All those hours on the treadmill might actually backfire.

If you're increasing the number of minutes you exercise a day or week, or starting a grueling new fitness program for the promise of a flat belly, be prepared: Those pounds may not only remain the same but even creep upwards.

You're probably thinking, "Hey, but exercise is supposed to help me lose weight and burn calories." And yes, in theory, it does. But in practice, many who start to an exercise regimen find themselves eating more as a reward for all those tough workouts-and putting on more pounds than they burn.

The Research

In fact, a recent study in The American Journal of Clinical Nutrition found that exercisers increased daily caloric intake and didn't lose the expected pounds in the time frame provided. Some even stayed at the same weight, much like the control group who didn't start a fitness program.

To recap: the researchers included 171 sedentary and overweight men and women aged 18 to 65, and measured their weight, resting metabolic rates, levels of hunger, required fitness, daily food intake and calories expended, for a six month period. They also asked participants before beginning to rate how they perceived exercise to corrolate to making "less healthy" choices. (As in, if you burned off calories at the gym, can you have one more boozy drink or a cookie?)

They then randomly assigned a control group where they didn't implement exercise and two other groups. In one, people exercised three times a week on treadmills or bikes until they had burned eight calories for every kilogram of body weight (which is about 700 calories a week). In the other, they exercised longer, burning 20 calories for every kilogram of body weight, or around 1,760 calories a week. And here's the catch: Every group could eat whatever they wanted.

The Results

Both the exercisers' and control group's weights and resting metabolic rates stayed the same. Some members lost weight, but around two-thirds of the participants in the less intensive workout group and around 90 percent of those in the more intensive group lost less weight than they should've in that time frame because of what they were putting in their mouths.

The excess calories the workout groups recorded eating, to be fair, were small. The less intensive group consumed about 90 extra calories each day, and the more intensive exercise group ate 125 extra per day. But even that small increase halted weight loss.

"In effect, [participants] felt that it's O.K. to trade behaviors," said Timothy Church, an adjunct professor at Pennington who led the new study. "It's the 'if I jog now, I deserve that doughnut' idea."

The good news is that almost everyone's resting metabolic rates remained high, rather than slowing down, which tends to happen with weight loss.

"There was only a small difference, over all," between those who compensated [with extra food] and those who did not, Dr. Church said. "We're talking about barely 100 calories. That's about four bites of most food."

The takeaway? Exercise can be an important part of weight loss, but it has to be be combined with a proper diet to be effective.