Conducted among 1,350 adults aged 53 to 69, the CARDIA study, published in The Lancet and led by the University of California at San Francisco, shows that a biological signature characteristic of Alzheimer’s disease is already associated with an increased risk of cognitive decline several years before any diagnosis of dementia. A discovery that could transform prevention, without constituting an infallible prediction tool.
When the brain begins to change well before the first forgetting
Alzheimer’s disease does not appear overnight. For several years, researchers have known that the biological mechanisms underlying it are gradually established in the brain, sometimes several decades before the first symptoms appear.
This is precisely what a large new American study has confirmed. Researchers analyzed data from 1,350 participants in the CARDIA study, followed for many years as part of a program dedicated to aging and cardiovascular health. None of them had dementia at the time of evaluation.
The scientists looked for two proteins in their blood closely linked to Alzheimer’s disease: amyloid beta and tau. These biomarkers are today considered the main biological signatures of the disease.
Five years later, the risk of decline multiplied by four
Result: around 6% of participants simultaneously had high levels of these two proteins. A modest proportion, but whose cognitive consequences have proven to be significant.
- Affected people already had lower scores in some essential brain functions, including information processing speed and executive functions, which enable planning, organizing or adapting to new situations. In contrast, no marked differences were observed regarding overall memory or language.
- The interest of this study also lies in its longitudinal follow-up. The researchers compared participants’ cognitive performance over a five-year period. And the results are striking. People with positive biomarkers were between 2.5 and 4 times more likely to experience a rapid decline in their verbal memory. Their risk of seeing their information processing speed decrease was multiplied by three to four.
In other words, the presence of these proteins in the blood seems associated not only with cognitive differences already perceptible in middle age, but also with a more rapid trajectory of decline over time.
These results suggest that the first cognitive changes associated with Alzheimer’s could be much more discreet than previously imagined, first manifesting as reduced mental speed or organizational difficulties even before the first perceptible memory problems.
Not a sentence but a chance for personalized prevention
However, researchers call for caution. Biomarkers were measured at a single time point during the study, at the end of the follow-up period. They therefore cannot predict with certainty that a person will develop Alzheimer’s disease in the years to come.
“The pathology of Alzheimer’s disease begins years before symptoms appear.explains Kristine Yaffe, vice chair of the department of psychiatry and behavioral sciences at the University of California, San Francisco. “Detecting the disease early means patients can target modifiable risk factors and perhaps seek alternative care.”.
This distinction is fundamental. A biomarker is not a verdict. It indicates the presence of a potentially concerning biological process, but does not alone predict an individual’s cognitive fate.
An opportunity to act before it is too late
If this blood test arouses so much interest, it is not because it would today make it possible to diagnose Alzheimer’s in asymptomatic people. Its interest lies elsewhere: offering an earlier window of action, to identify the people who would benefit the most from strengthening their cerebral lifestyle. Because contrary to what is still widely believed, a significant part of the risk of dementia depends on modifiable factors.
“For some people who discover they have these biomarkers, the tests could provide a window to undertake interventions that could delay the onset of Alzheimer’s disease.”says Kristine Yaffe.
According to the work of Kristine Yaffe and many other researchers, up to 40% of cases of dementia could be delayed, or even avoided, by acting on certain known determinants: insufficient physical activity, intellectual sedentary lifestyle, smoking, depression, hypertension, cardiovascular diseases, sleep disorders or even social isolation.
Currently, these blood tests are mainly used in patients already presenting symptoms suggestive of Alzheimer’s disease. They do not detect other forms of dementia, which represent around 30 to 40% of cases.
In a society where the number of people living with dementia continues to increase, this ability to identify the first biological signals could well mark a turning point. Not that of an announced cure, but that of more personalized, earlier and, perhaps, more effective prevention.