Obese people tend to have greater dopamine activity in the habit forming region of the brain than their lean counterparts and lesser dopamine activity in the region controlling rewards, the findings showed.

Dopamine is a neurotransmitter that helps control the brain's reward and pleasure centres. These differences could potentially make the obese people more drawn to overeat in response to food triggers and simultaneously make food less rewarding to them.

"Eating based on unconscious habits rather than conscious choices could make it harder to achieve and maintain a healthy weight, especially when appetizing food cues are practically everywhere," said lead author Kevin Hall from the National Institutes of Health in US.

The study involved 43 men and women with varying amounts of body fat. Study participants followed the same eating, sleeping and activity schedule. Tendency to overeat in response to triggers in the environment was determined from a detailed questionnaire.

Positron emission tomography (PET) scans evaluated the sites in the brain where dopamine was able to act.

The study appeared in the journal Molecular Psychiatry.

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