While the findings cannot be directly transferred to human obesity, it found that overconsumption of high-calorie food triggered addiction-like responses in the brain.
But the study, published online in Nature Neuroscience, suggests for the first time that our brains may react in the same way to junk food as it does to drugs.
Dr Paul Kenny, a neuroscientist who led the research, said the study, which took nearly three years to complete, confirmed the “addictive” properties of junk food.
The findings could partly explain the soaring obesity rates in Britain and the success of fast food outlets.
Experts studied rats fed on cheesecake, bacon and sausages. Soon after the experiments began the animals began to bulk up and show signs of addiction.
‘It presents the most thorough and compelling evidence that drug addiction and obesity are based on the same underlying neurobiological mechanisms,’ Professor Paul Kenny said.
‘In the study, the animals completely lost control over their eating behaviour and continued to over-eat even when they anticipated receiving electric shocks, highlighting just how motivated they were to consume the palatable food.’
During the trial the animals were rewarded with a pleasurable electrical stimulation.
The rats could control how much of the stimulation they got by running on a wheel. Animals living on junk food ran far more - suggesting they needed more brain stimulation to feel good, the researchers found.
The scientists at the Scripps Research Institute in Jupiter, Florida, also gave rats electric shocks on their feet when they ate high-fat food.
Rats on a normal diet quickly learned to avoid the unhealthy food. But those used to junk food refused to let the shock get in their way of their high calorie food.
‘They always went for the worst types of food,’ said Professor Kenny, who published his findings in the journal Nature Neuroscience.
‘When we removed the junk food and tried to put them on a nutritious diet they simply refused to eat.’
The researchers found junk food altered the chemical balance in the brain’s ‘reward circuits’ - the parts of the brain that handle the feel-good chemical dopamine.
Identical changes happen in the brains of rats given cocaine or heroin and are thought to play a key role in drug addiction.
The scientists fed the rats a diet modelled after the type that contributes to human obesity easy to obtain high-calorie, high-fat foods. Soon after the experiments began, the animals began to bloat.
Latest figures show that one in four people in Britain are obese with married people twice as likely to become obese than their single counterparts.
Eight in 10 men and almost 7 in 10 women will be overweight or obese by 2020.
Cases of devastating health conditions like heart disease, diabetes and stroke will increase with the nation’s waistlines, the recent Government-commissioned Foresight report warned.