Lower brain volume, or brain shrinkage, has been linked with an increased risk of memory decline and dementia. Of that group, 19 per cent were obese, according to their body mass index (BMI), calculated as weight in kilograms divided by height in metres squared.
The study looked at more than 9,600 people across Britain, with an average age of 55.
The scientists found that people with both higher BMI (defined as equal to or greater than 30 kg/m2) and higher waist-to-hip ratio measurements had lower grey matter volume in the brain compared to those who were leaner.
They then measured the participants' waist-to-hip ratio, determined by dividing waist circumference by hip circumference.
The findings held even after researchers took into account other factors that can affect brain volume, including age, smoking and high blood pressure. Fat accumulated around the middle, which would be represented by a high waist-to-hip ratio, tends to have more toxic effects as it tends to surround abdominal organs like the liver, stomach and intestines than subcutaneous fat, which forms under the skin, by triggering inflammation that can drive everything from heart disease to conditions like arthritis.
About 500 participants with a high BMI but not a high waist-to-hip ratio also had an average amount of grey matter.
"It may be possible that someday regularly measuring BMI and waist-to-hip ratio may help determine brain health", Dr Hamer added.
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The discovery points to a link between excess body fat and impaired mental ability or dementia.
This compared with a volume of 798 for around 3,000 people of healthy weight.
But in the meantime, the results confirm the benefits of staying lean: not just to lower risk of heart-related problems, but to possibly keep the brain healthy as well. The white matter, however, did not appear to be affected by obesity.