Published in Frontiers in Psychology, a new study from the University of NSW (UNSW) provides insight into how excessive consumption of junk food can change behaviour, weaken self-control and give rise to overeating and obesity.

The team of researchers led by UNSW Professor Margaret Morris, Head of Pharmacology from the School of Medical Sciences, divided rats drawn for the same genetic outbred strain into two groups, dubbed Chow and Cafeteria.

Each group was fed the standard laboratory diet designed for optimal rat health. However, the Cafeteria group was also given unrestricted access to nutrient-poor, kilojoule-dense human foods such as pies, dumplings biscuits, and cakes – items commonly served in commercial buffets. These foods all contained highly-processed ingredients such as sugar and flour, seed oils, and additives, preservatives, colourings and flavourings, and contained 150 percent more kilojoules.

Using Pavlovian conditioning, the researchers trained both groups of rats to associate two different sound cues with a two particular flavours of sugar water: cherry and grape. The Chow group stopped responding to cues linked to a flavour they had recently overindulged in. This inborn mechanism, widespread in animals, protects against overeating and promotes a healthy, balanced diet.

One the other hand, rats in the Cafeteria group were 10 percent heavier and became indifferent in their food choices after just two weeks. They no longer avoided the sound that advertised the overfamiliar sugar-water flavour, which suggests they lost their natural preference for novelty. Even when the rats returned to their normal healthy diet, this change continued for some time.

Lasting changes in the brain’s reward circuit

The researchers believe a junk food diet causes lasting changes in the reward circuit parts of the rats’ brains, for example the orbitofrontal cortex, an area responsible for decision-making. “Because the brain’s reward circuitry is similar in all mammals, this could have implications for a person’s ability to limit their intake of certain kinds of foods,” Professor Morris commented. “If the same thing happens in humans, eating junk food may change our responses to signals associated with food rewards. 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.”

The findings may also give weight to the argument that junk-food advertising should be banned because of its influence on poor food choices. Dr Amy Reichelt from the School of Psychology in UNSW Science, and first author of the paper, remarked: “In a world surrounded with advertising for sugar- and fat-rich foods and drinks, these images may have a greater effect on people who are overweight, making impulse purchases of snacks like chocolate bars harder to resist.”

So how does a study on rats translate to humans? Rats are valuable for studying human eating behaviour because, like humans, they’re omnivores, and they also use flavour conditioning, which is learning through taste and experience those foods that are good to eat and those to avoid. Moreover, rats and humans share many of the neurobiological and hormonal mechanisms of flavour-learning and appetite-regulation.

The study shows that while rats – just like humans – develop food preferences and show the same appetite-regulation processes, these systems can be impaired and dysregulated by eating highly palatable, kilojoule-dense, nutrient-poor foods.