Afterward, everyone returned to the lab for comprehensive remeasurements. As expected, the control group’s numbers, including their weights and resting metabolic rates, had not budged. But neither had those of most of the exercisers. A few had dropped pounds, but about two-thirds of those in the shorter-workout group and 90 percent of those in the longer-workout group had lost less weight than would have been expected.
They had compensated for their extra calorie burn.
But not by moving less, the scientists found. Almost everyone’s activity-monitor readouts had remained steady. Instead, the exercisers were eating more, other measurements and calculations showed. The extra calories were slight — about 90 additional calories each day for the some-exercise group, and 125 a day for the most-exercise set. But this noshing was sufficient to undercut weight loss.
Interestingly, the researchers also found that those exercisers who had compensated the most and lost the least weight tended to be those who had reported at the start that they thought some good health habits gave people license for other, insalubrious ones.
“In effect, they felt that it’s O.K. to trade behaviors,” says Timothy Church, an adjunct professor at Pennington who led the new study. “It’s the ‘if I jog now, I deserve that doughnut’ idea.”
In consequence, they lost little if any weight with exercise.
But the study produced other, more encouraging data, he says. For one thing, almost everyone’s resting metabolic rates remained unchanged; slowed metabolisms would encourage pounds to creep back. And those few exercisers who avoided an extra cookie or handful of crackers did lose weight.
“There was only a small difference, over all,” between those who compensated and those who did not, Dr. Church says. “We’re talking about barely 100 calories. That’s about four bites of most food.”
So, people hoping to lose weight with exercise should pay close attention to what they eat, he says, and skip those last four bites, no matter how tempting.