Rat studies with chocolate breakfast cereal reveal why animals overdo it; opioid drug study implicates brain's reward system
WASHINGTON -- Two studies in the October issue of Behavioral Neuroscience show that when animals are stressed, deprived and exposed to tempting food, they overeat, with different degrees of interaction. The powerful interplay between internal and external factors helps explain why dieters rebound and even one cookie can trigger a binge if someone's predisposed to binge.
The findings also implicate the brain's opioid, or reward, system in regulating overeating, especially when the food is extra-tempting – and not only in under-fed animals. This knowledge may help even non-stressed people to avoid overeating, keep their weight down and improve their health. Behavioral Neuroscience is published by the American Psychological Association (APA).
A study by M. Flavia Barbano, PhD, and Martine Cador, PhD, at the University of Bordeaux 2 in France, separated the distinct roles in consumption played by food deprivation and the "yum" factor, establishing that the interplay between internal and external factors regulates food intake, at least in mammals. Although much has been learned about human overeating, it is easier to untangle and verify the different variables involved in controlled animal studies.
Working with laboratory rats, the researchers tested three aspects of eating behavior: motivation (how bad did they want it), anticipation (how excited were they in advance), and intake (how much did they eat), all relative to homeostasis (satiety or deprivation) and food type (ordinary lab chow or "highly palatable" chocolate breakfast cereal, as verified by a pre-test of different foods).
For motivation, the researchers measured how fast 16 rats – who either had eaten freely or been put on a diet -- ran down an alley to a bowl of either chow or Choc and Crisp, a German-brand cereal. The animals ran faster when they were either food-deprived or presented with the chocolate cereal. However, when the food-sated animals were presented with Choc and Crisp, they ran just as fast as the hungrier rats.
The authors also measured anticipation in 32 rats by comparing activity levels when placed in individual cages where they would get either chow or cereal. First, the authors got the rats used to unpredictable feeding times; then for 10 days fed them a half hour after they went into the cage. Whether they expected chow or cereal, the food-restricted rats were more active, rearing up a lot more. Regardless of food type, only the deprived rats were more active, so the researchers concluded that anticipatory activity depends not on food type but on whether the animal has had enough to eat (homeostatic state).
As for actual intake, when presented with the Choc and Crisp, the food-sated group ate almost as much as the food-deprived group. But when presented with lab chow, they ate very little. Barbano and Cador concluded that highly palatable food motivates an animal to eat more than it really needs. When food type and satiety interact, attractiveness overrides satiety -- a phenomenon known to anyone who has ever stood in a buffet line.
In another key study, neuroscience psychologist Mary Boggiano, PhD, and her colleagues at the University of Alabama at Birmingham focused on the regulatory role of the brain's opioid system. Opioids or endorphins (the brain's "feel good chemicals") play a key role in our liking of food. Yet external substances such as heroin and morphine mimic endorphins by binding to the same receptors in the brain, produce a sense of reward (among other functions). The researchers compared how binge-eating rats versus non-binge eating rats responded to drugs that either turn on opioid receptors (butorphanol, which treats pain) or block them (naloxone, which treats heroin addiction).
From the rats' responses to these drugs, Boggiano and her colleagues inferred how stress and dieting change the brain's opioid control of eating. The binge eating occurred after rats experienced both foot shock (stress) and cyclic caloric restriction (dieting). Either caloric restriction or stress alone were not enough to produce changes in food intake, but stressed and underfed rats ate twice the normal amount of Oreo® cookies, which rats find rewarding. In other words, animals subjected to both stressors became binge eaters, confirming how strongly these outside factors interact to change eating behavior.
The findings also implicated opioids in the neurochemistry of binge eating. The highly rewarding butorphanol enhanced the binge eating; the reward-blocker naloxone suppressed how much the stress/deprived rats ate, to the level of the control rats. The authors say this pattern of findings in rats who were sated at the time of testing strengthens the evidence that reward, more than metabolic need, drives binge eating. Boggiano and her colleagues speculate that sensitized opioid-receptor signaling may be necessary to initiate binge eating. In the rats, stress and reduced calories seemed to sensitize those receptors to the presence of highly palatable food, in this case cookies. This, they write, "may underlie the common clinical observation that just a bite of highly palatable (often forbidden) food triggers binges and makes it difficult to abstain from binge eating."
The researchers speculate that the deprived and stressed rats may have been in a "hedonic deprivation state," essentially craving something good and rewarding. The research underscores how what is viewed as an unhealthy behavior (indulging in palatable foods, which are cheap, convenient and often high in fat and sugar) may have its roots in the need to survive. It suggests that binge eating is an adaptive response to abnormal environmental conditions. Boggiano cites other scientists' findings that among healthy people without eating disorders, dieting is the biggest predictor of stress-induced overeating.
In light of their findings, she says, "Highly palatable food can mimic opioid drugs by releasing opioids or activating sensitized receptors, so imagine so imagine what it can do in a human with a history of dieting. If only rat chow is available, even rats with a history of dieting when stressed rats don't binge -- but when they get a little bite of cookie first, they do." As a result, she says when treating bulimics and binge eaters, it may not be a good idea to introduce palatable (junk) food too early in therapy.
However, she thinks that binge eaters' sensitized opioid receptors should return to normal as long as they stay away from very-low-calorie diets and from trigger foods for a long time, perhaps relative to the amount of time they've had the disorder. In the meantime, scientists could perhaps develop a safe opioid blocker that could help binge eaters fight off food cravings. However, Boggiano believes that the main key is not drugs but behavioral change around food, recognizing stressors and avoiding restrictive diets.
"Binge eating is normal," she says. "It's your brain's best way to respond to expected starvation. It's restrictive dieting and stressing so much about your body weight and shape that is abnormal."
Source: Eurekalert & othersLast reviewed: By John M. Grohol, Psy.D. on 21 Feb 2009
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