Fat people are less intelligent than their thinner counterparts, a provocative study has claimed.
The research suggests overweight men and women have less grey and white matter in key areas of the brain. They also have greater impulsivity and "altered reward processing", the study found.
The researchers said their findings could explain why overweight people made poor diet choices - they did not have the mental capacity to control themselves. Nor are they able to stop themselves from making poor choices when they do eat something.
The theory is likely to prove controversial, with weight-loss campaigners saying that individuals have different reasons for their struggle with their body.
The research involved taking brain images of 32 adults - 16 men and 16 women - from the US city of Baltimore in Maryland. Anyone who had a history of brain damage, substance abuse or mental illness was excluded from the group.
The researchers measured the participants' body mass index and body fat percentages and compared them to differences in brain structure and function.
Lead researcher Chase Figley, from the University of Manitoba, said the brain scans covered changes across the whole organ but also "specific networks". In particular he was interested in the "salience network", which he described as the "seat of motivation, willpower, and the ability to persevere through physical and emotional challenges".
The overall results showed there was "no significant different" in terms of white matter between people who had a normal weight and people who were fat. People with a higher BMI actually had slightly more grey matter.
However, when researchers looked at specific networks in the brain a different picture began to emerge. In particular, heavier and fatter people had less white matter in the salience network. There were also differences in the dorsal striatum, an area of the brain involved with habitual behaviour. Professor Figley said: "It stands to reason that these changes could further affect the ability of overweight individuals to exert self-control and maintain healthy lifestyle choices".
He said it was not clear if the brain differences predispose certain individuals to becoming fat, or vice versa, but added: "There are previous studies that imply elevated body fat can cause these sorts of brain changes."