Overweight people are less intelligent than people who are do not have weight problems , a provocative study claims.

Overweight men and women have less grey and white matter in key areas of the brain, it suggests. They also have greater impulsiveness and "altered reward processing", the study said.

The researchers said that their findings could explain why overweight people make poor diet choices - they do not have the mental capacity to control themselves.

The theory is likely to prove controversial as weight loss campaigners have emphasised that the issues behind weight problems vary from individual to individual.

The research involved sophisticated brain images of 32 adults - 16 men and 16 women - selected 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.