In many cases these people are compensating for sadness and depression

Professor Ian Macdonald, Nottingham University

Dopamine has already been linked with an addiction to drugs like cocaine, alcohol and even to gambling.

But scientists admit it could be a chicken and egg situation, with the obesity triggering the chemical imbalance rather than the other way round.

The researchers from America's Department of Energy's Brookhaven National Laboratory, found that the extremely obese people they looked at had fewer receptors for dopamine.

And they said this could lead to binge eating as the body tries to stimulate the dopamine "pleasure" circuits.

Chronic over eating

In the study, published in The Lancet, lead scientist Gene-Jack Wang said he hoped the research could be used to help treat obese patients, although he admitted the findings were still in the preliminary stages.

"The results from this study suggest that strategies aimed at improving dopamine function might be beneficial in the treatment of obese individuals," he said.

"It is possible that obese people have fewer dopamine receptors because their brains are trying to compensate for having chronically high dopamine levels, which are triggered by chronic over eating.

"However it is also possible that these people have low numbers of dopamine receptors to begin with, making them more vulnerable to addictive behaviour including compulsive food intake," he added.

Obesity

Obesity affects up to a third of Americans and a growing number of people world-wide People with a body mass index (BMI) of over 30 are considered obese, the normal BMI is 18-25 The BMI is calculate by dividing a person's weight in kilograms by the square of their height in metres

Most of the drugs that alter dopamine levels are highly addictive and the US researchers say the only way to combat this might be good old fashioned exercise.

Exercise levels

They say that this is the body's natural way of boosting dopamine levels and giving the highs, and cuts down the obesity risk by burning calories.

Obese people might be addicted to food highs

Professor Ian Macdonald, Professor of Metabolic Physiology at Nottingham University, said the research provided interesting pointers.

But said he believed the reverse of this could in fact be true - with the obesity causing the chemical imbalance.

"It is interesting. The biggest problem is what comes first. Whether there is an abnormality of dopamine receptor or whether there is an obese state," he said.

But said that although the dopamine might not be the trigger that it could well exacerbate the problem.

Professor Macdonald said that his studies of binge eaters were at odds with the new research, which linked binge eating with pleasure highs.

"I think that does not fit with the behaviour, the psychological behaviour, of most people over eating. In many cases these people are compensating for sadness and depression," he said.

But he did admit that rich texture foods like chocolate or cakes could provide a pleasure sensation.