The sticky plaques associated with Alzheimer's
Heightened activity in an area of the brain that deals with memory may give a subtle early warning of dementia decades later, UK research suggests.
It was known that carrying a rogue version of a gene called ApoE4 raised the risk of Alzheimer's disease.
Now researchers have linked the same mutation with raised activity in an area of the brain called the hippocampus in people as young as 20.
The study appears in Proceedings of the National Academy of Science.
The researchers, from Oxford University and Imperial College London, believe over-activity in the hippocampus may effectively wear it out, raising the risk of dementia in later life.
They hope their work could be a first step towards developing a simple method to identify people at increased risk of developing dementia.
They could then potentially be offered early treatment and lifestyle advice.
Carrying one copy of the rogue ApoE4 gene raises the risk of Alzheimer's by up to four times the normal, two copies by up to 10 times.
But not everyone with the rogue gene will develop the condition.
The latest study used scans to compare brain activity in 36 volunteers aged 20 to 35.
In those who carried the rogue gene activity in the hippocampus was consistently raised, even at rest.
Researcher Dr Clare Mackay said: "These are exciting first steps towards a tantalising prospect: a simple test that will be able to distinguish who will go on to develop Alzheimer's."
Dr Peter Nestor, a neuroscientist at the University of Cambridge, said: "The findings of this study are of considerable interest but should not be over-interpreted to mean that Alzheimer's disease is already starting to develop in this young, healthy group of volunteers.
"Whether or not the differences seen in those with ApoE4 can offer a clue to what makes some brains more likely to develop Alzheimer's is a challenge for future studies."
Rebecca Wood, of the Alzheimer's Research Trust, said the research was a "significant development".
"It takes us a step closer to accurately predicting who will develop Alzheimer's before any symptoms become apparent.
"However, we are not yet at that stage; those with the ApoE4 genetic variant - while at a statistically higher risk of developing the disease than others - will still not develop Alzheimer's in most cases.
"The causes of Alzheimer's are complex - both genetic and environmental - and if we can understand these better, we can enhance efforts to help people lower their risks."
Professor Clive Ballard, director of research at the Alzheimer's Society, said: "This study paves the way for further research that could help us understand how brain function in younger adults may contribute to the development of Alzheimer's disease later in life."