The new findings, published January 9 in the journal Neurology, suggest that the combination of obesity (as measured by body mass index, or BMI) and a high waist-to-hip ratio may be a risk factor for brain shrinkage, the researchers said. Similarly, the waist-to-hip ratio is scored, and a high score - above 0.90 for men and above 0.85 for women - means a person has central obesity, or a bigger belly than hips.
"It may be possible that someday regularly measuring BMI and waist-to-hip ratio may help determine brain health", Dr Hamer added.
Participants had their body mass index and waist-to-hip ratio compared with their brain volumes for the research by Loughborough University.
Only five per cent of those invited to participate in the study took part, and that group tended to skew healthier. People with higher numbers on both BMI and waist-to-hip ratio had the lowest gray matter volume. Specifically, researchers found that 1,291 people who had a high BMI and a high waist-to-hip ratio had the lowest average gray matter brain volume of 786 cubic centimeters, compared to 3,025 people of healthy weight who had an average gray matter brain volume of 798 cubic centimeters and 514 people with a high BMI but without high waist-to-hip ratio who had an average gray matter brain volume of 793 cubic centimeters. While grey matter is involved in reward processing and certain aspects of controlling behavior, it's not clear whether body fat is a driver of these changes in grey matter, or the result of them. However, excess weight was associated with shrinkage in specific regions of the brain: the pallidum, nucleus accumbens, putamen (linked only to a higher BMI) and caudate (linked only to a higher waist-to-hip ratio).
"Existing research has linked brain shrinkage". Researchers measured BMI, waist-to-hip ratio and overall body fat and surveyed participants about their health. White matter contains nerve fiber bundles that connect various regions of the brain.
It was unclear whether obesity lead to brain structure abnormalities or the other way around, he said.
About 500 participants with a high BMI but not a high waist-to-hip ratio also had an average amount of grey matter.
They found no significant differences in brain volumes of white matter.
Though the study didn't look at potential mechanisms linking visceral fat and brain shrinkage, one hypothesis is that this type of fat is thought to produce inflammatory substances that may play a role in brain atrophy, the researchers said.