Mark A. Philbrick, BYU
PROVO — Exercise not only burns calories, but also may actually decrease interest in food temporarily. Brain scans showed people who had exercised for 45 minutes on a treadmill had less motivation for food afterward than those who had been sedentary.
The difference in interest in food appeared to be driven by the exercise, not by one’s body mass index, as the researchers had expected. And it was consistent for both those who were obese and those who were normal weight.
Besides that, those who exercised continued to outpace their counterparts in terms of physical activity for the rest of the day.
The study is published online in the journal Medicine & Science in Sports & Exercise.
The findings come at a time of high interest in obesity and what its toll may be. The New York City Board of Health recently approved a much-debated ban on sale of large, sugary drinks by restaurants, street vendors and movie theaters. McDonald's just announced that it will list the number of calories in each of its food items on its menus. And increasing efforts by fast-food restaurants to provide some alternative choices to high-calorie food offerings and increased public awareness campaigns on the benefits of walking and exercise are just a small sample of efforts to tackle the weight issue.
The Centers for Disease Control and Prevention reported recently that in 2009-2010, 16.9 percent of children and 35.7 percent of adults in the United States were not just overweight, but obese. For adults, that means those with a body mass index of 30 or above. Normal weight is 18.5 to under 25. Above that but below 30, a person is overweight.
For the study, Brigham Young University professors James LeCheminant and Michael Larsen randomly assigned 35 women, 17 obese (BMI 30 and above) and 18 normal weight (BMI under 25), to come in to walk on a treadmill for 45 minutes or perform sedentary activities like body mass composition testing. Then both groups were taken into the lab for neurological testing, where minute differences in electrical responses to pictures of food were measured through electrodes affixed to their scalp, LeCheminant, an associate professor in exercise sciences, said.
The next day, the group that was assigned to exercise did the sedentary activity followed by brain scan, and vice versa, said Larson, an assistant professor of psychology and neuroscience. The findings were the same.
They used pictures of flowers as a control to see if the response was neutral or not, LeCheminant said. And the researchers watched specifically for brain wave forms in a “window” associated with attention.
“We wanted to see how engaged people were by food pictures,” said Larson, who added that it’s unlikely the exercise made them tired and their brains less responsive, since there was no change in response to the floral photos. “That said it was not just general brain tiring, but specific to food."
Prior to either activity, all participants fasted overnight, then ate identical energy shakes, Larson said.
After, they were each given an accelerometer that measured their physical activity and that’s how researchers were able to see that those who started their day with exercise continued to be more active over the course of the day.
They also did not “compensate” for the exercise by consuming more calories that day. Consumption was similar for the two groups.
It’s a first-step study, interesting but not conclusive, the researchers said, and it has some limitations. They only looked at one intensity, type and duration of exercise — a moderate to vigorous 45-minute stint on a treadmill.
There are related studies underway or planned, as well, said Larson. They’re just finishing one on how sleep influences response to food. Evidence suggests inadequate sleep and obesity are highly correlated, so they’ve been looking at the interaction of sleep, exercise and diet. And they plan to follow the exercise/food response study with one that looks at whether the response to food changes a few hours after exercise.
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