The discovery points to a link between excess body fat and impaired mental ability or dementia.
Scientists looked at a total of 9,652 people with an average age of 55.
Of the whole group, 19 per cent were shown to be obese according to their body mass index (BMI), a measurement which relates weight and height.
Magnetic resonance imaging (MRI) was then used to determine the brain volumes of participants in different parts of their brain.
Among people with higher BMIs, those with thicker waists had lower grey matter brain volumes than those who were slimmer round the middle.
The lowest grey matter brain volume, seen in 1,291 participants, was 786 cubic centimetres (cc).
This compared with a volume of 798 for around 3,000 people of healthy weight.
Grey matter in the brain consists mostly of nerve cells while “white matter” is made up of connecting nerve fibres.
The study, published in the journal Neurology, found no significant differences in white matter volume.
“Existing research has linked brain shrinkage to memory decline and a higher risk of dementia, but research on whether extra body fat is protective or detrimental to brain size has been inconclusive,” said lead researcher Dr Mark Hamer, from Loughborough University.
“Our research looked at a large group of people and found obesity, specifically around the middle, may be linked with brain shrinkage.”
It was unclear whether obesity leads to brain structure abnormalities or the other way around, he said.
Links were also found between obesity and shrinkage in specific regions of the brain.
“It may be possible that, someday, regularly measuring BMI and waist-to-hip ratio may help determine brain health,” Hamer added.
A limitation of the study was that only 5 per cent of those invited to participate ended up taking part, the researchers pointed out. Those who did participate tended to be healthier than those who did not.
Therefore the results may not provide a true population-wide picture.
Dr James Pickett, head of research at Alzheimer’s Society, says: “We don’t know what precisely links obesity, brain health, and risk of dementia, but it is likely that poorer cardiovascular health plays a role.
“Our researchers are working day-in, day-out to try and further unpick this.”
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