Spinal fluid tests and brain scans could be used to detect the beginnings of Alzheimer's in people who appear to be healthy, a study has revealed.
Making an early diagnosis could mean that the disease is treated with more effective pre-emptive methods, experts believe.
The study was conducted on 105 people in their 70s and 80s who did not show any signs of suffering from dementia.
Researchers split the group by whether an individual had high or low levels of cerebrospinal fluid (CSF) amyloid, which is normally reduced in patients with Alzheimer's.
The rate of brain shrinkage was then monitored with magnetic resonance imaging scans over a 12 month period.
Results of the scans revealed that the brains of the 38% of participants with low CSF amyloid levels shrank twice as quickly as those with higher levels.
These individuals were also five times more likely to possess the Alzheimer's risk factor gene, APOE4, and had higher levels of another protein linked to the disease, called tau.
Study leader Dr Jonathan Schott, from University College London's Institute of Neurology, said: "In this study of healthy people in their 70s and 80s we found that about one in three had a spinal fluid profile consistent with Alzheimer's disease.
"Using MRI scanning, we showed that these individuals also had increased brain shrinkage over the following year.
"The significance of these findings will only be clear with longer clinical follow-up but may suggest that these individuals are at increased risk of developing dementia. If so these results add to a growing body of work suggesting that Alzheimer's disease starts many years before the onset of symptoms."
The findings are published online in the journal Annals of Neurology.