Research by the University of New South Wales (UNSW) has found that a diet of junk food can change rats' eating behaviour and trap them in a cycle of unhealthy eating.
The team of scientists, led by head of pharmacology Margaret Morris, taught male rats to associate two different sound cues with two flavours of sugar water - cherry and grape.
Usually rats, and most other animals, will avoid a flavour or food that they’ve recently over indulged in. This is an inborn mechanism that protects against overeating and promotes a healthy, balanced diet in animals.
To test the effect junk food had on this mechanism, they fed one group of the rats a healthy diet for two weeks, and the other a diet of cafeteria foods, including dumplings, cookies and cakes, which had around 150 percent more calories than the healthy diet.
The researchers then tested how the different groups responded to sound cues for the two types of flavoured water. They found that the healthy rats would stop responding to cues for a flavour they’d recently overindulged in and try the new flavour instead, as they normally would.
But the rats who had been living on junk food for a fortnight changed their behaviour dramatically. Not only did their weight increase by 10 percent, they also became indifferent to their food choices and didn’t avoid the sound cues for the water they’d recently overindulged in. Instead, they continued to drink the overfamiliar taste, and lost their natural preference for new flavours.
Even one week after the rats returned to their healthy diet, the changes persisted. The results are published in Frontiers in Psychology.
“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,” said Morris in a press release.
“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 said.
This biological response could explain why it’s harder for people who are overweight to avoid purchasing high calorie foods, explained a co-author of the paper, Amy Reichelt from the UNSW Science school of psychology.
“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,” she said.
Love science? Find out more about the ground-breaking research from UNSW.