Scientists have discovered a chemical in blood that indicates whether people will have declining brain function.
Looking for the earliest signs of Alzheimer’s disease, they analysed levels of 1,129 proteins circulating in the blood of more than 200 twins.
These were compared with data from cognitive-function tests over the next decade, in Translational Psychiatry.
And levels of one protein, MAPKAPK5, tended to be lower in those people whose brains declined.
MAPKAPK5 is involved in relaying chemical messages within the body, although its connection with cognitive decline is unclear.
Dementia cases are expected to treble globally by 2050, but there is no cure or treatment.
It can take more than a decade from the first changes in the brain to culminate in symptoms such as memory loss, confusion and personality change.
And drug companies believe they need to treat patients years before symptoms appear in order to protect the brain.
‘Long way off’
Dr Steven Kiddle, a Medical Research Council scientist at King’s College London, told the BBC News website: “People think it may be hard to reverse 20 years of potential damage to your brain.
“But if you could start much earlier in that process, then you might be able to find something that works.”
He said a blood test could help identify people for clinical trials.
But he added: “A test you could go in to your doctor to say, ‘Do I have Alzheimer’s disease or not?’ I think that’s a long way off.”
The twins in the study will have to be followed for many more years to see whether levels of the protein predict dementia.
Dr Eric Karran, from the charity Alzheimer’s Research UK, said: “This study associated blood levels of a protein called MAPKAPK5 with cognitive decline over a 10-year period, but it will be necessary to investigate more about a possible mechanism linking this protein to changes in memory and thinking.
“Current diagnosis of diseases like Alzheimer’s is not an exact science, and we urgently need to improve approaches to deliver more timely and accurate diagnosis.
“Accurate and early diagnosis of Alzheimer’s will be essential for the development of new treatments.”