Exercise has many health benefits and is important in maintaining a healthy body weight. However, since weight loss happens when more calories are expended than taken in, increasing the amount we exercise is beneficial for weight loss assuming that it does not affect what we consume. Unfortunately, this is often not the case. Indeed, sometimes exercise may be counter-productive to our goal of losing weight.
Brian Wansink, a professor at Cornell University had a group of people participate in a study: Before dinner, the group was told that they would do a one-mile walk before dinner. Half of the participants were told the walk was an "exercise walk" and half were told it was a "scenic walk." The walks, of course, were identical: same distance, same pace, same time. The only difference was whether it was labeled "exercise" or "scenic." After the walk came dinner, and Wansink measured what individuals in each group consumed. Those who first did the exercise walk consumed significantly more calories. Further, these calories largely came from dessert, suggesting that people were rewarding themselves for their "exercise."
We are notoriously poor judges of both how many calories we eat and how many calories we consume. Wansink's participants estimated they burned more calories during the exercise walk than on the scenic walk. This gave them a justification for extra consumption. Couple this with the fact that we also underestimate how many calories we consume with our reward, and then a problem emerges. If we overestimate how much we burn, and then reward ourselves with a treat that we underestimate the calories of, we may end up consuming more calories than if we did not exercise in the first place.