Scientists say they have identified a section of the brain which, when damaged by stress hormones, can cause the onset of dementia.
A normal sized cingulate (top) and a shrunken cingulate (bottom)
High levels of stress hormone have been found to shrink the anterior cingulate cortex at the centre of the brain.
Researchers at the University of Edinburgh say brain scans showing a shrunken cingulate indicates you could develop Alzheimer's in the future.
It is the first time the link has been found in human brains.
The study's author, Dr Alasdair MacLullich said he was "very excited" by the £130,000 study's findings.
"If this part of the brain is smaller then you are likely to have higher levels of stress hormone and are at higher risk of developing dementia and depression," he said.
"This could be a marker, an indicator that your brain might go wrong in the future."
A team of six researchers looked at stress hormone levels in 20 healthy male volunteers aged between 65 and 70 for the study.
It found that people with a smaller anterior cingulate cortex had higher levels of stress hormones.
Doctors have known for years that certain diseases common in ageing like Alzheimer's disease and depression can be associated with shrinkage of the brain.
But Dr MacLullich, from the university's geriatric medicine department, said: "This is the first study in the world that links high stress hormones with the cingulate.
"We've had a lot of interest from psychologists, as this hasn't been looked at before.
"The discovery deepens doctors' understand of ageing, depression and Alzheimer's diseases, and will help in the development of treatments based on reducing high levels of stress hormones."