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.