's been a few years since the 5:2 diet became the hit diet for slimmers everywhere and while many people have now switched to a more clean-eating based diet plan, or upgraded to the 16:8 diet, four years later, it still has a cult following. But, does it work?
published in the JAMA Internal Medicine journal shows that yes, it does aid weight-loss but it's not much more effective than other means of calorie restricting.
The researchers took 100 obese people from Chicago and put them on year-long diets. Randomly assigned, some people were put on alternative-day fasting diets, some on daily calorie restrictive diets and some on no diet at all. By the end of the yearlong diet sessions, those in the alternate-day fasting group lost around 6% of their original body weight, whilst those in the calorie restriction group lost 5.3%.
Although both the alternate-day fasting and calorie restriction groups had similar weight loss on average, the researchers found that a higher percentage of people in the fasting group cheated on their diets compared with the calorie-restriction group. The dropout rate in the alternate-day fasting group was 38%, versus 29% in the calorie-restriction group.
"We basically showed that they both produce a clinically significant amount of weight loss," said Krista Varady, a professor of nutrition at the University of Illinois in Chicago and lead author of the study.
"Instead of being better than calorie restriction, it's the same," she said of intermittent fasting. "So it's kind of like an alternative to calorie restriction."
Don't be too disappointed though, because while alternate day fasting may not be living up to its hype, researchers at Harvard University have commented that intermittent fasting may still be very effective, but only under the right conditions. "A growing body of research suggests that the timing of the fast is key, and can make IF a more realistic, sustainable, and effective approach for weight loss, as well as for diabetes prevention," reads a post on the .
. Apparently, it's all down to "early-time restricted feeding", where you restrict the hours in the day in which you eat to 7am - 3pm - a new interpretation of the 8:16 diet.
So, why is fasting so popular and where did it come from?
Originally brought on the scene as an alternative to the Atkins diet or the more popular Dukan diet, people couldn't get enough of the idea of intermittent fasting to lose extra pounds after Michael Mosley's 2012 documentary, Eat, Fast And Live Longer, raved about it. His spin-off book, The Fast Diet, went on to sell over 400,000 copies in the UK alone.
Find out more about it below but remember, you should never calorie restrict without consulting your doctor first. If you're not eligible to do so, you could be putting your health at risk.
So how does 5:2 really work?
"The basic rules of the 5:2 go like this," says nutritionist Ruth Tongue. "Two days a week you eat just 500 calories (600 for men). This can be consumed at any time, and made up of any food you like. The rest of the week, you eat and drink normally. Nothing is off limits."
Well, that sounds pretty good to us. Ruth says: "It sounds amazing, until you're standing in Pret on a fasting day and realise a chicken and avocado sandwich contains 469 calories."
Ruth says it's true that some studies "have shown intermittent fasting can have health benefits and lead to weight loss."
"In Mosley's documentary," adds Ruth, "he lowered his risk of getting two of the most common life-threatening diseases, cancer and diabetes, by reducing not only his blood glucose and cholesterol levels to within the normal range, but also levels of a hormone called IGF-1 - known to be a key player in ageing and age-related diseases. As an added bonus, he lost around 19lb of fat in the process."
So it can't be all bad then?
"But what's not being publicised is that those benefits were mostly only seen in men," says Ruth.
Listen up, women - you could be doing serious damage to your health.
Ruth says: "What if we were to tell you that most of the (little) research out there has been done on men? That if you look past the intoxicating endorsements like, "revolutionary and clinically proven" or "amazing weight loss and optimum health", you discover that men and women seem to react quite differently to this type of eating plan?"
And worst of all: "That buried beneath the amazing claims, there's a risk it could even affect your chance of having a family in the future?"
According to Ruth, "in women, there was a negative impact on blood sugar levels and insulin control - risk factors for diabetes. What's also not being talked about is the potentially harmful effect that this type of dieting could have on your fertility. "68-day cycles", "bleeding between periods", "late periods and ovulation pain", "lack of periods completely"… these are just a few of the responses to a question about menstrual cycles on the 5:2 Facebook forum."
It seems crazy, as women, to toy with our health and ability to have children just for a slimmer body. Pregnancy and fertility expert Zita West warns: "any type of strict dieting can make women extremely tired - and it puts ovulation way down the body's list of priorities," she says.
Ruth agrees: "It may be only two days a week, but on those days you're denying your body an extra 1,500 calories recommended for the average woman."
Studies conducted on animals are scarier still, with biotechnologists in India finding decreases in ovary size, irregular menstrual cycles and infertility, all due to intermittent fasting. These changes in menstrual cycle were seen after just ten days, while a further animal study in the American Journal of Physiology found that fasting stopped ovulation in 80% of the test sample and reduced ovarian follicle size.
If these results on animals are at all similar to what happens to human bodies on the 5:2, it seems downright dangerous to continue with this harmful yo-yo diet - though effective it may be at helping to shed excess fat.
So should we quit 5:2, with its potential risk of infertility, diabetes and loss of periods? Ruth thinks so: "The really scary thing is that there is so little research into this, and any of the other potential side effects of intermittent fasting."
"It may be one of the most achievable diet plans to stick to, but no one knows exactly what the future consequences will be," adds Ruth. "For now, eating a balanced diet still remains the most efficient, reliable way to stay healthy. It might not promise a quick fix or be hyped in the headlines, but it works."
THIS is what balanced looks like
A healthy body without deprivation? Sign us up! Dr Amelia Lake, dietician and food lecturer at Durham University, shows the basics of a great, sensible diet.
- 15% Dairy "You want the equivalent of a pint of milk a day. It's easy to get: a yoghurt + a matchbox-sized piece of cheese + a day's worth of milk in your tea = done."
- 30% fruit & veg (5 portions) "Fresh, frozen, tinned, dried, juices… they all count. The more colours you eat, the more different vitamins and minerals you'll get."
- 15% protein (meat, fish, eggs, beans, pulses) "You want 2-3 portions a day: a portion is an egg or a deck-of-cards-sized amount of meat/fish."
- 10% fat and sugar "One portion a day (eg 2 biscuits) is fine. Keep rest of intake unsaturated fat (eg olive oil, nuts, avocados)."
- 30% starchy foods "Starch - so, potatoes, bread, rice, pasta - should make up a third of every meal we eat as it's our main energy source. Keep it wholegrain, and leave the skins on potatoes."