Overweight people's brains have aged ten years faster than those of healthy people by middle age, experts have found.
The research, by scientists at Cambridge University, suggests that being overweight also raises the risk of cognitive decline, which in turn increases the chance of dementia and other neurological conditions.
The team found that the brains of obese people had lost a surprising amount of white matter - the material that connects areas of the brain.
In healthy people, this degree of decline would only usually be seen in those ten years older, they said.
The academics studied brain scans of 473 people aged between 20 and 87. Their findings revealed overweight people had a widespread reduction in white matter, compared to those who were slim.
The team discovered that an overweight person at 50 had a comparable white matter volume to a healthy person aged 60.
They only found these differences from middle age onwards, suggesting brains may be most vulnerable during this period of ageing.
They found no difference in brain power or thinking skills among the people they studied, but white matter degradation is known to lead to cognitive decline at an older age.
The experts are not sure exactly why obesity is so strongly linked to the brain, but one theory is that being overweight changes the body's hormonal balance, which has implications for white matter levels. They suspect fat tissue might trigger the over-production of leptin hormones and molecules called cytokines, which prompts an inflammatory response that harms the wiring of the brain.
Writing in the Neurobiology of Aging journal, the scientists said: 'This biological mechanism suggests that the initial insult of obesity may lead to self-perpetuating damage.' Dr Lisa Ronan, of the department of psychiatry at the University of Cambridge, said: 'As our brains age, they naturally shrink in size, but it isn't clear why people who are overweight have a greater reduction in the amount of white matter.'
Professor Paul Fletcher added: 'We're living in an ageing population, with increasing levels of obesity, so it's essential that we establish how these two factors might interact, since the consequences for health are potentially serious.
'The fact that we only saw these differences from middle-age onwards raises the possibility that we may be particularly vulnerable at this age.
'It will also be important to find out whether these changes could be reversible with weight loss, which may well be the case.'