Early signs of Alzheimer's can be detected in apparently healthy people with a combination of brain scans and spinal fluid testing, a study has shown.
Experts believe early diagnosis could lead to much more effective pre-emptive treatment of the disease.
The research involved 105 people in their 70s and 80s who appeared outwardly free of dementia.
First, the group was split into individuals with high and low levels of cerebrospinal fluid (CSF) amyloid, which is typically reduced in Alzheimer's patients.
Magnetic resonance imaging scans were then conducted over 12 months to measure rates of brain shrinkage.
The results showed that the brains of people with low CSF amyloid levels, 38% of the group, 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.
Rebecca Wood, chief executive of the Alzheimer's Research Trust which part-funded the study, said: "We are hamstrung by our inability to accurately detect Alzheimer's but these findings could prove to be pivotal.
"Spotting Alzheimer's early is essential to the global research effort to beat the disease. We know that treatments for many diseases can be more successful if given early and this is likely to be true for Alzheimer's.
"It will be crucial to keep following the study group to see how many develop Alzheimer's, and to expand the research to test the approach further."
Dr Anne Corbett, from the Alzheimer's Society, said: "Detecting dementia early is really important. It can open doors to new treatment targets and could one day go hand in hand with an Alzheimer's vaccine that scientists are edging slowly towards.
"Testing spinal fluid is a good way of detecting Alzheimer's early but it is desperately under used in the UK. If we change our attitudes and invest more in research we could give hope to the million people who will develop dementia in the next 10 years. We particularly want to see this research repeated over a longer period of time to confirm the findings."