A diet of junk food not only makes rats fat, but also reduces their appetite for novel foods, a preference that normally drives them to seek a balanced diet, according to UNSW research.

The study, published in Frontiers in Psychology, helps to explain how excessive consumption of junk food can change behavior, weaken self-control and lead to overeating and obesity.

The team of researchers, led by UNSW Professor Margaret Morris, Head of Pharmacology from the School of Medical Sciences, taught young male rats to associate two different sound cues with a two particular flavours of sugar water – cherry and grape.

Healthy rats, raised on a healthy diet, 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.

But after two weeks on a diet that included daily access to cafeteria foods, including pies, dumplings, cookies and cake – with 150 percent more calories – the rats’ weight increased by 10 percent and their behaviour changed dramatically. They became indifferent in their food choices and no longer avoided the sound advertising the overfamiliar taste, indicating they had lost their natural preference for novelty. The change lasted for some time after the rats returned to a healthy diet.

“We think that a junk diet causes lasting changes in the reward circuit parts of the rats’ brains, for example, the orbitofrontal cortex, an area responsible for decision-making,” says UNSW Professor Morris. “Because the brain’s reward circuitry is similar in all mammals, this could have implications for people’s ability to limit their intake of certain kinds of foods.

“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 comes by," she says.

First author of the paper, Dr Amy Reichelt from the School of Psychology in UNSW Science says: “As the global obesity epidemic intensifies, advertisements may have a greater effect on people who are overweight and make purchases of snacks like chocolate bars harder to resist.”

“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.” 

Being overweight or obese is a major risk factors for chronic diseases including diabetes, cardiovascular diseases and cancer. The World Health Organization estimates that over 10 percent of the world’s adult population is obese and at least 2.8 million people die each year as a result of being overweight or obese.

This work has been supported by an NHMRC Project Grant. Dr Amy Reichelt is the recipient of an ARC Discovery Early Career Research Award.