Scientists now know why people keep on eating junk food they know is unhealthy.

No, it isn't because you're an undisciplined slob. It's because junk like burgers, fries, cakes and sweets suppress your desire to eat a balanced diet.

Your spirit is willing but the chemicals you consume in junk food weakens your will to resist.

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Scientists have also discovered an excessive consumption of unhealthy junk foods can change a person's behavior; weaken his self-control and lead to overeating and, ultimately, obesity.

"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 Professor Margaret Morris, from the University of New South Wales.

"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 results of her team's study was recently published in the peer-reviewed academic journal, Frontiers in Psychology.

In the study, researchers taught young male rats to associate each of two different sounds with a specific flavor of sugar water, grape and cherry.

The rats were split into two groups: Chow and Cafeteria.

Both groups received standard laboratory food perfect for healthy rats. The Cafeteria group, however, was also fed a set of human foods deemed highly palatable such as cookies and cakes.

These palatable foods contained highly-processed ingredients such as sugar, flour and preservatives.

Researchers said the Chow rats raised on a healthy diet stopped responding to sounds linked to a flavor they had overindulged.

Researchers said this response in an inborn mechanism widespread in animals that protects against overeating and helps promote a healthy, balanced diet.

After two weeks eating a daily diet packed with sugars such as pies, dumplings, cookies and cake, the Cafeteria rats' weight rose by 10 percent.

More surprising, researchers said the Cafeteria  rats' behavior towards food also changed dramatically.

Researchers found that after two weeks on this sweet diet, the Cafeteria rats lost motivation to do tasks. These rats also appeared unresponsive to normal sensory cues about what to eat.

Researchers concluded the Cafeteria rats had lost their natural preference for healthy foods.

They also found out 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. These parts include the orbitofrontal cortex, or the area of the brain responsible for decision-making.

They believe the findings might have implications for a person's ability to limit his intake of certain kinds of foods because the brain's reward circuits are similar in all mammals.

The World Health Organization said 2.8 million people die each year as a result of being overweight or obese.