Fatter people had lower levels of white matter in this part of the brain

Looked at salience network of brain linked to motivation and willpower

Researchers compared BMI and levels of grey and white brain matter

Fat people are less intelligent than people with a normal weight, a provocative study claims.

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 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.

Nor are they able to stop themselves from making poor choices when the do eat something.

The theory is likely to prove controversial as weight loss campaigners have emphasised that each individual has different reasons for their struggle with their body.

Overweight men and women have less grey and white matter in key areas of the brain, the study claims

The research involved sophisticated brain images of 32 adults recruited from the city of Baltimore in Maryland, 16 men and 16 women

Anyone who had a history of brain damage, substance abuse or mental illness was excluded from the group.

Outlining the object of the study, the authors said: ‘It has been suggested that body composition itself might somehow affect the neural systems that underlie cognition, motivation, self-control and salience processing, which would in turn affect one’s ability to make better lifestyle choices, forgoing immediate and/or highly salient rewards for the sake of longer-term health and wellness goals’.

The researchers measured Body Mass Index, a commonly measure of how overweight a person is, and body fat percentages and compared them to differences in brain structure and function.

Lead researcher Chase Figley, an assistant professor in the department of radiology at the University of Manitoba, said that the brain scans were very thorough.

He said they covered changes across the whole brain 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 results showed that there was ‘no significant difference’ in terms of white matter between people who had a normal weight and people who were fat.

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

In a surprise twist, people with a higher BMI actually had slightly more grey matter overall.

However looking at specific networks on 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 told the National Post, a Canadian newspaper: ‘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 added that it was not clear if the brain differences predispose certain individuals to becoming fat, or vice versa.

However he said: ‘There are previous studies that imply elevated body fat can cause these sorts of brain changes’.

Britain is the fattest country in Europe and 67 per cent of men and 57 per of women are overweight or obese.

Some 26 per cent of boys and 29 per cent of girls are overweight or obese, compared to 17.5 per cent and 21 per cent in 1980.

The costs of treating overweight people for conditions like diabetes and heart problems is already costing billions with some saying that it could bankrupt the NHS in the future.

Obesity has also been linked to dementia and early onset brain shrinking.