Dieters who drop a lot of weight quickly could be more likely to put all the pounds back on, and then some, because of changes in the way their brains respond to stress, a study published Tuesday shows.
Researchers at the University of Pennsylvania, led by Tracy Bale, studied the behavior and hormone levels of mice on restricted diets and found that stressed-out mice with a history of dieting ate more high-fat foods than similarly stressed mice that had not been on a diet.
After three weeks on their reduced calorie diet, the mice in the study lost 10-15 percent of their body weight, similar to what humans can lose on a diet, says the study published in the Journal of Neuroscience.
Bale and her colleagues found that the mice had increased levels of the stress hormone corticosterone, and in a test where they were hung by their tails, they spent more time just hanging there, immobile, than the control mice. The researchers called that "depressive-like behavior."
The researchers also found that several genes that play a key role in regulating stress and eating patterns had changed in the dieting mice.
Previous research has shown that experiences can alter the form and structure of DNA, an effect known as epigenetics.
Even after the mice were allowed to eat their fill and had returned to their normal weights, the epigenetic changes remained, the study found.
When the researchers put the rodents in stressful situations and monitored how much fatty food they ate, they found that the mice that had been on a diet ate more than the control group of rodents, that hadn't been on a weight-loss regime.
The findings illustrate why a piece of pizza is so appealing after a stressful day at work, and also help to explain why so many people who want to lose weight get trapped in the "yo-yo diet" cycle, where they lose weight, only to gain it all back and then some, the study says.
"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.
Jeffrey Zigman, an expert in endocrinology, diabetes and metabolism at the University of Texas Southwestern Medical Center, said the mice in the study experienced conditions that mimicked the stress that people often experience.
"This study highlights the difficult road that human dieters often travel to attain and maintain their weight loss goals," said Zigman, who was not involved in the study.
"It also suggests that management of stress during dieting may be key to achieving those goals."
One in three Americans is considered obese, according to official health figures.
Dieting is big business in the United States, where consumers dish out between 33 and 100 billion dollars per year on weight loss diets, supplements, books and other programs.