Scans show greater shrinkage in volume of white matter in overweight and obese people, although it does not appear to affect cognitive performance

The brains of people who are obese or overweight appear to have aged an extra 10 years compared to their lean peers from middle age onwards, brain scanning research has revealed.

The difference, scientists say, corresponds to a greater shrinkage in the volume of white matter, although they don’t know the cause. It might be down to genes causing both brain-shrinking and obesity, or it could be that changes occurring in the brain lead to overeating. Either way, it does not appear to affect cognitive performance.

White matter is tissue, composed of nerve fibres, that aids communication between different regions of the brain. The volume of white matter in a human brain increases during youth and then decreases with age for both lean people and those who are overweight or obese.

But researchers have discovered that this shrinkage differs depending on a subject’s BMI.

“The overall message is that brains basically appear to be 10 years older if you are overweight or obese,” said Lisa Ronan, first author of the study from the University of Cambridge.

Despite a higher BMI being linked to a smaller volume of white matter, it did not appear to have any link to mental prowess, with no difference seen between lean and overweight or obese participants when they were subjected to IQ tests. “While cognition does change in the general population as you age, that is just perfectly normal; there were no differences in these changes between lean and overweight or obese [people],” Ronan said.



While the new research involved only cognitively healthy individuals, Ronan adds that further work is needed to probe the influence of BMI-linked brain ageing and neurodegenerative conditions. “This study raises the possibility that if you are overweight or obese you may be more susceptible to diseases [linked] to age-related decline such as dementia and Alzheimer’s,” she said.

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Writing in the journal Neurobiology of Aging, scientists from the University of Cambridge and Yale University describe how they sought to unpick links between obesity and brain structure by analysing MRI scans from 473 individuals aged between 20 and 87.

The researchers split the participants into two categories based on their BMI, with 246 classified as lean and 227 as overweight or obese. They then analysed the brain scans for the two groups, taking into account sex and various self-reported health issues such as high blood pressure and diabetes, to create a computer model that explored how the volume of white matter changed with age among the participants.



When the researchers looked at those over the age of 37, the age at which the volume of white matter starts to decrease, they discovered those who were overweight or obese had a smaller volume of white matterthan lean subjects. This difference in white matter volume was greatest around the age of 40 and then stabilised, corresponding to an increase in brain age of around a decade compared to lean participants.

But, Ronan warns, it is not yet clear whether an increased BMI is driving the effect. “It could be that the genes that are responsible for obesity could also be responsible for smaller brains, or it could be that if you have a brain change that could lead to overeating,” she said.

“Everybody understands the impact of obesity on your body, but on your brain we are at a loss really, we don’t understand yet,” she added. “This is part of the puzzle.”

Claudia Metzler-Baddeley, from Cardiff University’s Brain Research Imaging Centre, said the research backs up previous suggestions that obesity and brain structure are linked. But, she added, “It is cross-sectional study - so it is not following people up over time. That is always a limiting factor. It doesn’t allow you to make any inferences about cause and effect.” What’s more, she says, self-reports of health and lifestyle factors are prone to inaccuracies, while the use of BMI also has drawbacks. “You can have a very high BMI just simply because of high muscle mass,” she said.



Dr James Pickett, head of research at Alzheimer’s Society, said: “The relationship between obesity and dementia is complex and studies on the subject have shown contradictory results. While this new research indicates that obesity may have an impact on how the brain ages, it does not show a link between obesity and increased risk of memory and thinking problems.”

“The jury is still out regarding any potential links between obesity and risk of memory loss and dementia – but we do know that what is good for the heart is also good for the head. Evidence shows that the best ways to reduce your risk of dementia include following a healthy diet, taking plenty of exercise and stopping smoking.”