How being heavy or lean shapes our view of exercise

By Gretchen Reynolds

Overweight women’s brains respond differently to images of exercise than do the brains of leaner women, a sophisticated new neurological study finds, suggesting that our attitudes toward physical activity may be more influenced by our body size than has previously been understood.

For the study, which was published last month in the International Journal of Obesity, scientists affiliated with the Key Laboratory of Cognition and Personality at Southwest University in Chongqing, China, recruited 13 healthy, young, normal-weight women and 13 who were overweight or obese.

The scientists asked their volunteers to complete two questionnaires, one of which probed the extent to which they considered exercise desirable; would they agree, for instance, that, “if I were to be healthy and active, it would help me make friends”? The other set of questions examined whether they expected exercise to be unpleasant; if they were to be physically active on most days, for example, would they expect to wind up feeling sore, or maybe even embarrassed by exercising in public?

The researchers next had each woman lie inside a functional magnetic resonance imaging machine, which scans blood flow to specific areas of the brain, indicating areas of increased activity. Then they started a slide show.

For some time, scientists have known that many overweight people’s brains operate differently than the brains of thinner people when they look at images related to eating. In previous neurological studies, when heavier volunteers viewed pictures of food or food preparation, they typically developed increased activity in portions of the brain involved in reward processing, or an urge to like things, including in an area called the putamen. At the same time, their brains showed relatively blunted activity in areas that are thought to induce satiety, or the ability to know when you are full. These changes generally are reversed in the brains of thinner people shown the same images.

But no brain-scanning studies had examined whether being heavy might also affect people’s brain responses — and presumably their attitudes — toward physical activity.

So, to address that gap, the researchers now flashed a series of photographs before their volunteers. Ninety of the images showed people being joyously active by running, dancing, leaping, playing tennis, and similar activities. The women were asked to vividly imagine themselves performing the same actions, using hand gestures and limited bodily contortions, to the extent possible within the confines of the scanner.

Ninety additional images featured relaxed, sedentary behaviors, including stretching out on a sofa and sitting in a desk chair. Again, the women were directed to imagine themselves similarly lounging. The various images of activity and quiet were interspersed with photographs of landscapes.

While the women viewed the pictures, the functional MRI machine monitored their brain activity.

The resulting readouts revealed that overweight women’s brains were put off by exercise. Shown images of people being active, these women developed little activation in the putamen region of the brain, suggesting that they did not enjoy what they were seeing. At the same time, a portion of the brain related to dealing with negative emotions lit up far more when they viewed images of moving than of sitting. Emotionally, the brain scans suggested, they anticipated disliking physical activity much more than they expected to disdain sitting.

Leaner women’s brain activity, by and large, was the opposite, with the putamen lighting up when they watched others work out and envisaged doing the same themselves.

Such data might at first seem discouraging, underscoring the possibility that being obese or overweight is self-reinforcing, although it is impossible to know from this study whether a dislike of exercise contributed to or resulted from weight gain.

A final and unexpected finding from the study provides a basis for hope, though. The scans also showed that when overweight volunteers viewed images of exercise, a portion of their brain related to movement memory remained stubbornly silent. Their bodies were unfamiliar with how to be active, which might have contributed, the study’s authors speculate, to the women’s negative emotional response to activity. They didn’t know how to exercise and anticipated not enjoying trying to learn.

Interestingly, these women had also said at the study’s start, when answering the questionnaires, that they expected exercise to end in embarrassment (while also believing that if only they could exercise, they would be more popular).

The practical takeaways of the findings are obvious and almost poignant.

“Encourage people to pursue physical activities and exercise that they actually find pleasurable and might enjoy,” said Todd Jackson, a professor of exercise science at Southwest University, who led the study. Hire a kind, nonjudgmental coach or personal trainer to lead you through a manageable exercise routine.

And if you continue to find yourself drawn to the couch instead of the gym, use that inclination strategically “as an incentive or reward for increasing exercise,” Dr. Jackson said. Swim for 45 minutes and then allow yourself to surf the Internet, for instance, he suggests. Don’t fight your brain’s unenthusiastic attitude toward exercise, he said. Embrace it.

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