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.