A person with a body mass index (BMI) of 30 or greater is considered obese. BMI is calculated as weight in pounds divided by height in inches squared and multiplied by 703.
Dieters often struggle to keep off the pounds they've lost, and a new study involving mice offers a possible explanation. Dieting may change how the brain responds to stress, so that the next time dieters feel frazzled, they eat more, researchers say.
In the study, mice that previously had been put on a diet ate more high-fat foods than did mice with no history of dieting.
The altered eating behavior may be due, at least in part, to changes in genes that control the stress response and feeding habits, the researchers said.
The findings make sense from an evolutionary standpoint, said Tracy Bale of the University of Pennsylvania and other study researchers. If an animal has experienced famine, it would be advantageous for its brain to change in ways that would protect the animal against another drastic loss of calories, by promoting the consumption of high-fat foods.
But in today's environment, where high-fat foods abound, such a response encourages weight gain.
"This study highlights the difficult road that human dieters often travel to attain and maintain their weight- loss goals," said Dr. Jeffrey Zigman, an endocrinologist at the University of Texas Southwestern Medical Center, who was not involved in the study. "It also suggests that management of stress during dieting may be key to achieving those goals."
Because the study was conducted in mice, research is needed to determine if the findings also apply to humans.
Researchers restricted the food intake of the mice in the study until the mice had lost 10 percent to 15 percent of their weight, a reduction typical of people on diets.
These mice showed elevated levels of the stress hormone corticosterone and exhibited depression-like behavior, suggesting dieting itself induces mood changes.
There also were chemical changes in the way the mice's bodies regulated the release of another stress hormone, called corticotropin-releasing factor, as well as melanin-concentrating hormone and orexin, hormones thought to control eating behavior. These changes — which are called epigenetic changes and don't alter the genes' DNA sequence, but alter the way that sequence is read by cells — remained even after the animals were allowed to regain the weight.
Once the mice were back to their normal weight, they were subjected to a series of stressful situations, including being placed in total darkness, hearing new sounds at nighttime, and seeing a predator. A group of control mice that had never been placed on a diet also were exposed to these stressors.
All that stress led both groups of animals to binge when they were later presented with high-fat food. However, more was consumed by the mice that had dieted.
"These results suggest that dieting not only increases stress, making successful dieting more difficult, but that it may actually 'reprogram' how the brain responds to future stress and emotional drives for food," Bale said in a statement.
The study is published in the Dec. 1 issue of The Journal of Neuroscience.
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This story was provided by MyHealthNewsDaily, a sister site to LiveScience.
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