One of the first rules of dieting is that to lose weight, you have to burn off more calories than you take in. But cut back on calorie intake too much or for too long, and the body responds by going into energy-conservation mode—slowing down the rate at which those calories burn, which can counteract those good intentions.
Now, Australian researchers say they may have a way to make dieting more efficient and to keep the body’s metabolism humming along at its normal clip—which means more pounds lost (and kept off) in the long run. The secret, they say, is taking a break from dieting every few weeks.
In their new study, published in the International Journal for Obesity, researchers from the University of Tasmania found that obese men who dieted continuously for 16 weeks lost less weight overall—20 pounds versus 31—than those whose diets followed a 2-weeks-on/2-weeks-off cycle for 30 weeks. The continuous dieters also lost less body fat than those in the intermittent group.
The intermittent dieters kept more of their weight off for the long-term, too. Six months after their diets had ended, the on-and-off group had maintained the most total weight loss since the start of the study—about 24 pounds versus only about 7.
So why did the on-again, off-again diet work so much better? The researchers think it has to do with something called adaptive thermogenesis—a process by which a person’s resting metabolism decreases when calorie intake is slashed. It’s a survival mechanism that’s helped humans stay alive during lean times (it’s sometimes called the “famine reaction”). But when an overweight person tries to lose weight, it can also work against them.
By limiting periods of calorie restriction to two weeks at a time, the authors believe they kept the famine reaction at bay—which allowed the study participants to burn more calories during those dieting periods.
To conduct the study, the researchers provided meals during the study period. Overall, each group was assigned to 16 weeks of dieting, during which the men reduced their daily weight-maintenance calorie requirements by 33%. (On average, participants ate about 900 to 1,000 fewer calories per day during diet weeks.)
But while men in the continuous diet group stuck with their plan for 16 weeks straight, those in the intermittent group cycled on and off their diet every two weeks. During their off weeks, they ate their full caloric requirement—the number of calories required for weight to stay the same day-to-day, based on resting metabolic rate and self-reported physical activity levels.
Because of that, weight loss (or gain) during those off weeks was minimal. “Therefore, the greater weight loss in the [intermittent] group can be attributed to a higher rate of weight loss during the 8 x 2-week [energy-restriction] blocks, and not simply continual eight loss over a longer (30-week) intervention period,” the authors wrote in their paper.
Before you try the two-weeks on, two-weeks off diet strategy, though, know this: The authors were quick to point out that strict calorie-counting was also important during the non-diet weeks. Participants didn’t just eat whatever they wanted; they ate only what they needed to maintain a stable weight.
And that may be why the back-and-forth approach worked so well in this study, the authors say. In real life, taking a break from dieting could lead to an abnormally large appetite and overeating, “which may compromise weight loss,” they wrote.
They also point out that intermittent-fasting diets—programs that alternate no-holds-barred eating with several days very little or no food at all—don’t seem to work any better than continuous, steady dieting. “As such, incorporating periods of controlled energy balance, not simply variations in energy intake, may be necessary to realize the beneficial effects” of on-again, off-again dieting, they wrote.
The study was small (32 participants completed it), and it could not determine whether two weeks on, two weeks off is an optimal pattern—just that it worked better than continuous calorie-cutting. And because the study only included men, it’s unclear whether the same would be true for women. More studies are needed, the authors say, to see if this plan would still be effective outside of a tightly controlled lab setting.
Still, the authors concluded, their findings provide preliminary support for an on-and-off calorie restriction, and suggest that it may be a “superior alternative” to continuous diet plans.