The findings help to explain how excessive consumption of junk food can change behaviour, weaken self-control and lead to overeating and obesity.

The team of researchers, led by Professor Margaret Morris, Head of Pharmacology from the School of Medical Sciences, University of New South Wales, Australia, taught young male rats to associate each of two different sound cues with a particular flavour of sugar water - cherry and grape.

Healthy rats, raised on a healthy diet, stopped responding to cues linked to a flavour in which they have recently overindulged.

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 pie, dumplings, cookies, and cake - with 150 percent more calories - the rats' weight increased by 10 per cent and their behaviour changed dramatically.

They became indifferent in their food choices and no longer avoided the sound advertising the overfamiliar taste. This indicated that they had lost their natural preference for novelty. The change even lasted for some time after the rats returned to a healthy diet.

The researchers believe that a junk diet causes lasting changes in the reward circuit parts of the rats' brain, for example, the orbitofrontal cortex, an area of the brain responsible for decision-making.

They said these results may have implications for people's ability to limit their intake of certain kinds of foods, because the brain's reward circuitry is similar in all mammals.

"The interesting thing about this finding is that if the same thing happens in humans, eating junk food may change our responses to signals associated with food rewards," said Morris.

"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 come by," Morris added.

The study was published in the open-access journal Frontiers in Psychology.

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