Researchers from the U.K.'s Loughborough University and UCL discovered that people with a high body mass index (BMI) and high waist-to-hip ratio had brains that were 12 cubic centimetres smaller than people of a healthy weight.
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.
The study found that 1,291 people who had a BMI of 30 or higher and a high waist-to-hip ratio had the lowest average grey matter volume, at 786 cubic centimetres; 514 people with a BMI of 30 or higher but without central obesity had an average grey matter volume of 793 cubic centimetres.
Grey matter contains most of the nerve cells, and includes regions of the brain involved in muscle control, and sensory perception such as seeing, hearing, memory, speech, decision making, self-control etc. People with higher numbers on both BMI and waist-to-hip ratio had the lowest gray matter volume. A 2018 study, for instance, has shown that people with excess belly fat have twice the risk of suffering from heart attack or die from cardiovascular problems regardless of their weight.
Dr. Gayatri Devi, a neurologist at Lenox Hill Hospital in NY, who was not involved with the study, agreed with the findings.
After using a tape measure to calculate people's waist-to-hip ratio, scientists put them in MRI scanners to look at their brains. Obesity, low BMI linked to increased risk of death, study reveals The study also showed no real differences in white matter brain volume linked to obesity.
It could be that people with lower volumes of gray matter in certain brain areas are at a higher risk of obesity.
Looking at both BMI as well as waist-to-hip ratio clarifies what role different types of body fat may play in affecting the brain, Hamer says.
"Our research looked at a large group of people and found obesity, specifically around the middle, may be linked with brain shrinkage". Because the study only analyzed body fat and brain images at a single point in time for all of the participants, it's also possible that the reverse effect could be occurring, and that brain changes are responsible for changing eating habits and satiety and therefore driving the increase in body fat.
A limitation of the study was that only 5% of those invited to participate ended up taking part, the researchers pointed out.