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When Working Out Doesn’t Work

A Danish study reveals that, when it comes to working out and losing weight, unintended consequences can hamper even the best of intentions.

There’s a park near my house where I like to run just before dinner. Afterward, my favorite meal to cook—especially when my girlfriend is out of town—has a kind of simplicity that would make even a chef like Mario or Martha envious: Prepare one box of macaroni-and-cheese, with plenty of butter. Add one can of baked beans. Stir and serve. The Redmon Running Special is sweet, salty, and best eaten in bed, while watching back-to-back episodes of “The Wire.”

I’ve been a dedicated runner for some time now, and my heart-rate monitor logs several hour-long sessions a week, yet my jean size hasn’t changed an inch. A recent Danish study, published in the American Journal of Physiology, reveals that I have plenty of company—and that the Redmon Running Special is probably to blame.

Physiologists have long understood that humans “compensate” in response to big workouts: the body protects energy stores (i.e. fat) by increasing appetite, operating more efficiently, and decreasing what doctors call “non-exercise activity thermogenesis,” or NEAT—essentially, all the little calorie-burning activities we do when we’re not sleeping, like climbing stairs, hauling groceries, and raking leaves. (Not surprisingly, an overall decrease in neat is one of the reasons Americans, especially those who hold sedentary jobs and depend heavily on their cars, are thought to be packing on pounds at an alarming rate.) In other words, our animal brains begin to treat trips to the gym as a reason to eat more than usual (hello, R.R.S.) and veg out.

Researchers at the University of Copenhagen were surprised to discover just how pronounced that compensation effect was. Sixty moderately overweight Danes, younger than 40 and living sedentary lifestyles, were recruited for the three-month study. Twenty were assigned to do nothing; 20 were prescribed 30 minutes of strenuous daily exercise; the remainder were told to work out twice as long, for a full hour. Heart-rate monitors kept close track of the participants’ adherence to the regimen, as did daily food logs and follow-up visits to the lab, where investigators measured their body fat and peak oxygen consumption.

“High” exercisers were burning twice as many calories each session as “moderate” exercisers, and both groups showed equal levels of commitment to the workouts. After 13 weeks, however, they showed similar reductions in weight and body fat; while the high exercisers were burning twice the calories&mdfash;and their monitors showed good compliance with the workouts—it was the moderate exercisers who achieved slightly better overall results.

Both groups were expected to produce a “negative” energy balance—that is, they were expected to burn more calories than they consumed during the study. And that happened. But when the researchers looked deeper, they found that the moderate group had burned far more calories—83 percent more—than could be explained by exercise alone, while the high group burned 20 percent fewer.

Compensation was clearly hard at work: the hour-long workouts likely revved up subjects’ appetites and depressed their neat, while the more sane sessions didn’t leave participants voraciously hungry and too exhausted to do anything but lounge on the couch. In other words, the moderate exercisers reaped both the rewards of their workouts and a NEAT “bonus” (they stayed active even when they weren’t in the gym) which gave them better results than the turbo exercisers, even though the latter were working twice as hard.

The takeaway for us well-intentioned-but-fallible individuals is to always mind the compensation effect. For public health officials designing anti-obesity campaigns, the message is an even more important one: smaller efforts may mean bigger results.