Is this why we’re all so fat? Junk food can kill your appetite for anything healthy, scientists warn

A diet laden with burgers and fries, dripping with sweet treats and saturated in pies could kill our appetite for healthy foods, scientists warn.

A new Australian study has found living on junk food not only makes rats fat, but also suppresses their desires to eat a balanced diet.

Scientists believe their findings help explain how excessive consumption of unhealthy foods, can change a person’s behaviour, weaken their self-control and lead to overeating and obesity.

Professor Margaret Morris, from the University of New South Wales, said: ‘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.

‘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.’

The team of researchers taught young male rats to associate each of two different sound cues with a particular flavour of sugar water, cherry and grape.

Rats raised on a healthy diet stopped responding to cues linked to a flavour, in which they had recently overindulged.

This inborn mechanism, widespread in animals, protects against overeating and helps promote a healthy, balanced diet.

But after two weeks eating a daily diet rich with cafeteria foods, including pie, dumplings, cookies and cake – with 150 per cent more calories – the rats’ weight increased by 10 per cent.

At the same time, researchers noted their behaviour had also changed dramatically.

They became indifferent in their food choices and no longer avoided the sound advertising the over-familiar taste.

The team of researchers deduced this indicated they had lost their natural preference for healthy foods.

They noted the change lasted for some time after the rats returned to a healthy diet.

The team concluded a diet of junk food causes lasting changes in the reward circuit parts of the rats’ brain, including the orbitofrontal cortex – an area of the brain responsible for decision-making.

They said the findings may have implications for people’s ability to limit their intake of certain kinds of foods, because the brain’s reward ciruits are similar in all mammals.

The World Health Organisation has estimated that more than 10 per cent of the world’s adult population is obese.

WHO said at least 2.8 million people die each year as a result of being overweight or obese – which are major risk factors for a number of chronic diseases including diabetes, heart disease and cancer.

Dr Amy Reichelt, lead author of the paper, said: ‘As the global obesity epidemic intensifies, advertisements may have a greater effect on people who are overweight and make snacks like chocolate bars harder to resist.’

The findings were reported in the journal Frontiers in Psychology.