The brains of overweight people look "10 years older" than those of leaner peers, a study has found.
Brains naturally lose white matter - the part of the brain that transmits information - as people age.
But a Cambridge University team found that loss was exacerbated with extra weight - so an overweight 50-year-old had a lean 60-year-old's brain.
Researchers said it shows we need to know relatively more about how extra weight affects the brain.
The team, from the Cambridge Centre for Ageing and Neuroscience, looked at the brains of 473 people aged between 20 and 87, dividing them into lean and overweight categories.
Their findings, published in the journal Neurobiology of Aging, found significant differences in the volume of white matter in the brains of overweight people compared with leaner individuals.
Those in the overweight group had much less white matter than their thinner counterparts.
The difference was only evident from middle-age onwards, suggesting that our brains may be particularly vulnerable during this period of ageing.
However there was no difference in how the groups fared in tests of knowledge and understanding, so the researchers say more work is needed to follow people and see who develops conditions such as dementia.
Dr Lisa Ronan, from the Department of Psychiatry at the University of Cambridge, who led the study, said it was not clear if obesity affected the brain, or vice versa.
She told the BBC: "Obesity is so complex. We know an awful lot about what it does to the body.
"But what it does to the brain and how it interacts with obesity - we're at the beginning of understanding that."
Prof Sadal Farooqi, from the Wellcome Trust Medical Research Council Institute of Metabolic Science at Cambridge University, who also worked on the study, said the work suggested the middle-aged brain could be particularly vulnerable.
"It will also be important to find out whether these changes could be reversible with weight loss, which may well be the case.
"This must be a starting point for us to explore in more depth the effects of weight, diet and exercise on the brain and memory."