Over 10 percent of the global population is considered obese, according to the World Health Organization (WHO), and a majority of the people struggling with being overweight or obese run the risk of developing diabetes, cardiovascular disease, or certain types of cancer. A recent study published in the journal Frontiers in Psychology has revealed that exposing rats to junk food obviously causes them to gain weight, but it also reduces their appetite for foods that are included in a balanced diet.

"The interesting thing about this finding is that if the same thing happens in humans, eating junk food may change our responses to signals associated with food rewards," Professor Margaret Morris, lead researcher and Head of Pharmacology from the School of Medical Sciences, UNSW Australia, said in a statement. "It's like you've just had ice cream for lunch, yet you still go and eat more when you hear the ice cream van come by.”

Morris and her colleagues examined a natural mechanism in most animals that promotes a healthy diet by preventing overeating. Healthy male rats were exposed to two different sound cues that were related to either cherry or grape sugar water. After being raised on a healthy diet, the rats stopped responding to the sound cue that caused them to overindulge on that particular flavor. For two weeks, the research team changed the rats’ diet to add 150 percent more calories from cafeteria food such as pie, dumplings, cookies, and cake.

Following the two weeks of unhealthy dieting, not only did the rats’ weight increased by over 10 percent, but they also exhibited behavioral changes. Their capacity to overindulge returned seeing as they stopped avoiding sound cues of a certain flavor and they were less picky about their food choices. Even after researchers returned the rats to a balanced diet, their initial preferences for healthier food options did not return and overindulgence persisted.

"As the global obesity epidemic intensifies, advertisements may have a greater effect on people who are overweight and make snacks like chocolate bars harder to resist,” added Dr. Amy Reichelt, lead author of the paper and UNSW postdoctoral associate.

The Australian research team believes that an all junk food diet caused major changes to the reward circuits in the rats’ brain, especially in the decision-making area of the brain known as the orbitofrontal cortex. Considering the brain’s reward circuit is similar in all mammals’ including humans, their findings may have implications for overweight and obese people who are unable to control overindulging in unhealthy food options.

Source: Morris M, Reichelt A, et al. Frontiers in Psychology. 2014.