A diet of junk food causes weight gain in animals, by blocking the normal regulation of appetite for a variety of foods. This leads to consumption of an unbalanced diet high in fat and calories. Researchers at the School of Medical Sciences, UNSW Australia taught young male rats to associate two different sound cues with a particular flavor of sugar water – cherry and grape. Healthy rats raised on a healthy diet stopped responding to cues linked to the flavor they had just consumed. This inborn response is characteristic of most animals and protects against overeating as well as promotes a healthy, balanced diet. After two weeks on a diet that included cafeteria-style foods, such as dumplings, cookies, pie, and cake, the rats’ weight increased by 10% and their eating behavior changed. They no longer avoided the sound cues that announced an overfamiliar taste. In essence, they had lost their natural preference for novelty. This change persisted even after the animals returned to their normal diet. The researchers speculate that highly palatable junk food causes lasting changes in the reward circuitry of the decision-making areas of the brain, such as the orbitofrontal cortex. This results in an animal’s inability to limit intake of junk food.
Reichelt A, Morris M, Westbrook R: Cafeteria diet impairs expression of sensory-specific satiety and stimulus-outcome learning. Frontiers in Psychology 2014; August 27th.