A diet of junk food not only makes rats fat, but also reduces their drive to seek a balanced diet, according to researchers in Sydney.
The study by a University of New South Wales team, published in Frontiers in Psychology, has helped to explain how the excessive consumption of junk food can change behaviour, weaken self-control and lead to overeating and obesity.
Led by Professor Margaret Morris, the team taught young male rats to associate two different sound cues with two particular flavours of sugar water—cherry and grape.
Dramatic behavioural change
Rats that had been raised on a healthy diet soon stopped responding to the cues linked to a flavour they had 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, the rats’ weight increased by 10% 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, explained Prof Morris. The change lasted for some time after the rats returned to a healthy diet.
“We think that a junk diet causes lasting changes to the reward circuit parts of the rats’ brains—for example, the orbitofrontal cortex, an area responsible for decision-making,” she said.
“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 happened in humans, eating junk food could 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 coming by.”
Dr Amy Reichelt, the first author of the paper, added: “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 Organisation estimates that over 10% of the world’s adult population is obese, and at least 2.8m people die each year as a result of weight issues.