Alzheimer's may start developing at least 20 years before symptoms first occur, writes Channel 4 News's Science Editor Tom Clarke.
That's according to a new study which has detected the ealiest-ever signs of the condition in the brains of people with a rare inherited form.
Last year Channel 4 News visited families in northern Colombia, many of whom carry the "Paisa" mutation - a genetic fault which means a carrier is certain to develop Alzheimer's. We then followed them as they were recruited into a clinical trial in the US, the first to investigate whether Alzheimer's can be prevented.
Now new research published in the journal Lancet Neurology reveals the results from preliminary tests conducted on the families.
Most carriers show symptoms of the disease around the age of 40. But some of the Colombian volunteers already had a key Alzheimer's protein in their spinal fluid from as young as 18.
"These findings suggest that brain changes begin many years before the clinical onset of Alzheimer's disease," said Dr Eric Reiman of the Banner Alzheimer's Insititute in Arizona.
They raise new questions about the earliest brain changes involved in the predisposition to Alzheimer's and the extent to which they could be targeted by future prevention therapies. Eric Reiman
"They raise new questions about the earliest brain changes involved in the predisposition to Alzheimer's and the extent to which they could be targeted by future prevention therapies."
Nearly all new drugs that target the build-up of clumps of protein called amyloid in the brains of Alzheimer's sufferers have failed in clinical trials.
Though they seem to reduce the amount of the protein, they fail to improve the classic symptoms of Alzheimers: long-term memory loss, confusion, irritability and depression.
The consensus now among Alzheimer's experts is that the best hope for curing the disease is by preventing or slowing damage to the brain before the worst symptoms take hold. But right now, there is no way of predicting who will develop Alzheimer's.
It is for that reason that these Colombian families are so important. The study volunteers come from a group of around 5,000 closely-related people from the province of Antiochia in the north of the country.
Around 30 per cent of them carry the mutation in a gene called presenilin 1. This mutation causes rare cases of inherited Alzheimer's in other families around the world, but it is most concentrated in the Colombian familes.
Because carriers of the mutation are certain to go on to develop the disease, they provide a unique oportunity to study Alzheimer's as it develops.
In the latest study, 44 volunteers between the age of 18 and 26 were tested. Twenty were carriers of the mutation and 22 were not. Before entering the trial they agreed not to find out whether they were carriers of the gene to ensure the investgation wasn't biased.
"By generously agreeing to take part in this research, these affected families are helping scientists get a unique insight into how this devastating disease develops," said Dr Simon Ridley, Head of Research at Alzheimer's Research UK.
The next stage of the research is to give the volunteers Alzheimer's drugs which have failed to reverse the disease in previous trials, with the hope that it may prevent the disease from progressing.
It's not yet certain if the inherited form of Alzheimer's follows the same course as the common "sporadic" form of the illness, which affects around one in 10 people.
But if it does, the new study suggests it might be possible to measure the first signs of the disease when people are in their 50s. If a reliable test is found, it might then be possible to test drugs on people to see if it will prevent Alzheimer's from developing in the wider population.
Dr Ridley said: "New drugs are being developed and tested to stop amyloid from taking hold, but studies like these show that timing could be crucial for whether these drugs are successful."