(CNN) - Big waist, small brain? If you're too heavy, especially around your middle, you probably have shrunken gray matter volume in your brain, according to a study published Wednesday in the journal Neurology. Gray matter contains most of your brain's 100 billion nerve cells, while white matter is filled with nerve fibers that connect the brain regions.
"Previous studies have shown associations between gray matter atrophy and risk of developing dementia," study author Mark Hamer, a professor of exercise as medicine at Loughborough University in England, wrote in an email.
Obesity linked to reduced brain volume in four regions
The study of 9,652 middle-age people in the UK measured body mass index (BMI) and waist-to-hip ratio. BMI is a formula involving a person's weight and height; a BMI score between 18.5 and 24.9 is considered healthy, while above 30 is considered obese. 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.
Based on these criteria, nearly one in five of the study participants was found to be obese.
Hamer and his co-researchers also used an MRI to scan participants' brain volume. The researchers factored in age, physical activity, smoking and high blood pressure, all of which might lead to reduced volume.
What did they find? People with higher numbers on both BMI and waist-to-hip ratio had the lowest gray matter volume. "The reductions in brain size increase in a linear fashion as fat around the middle grew larger," Hamer wrote.
In hard numbers, 1,291 people who had a BMI of 30 or higher and a high waist-to-hip ratio had the lowest average gray matter volume, at 786 cubic centimeters; 514 people with a BMI of 30 or higher but without central obesity had an average gray matter volume of 793 cubic centimeters. Meanwhile, 3,025 people with overall healthy scores had an average gray matter volume of 798 cubic centimeters.
The study also showed no real differences in white matter brain volume linked to obesity. 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). All of these brain regions are involved in motivation and reward.
"It's unclear if abnormalities in brain structure lead to obesity or if obesity leads to these changes in the brain," Hamer said.
Potential causes of lower brain volume
Cara Bohon, an assistant professor in the Department of Psychiatry and Behavioral Sciences at the Stanford University School of Medicine, wrote in an email that the study's findings are "not particularly new or surprising."
The study's strength is its size with "a sample of almost 10,000 individuals," wrote Bohon, who was not involved in this research. Previous studies had come to similar conclusions, but few participants were involved, so scientists could not feel confident in the results, she explained.
"One particularly interesting finding is that, among individuals with obesity, those with greater waist to hip ratio (a marker of visceral fat around the abdomen), showed even lower gray matter volume," Bohon said. This connection between reduced brain volume and abdominal fat could suggest that inflammation and vascular factors may be at work.
Because the cause of a connection between brain volume and obesity is still unclear, perhaps something may be causing both, she wrote: "For example, if there are nutritional factors impacting brain volume, these same nutritional factors could be a direct cause of obesity."
Future research should explore inflammation, nutrition and vascular health to better understand potential links between brain health and obesity, she said. Bohon's own research suggests that weight loss can reverse brain changes, as it showed that when people lost weight, whether a lot or a little, brain volume improved.
Hamer said his research will help scientists learn more about how obesity relates to the risk of neurodegenerative disease. "Obesity can have a detrimental impact on a wide range of health parameters," he wrote. "People should strive to maintain normal body weight."
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