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New research suggests telling people to exercise just makes them snack more.
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msnbc.com contributor
updated 5/13/2009 7:40:11 AM ET 2009-05-13T11:40:11

Ever make a resolution to go out and exercise and end up grabbing a gooey chocolate cupcake instead?

No matter how good our intentions are, sometimes it seems like our stomachs are out to sabotage us. Scientists are now starting to understand why this happens.

As it turns out, the issue is often not insatiable stomachs, but diet-undermining brain chemistry. At labs around the country, researchers are finding that our brains behave in just the opposite way we would expect them to when it comes to diet and exercise.

Researchers recently discovered that public service announcements exhorting the fat and flabby among us to get more exercise might have an unfortunate and unexpected side effect: They can inspire people to eat more, according to a study published in the journal Obesity.

To learn a little more about the impact of campaigns designed to get couch potatoes moving, scientists from the University of Illinois rounded up 53 college students and asked them to judge a series of posters promoting exercise. After they rated the exercise posters, the students were then asked to evaluate some raisins. They were told they could eat as many raisins as they needed to make the evaluation.

The researchers then ran the same experiment but substituted posters that promoted goals such as joining a group or togetherness for the exercise posters. Again the students were asked to rate some raisins after scoring the posters. The students scarfed down more raisins after scrutinizing posters that promoted exercise than after looking over the other set of posters.

Study author Delores Albarracin, a professor of psychiatry at the university, suspects that the exercise posters simply inspired the students to do something — and because food was available, eating became the thing to do.

What this means, she says, is that we need to be careful about when and where we encourage people to work out. We shouldn’t be showing ads touting the benefits of exercise when people are sitting in front of the TV with a bag of chips in their hands. 

The study brings up the intriguing possibility that these ads could be doing more harm than good if they’re not targeted correctly, says Dr. Louis Aronne, clinical professor of medicine and director of the Comprehensive Weight Control Program at the New York-Presbyterian Hospital/Weill Cornell Medical Center.

A matter of taste
Low-fat labels may also backfire by making food seem less tasty, according to scientists at the Oregon Research Institute.

Young women were asked to sit in a brain scanner while gulping a chocolate milk shake. Half the people were told that the yummy concoction was a regular shake; the other half were told it was a low-fat version, says Eric Stice, a senior scientist at the institute. The twist: everyone got the same thing.

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As researchers watched, the reward centers of the brain lit up when people consumed the shake. But people who thought they were getting a low-fat shake had much less activation, Stice says. In other words, “knowing” that the shake was low fat took a lot of the fun out of the experience, the researchers concluded in the journal Science.   

“It’s really interesting when you think of this enterprise of the food industry to come up with lower-calorie alternatives,” Stice says. “This study shows that it may not be such a good idea to have all those low-fat alternatives since people may be experiencing less of a sense of reward when they eat — and that would make these low-calorie foods completely useless.”

In another study, published in the International Journal of Eating Disorders, Stice and his colleagues compared the brain chemistry of emotional eaters — people who turn to sweets for a lift when they feel sad — to that of others who said they didn’t eat to improve their mood.

Sure enough, Stice says, when the emotional eaters felt bad and then consumed something sweet, their reward centers lit up. People in the control group showed no change when given something sugary to brighten a dark mood.

Ultimately, all this research shows that we need to understand the brain chemistry behind overeating a lot better, says Dr. David Heber, a professor of medicine and director of the Center for Human Nutrition at the University of California, Los Angeles. And until we do, there won’t be any really significant advances in medications to help people lose weight.

In the meantime, people need to be very conscious about the calories they’re consuming and realize that their brains may be undermining attempts at weight loss, says Madelyn Fernstrom, founding director of the Weight Management Center at the University of Pittsburgh.

“We need to get a better understanding of the complex interaction between behavior and biology and to recognize that most of the time we’re not eating because we have physical hunger,” she says.

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