You almost certainly have a friend, colleague, or aunt on Facebook who raves about intermittent fasting — a weight-loss trend that, according to its devotees, helps you shed fat without any of that pesky dieting and exercising.
There are many forms of intermittent fasting (also known as time-restricted feeding), but they're all characterised by periods where you eat a lot, followed by periods where you eat very little, or nothing. Popular methods include:
The 5:2 diet: On five days of the week you eat normally, but on two non-consecutive days, you cut calories to about a quarter of your regular intake.
Alternate-day fasting: Similar to 5:2, except you eat normally one day then cut to about about a quarter of your regular calories the next.
The 16:8 diet: Within every 24-hour period, you fast for 16 consecutive hours of your choice, then eat in the remaining eight. So you might wait until 10am to have breakfast and finish your dinner by 6pm, and only consume water (or some other zero-calorie beverage, such as black tea and coffee) outside that time window.
Preliminary studies into intermittent fasting diets suggest they do indeed help some people lose significant amounts of weight.
But there are a couple of caveats, and a big one is that intermittent fasting doesn't appear to work any better than old-fashioned calorie restriction — that is, you'd lose the same amount of weight just by eating less overall.
That's backed up by new German research published in the American Journal of Clinical Nutrition, touted as the largest study into intermittent fasting so far.
A research team from the German Cancer Research Center (DKFZ) and Heidelberg University Hospital recruited 150 overweight people and put them on one of three diet plans for a year:
One group cut their overall daily calories by 20 percent; a second group followed the 5:2 diet, effectively cutting their overall weekly calories by 20 percent; and a third group was simply advised to follow government dietary guidelines.
At the end of the 12 months, both the calorie-cutters and the 5:2 dieters lost about the same percentage of body weight on average, including similar amounts of visceral fat — the fat stored deep inside the abdomen, which is particularly bad for your overall health.
Nor did the researchers find any differences in any metabolic values, biomarkers or gene expression between the two groups.
The study's results suggest there's no magic secret to the 5:2 diet, which doesn't work by sparking changes in your body that magically accelerate fat loss — it essentially just tricks you into eating less, which is ultimately the most important factor to lose weight.
"For some people it seems to be easier to be very disciplined on two days instead of counting calories and limiting food every day," explained the study's lead author, DKFZ epidemiologist Tilman Kühn, in a statement.
Note that use of "some people", because there's another big caveat to intermittent fasting: it won't help everyone lose weight.
In Kühn's study, both the calorie-cutters and the 5:2 dieters lost weight as a group — but there were significant individual variations within those groups, and some participants on both diet regimens even gained small amounts of weight.
The "best" diet for weight management is ultimately one that's balanced, not too restrictive and sustainable over the long term. If intermittent fasting ticks all those boxes for you, give it a try. If it doesn't, keep experimenting to find something that does.