A team of researchers have found that adherence to junk diet not only makes rats fat, but also lowers their appetite for a balanced diet.

It is estimated by the World Health Organization that more than 10 percent of the world's adult population is listed as obese. Obesity or overweight is the cause of death of nearly 2.8 million people each year. Overweight and obesity together trigger several chronic diseases such as diabetes, cardiovascular diseases and even cancer.

"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," adds Dr Amy Reichelt, lead author of the paper and UNSW postdoctoral associate.

In the study, led by Professor Margaret Morris - who is the Head of Pharmacology from the School of Medical Sciences, UNSW Australia, the researchers revealed how excessive intake of junk food can alter behavior and disrupt self-control eventually leading to overeating and obesity.

To prove the hypothesis, they trained a set of young male rats to link each of the two sound cues to a particular flavor of sugar water i.e. cherry and grape. They found that apart from making the rats fat, junk food also lowered their appetite for novel foods - a preference that drives them to seek a balanced diet.

During the study, the rats that were raised on a healthy diet stopped responding to cues to associate to a flavor in which they overindulged. This innate mechanism present in all animals offers protection against overeating and boosts intake of healthy and balanced diet.

But, after weeks of diet that included daily access to cafeteria food that included pie, cookies, cake and dumpling with more than 150 percent calories, there was a 10 percent rise in the rats' weight and also a dramatic change was witnessed in their behavior.

The researchers noticed that the rats had become indifferent to their food choices and no longer avoided the sound advertising the over-familiar taste, indicating they lost their natural preference for novelty. This change lasted till they returned to their healthy diet.

The researchers believe that junk diet triggers lasting changes in the reward circuit part of the rats' brain.

"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 UNSW Professor 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."

The finding is documented in the journal Frontiers in Psychology.