I have been writing about intermittent fasting, particularly the 5:2 approach, for some years now. In my book, the Fast Diet, I originally suggested that people doing 5:2 should cut their calories on their two “fasting” days to around 600 calories, I have since (the new 5:2) been recommending a more generous 800 calories a day as well as eating a healthy Mediterranean-style diet , whether you are fasting or not.
Since I wrote the Fast Diet there have been further human studies showing its safety and effectiveness, which I will go into in a moment. But there has also just been a negative rat-based study, presented at the European Congress of Endocrinology in Spain this week, which suggests that doing intermittent fasting might increase the risk of getting type 2 diabetes.
In this particular study young rats were put them on an absolute fast (nothing to eat at all) every other day for 3 months. The rats lost weight but, worryingly, they put on fat, particularly around the abdomen. The researchers note that their stomachs “greatly increased in size”
As a result of this swelling girth the rat’s pancreases became increasingly full of fat and their blood sugar control got worse.
This is a surprising finding because it flies in the face of so many other animal and human studies of intermittent fasting. The poster does not say what the rats were allowed to eat on their “non-fast” days, but if they were allowed to gorge this may have skewed the results.
I would not recommend this particular regime as doing an absolute fast (going without any food) every other day for months on end would be very hard to stick to. As this study suggests it could also be unhealthy; you need adequate levels of protein in your daily diet to maintain your muscle mass.
So that was a rat study, using an extreme version of intermittent fasting. What about human studies using more sensible approaches to intermittent fasting, in particular the 5:2 approach? Well there have been a number of studies in recent years and they have been consistently positive.
A randomised controlled trial published in 2013 with 115 women comparing two days a week of calorie or carb restriction with a standard steady weight loss approach showed that the two day fasters lost nearly twice as much weight over the first three months as steady dieters and an average of 7.7kg by the end of six months. Most of the weight loss was fat and they saw big improvements in their insulin sensitivity (a measure of diabetes risk).
In a more recent study from Surrey University, published in the British Journal of Nutrition this March, researchers got a group of overweight volunteers (41 men and women) and randomly allocated them to either the 5:2 diet or a standard calorie restricted diet (the women ate 1400 calories a day, the men 1900) . Both groups were then asked to lose 5% of their body weight.
So what happened? Well more of the 5:2 dieters hit their 5% target and they did it faster: they took 59 days while the steady dieters took 73 days. And when they were re-measured at the end of the study there were some interesting differences between the two groups.
The people who had been following the 5:2 diet were now able to clear the fat from their blood faster and more efficiently after a meal than those who had lost weight through daily dieting. The 5:2 dieters also saw an average drop in blood pressure of 9%, compared to a small increase of 2% among the daily dieters. There were no significant negative effects reported in either group.
Another very recent study, carried out at the Karolinska Institute in Sweden (home of the Nobel Prize) with 94 overweight and diabetic patients over a two year period also saw big falls in weight amongst the 5:2 dieters, as well as significant improvements in blood pressure, insulin resistance and cholesterol scores.
More anecdotally, I have received thousands of messages over the years from people who have done the 5:2. Many have reported fantastic results, others have struggled. None have reported significant side effects beyond the usual things you encounter when dieting like hunger, irritability or headaches.
Finally, I would point to a really important randomised controlled trial of more than 300 type 2 diabetics published a few months ago in the Lancet. Many of those allocated to an 800 calorie diet every day for 12 weeks not only lost large amounts of abdominal fat but were then able to come off all medication. Scans of the pancreas and liver showed they were far healthier than at the start of the trial.
So the moral of this story is that intermittent fasting can be a safe and effective way to lose weight but do choose a well tested version!