Loss of memory and other brain function can start as early as age 45, posing a big challenge to scientists looking for new ways to stave off dementia, researchers said.
The finding from a 10-year study of more than 7,000 British government workers contradicts previous notions that cognitive decline does not begin before 60 years of age, and it could have far-reaching implications for dementia research.
Pinpointing the age at which memory, reasoning and comprehension skills start to deteriorate is important because drugs are most likely to work if given when people first start to experience mental impairment.
A handful of novel medicines for Alzheimer's disease, the most common form of dementia, are currently in clinical trials, but expectations are low and some experts fear the new drugs are being tested in patients who may be too old to show a benefit.
Companies with products in development include Eli Lilly, working on a drug called solanezumab, and Elan and Johnson & Johnson, developing bapineuzumab.
The research team led by Archana Singh-Manoux from the Center for Research in Epidemiology and Population Health in France and University College London found a modest decline in mental reasoning in men and women aged 45-49 years.
"We were expecting to see no decline, based on past research," Singh-Manoux said in a telephone interview.
Among older subjects in the study, the average decline in cognitive function was greater, but there was a wide variation at all ages, with a third of individuals aged 45-70 showing no deterioration over the period.
"It doesn't suddenly happen when you get old. That variability exists much earlier on," Singh-Manoux said. "The next step is going to be to tease that apart and look for links to risk factors."
HEALTHY LIFESTYLE
Participants were assessed three times during the study, using tests for memory, vocabulary, and aural and visual comprehension skills.
Over the 10-year period, there was a 3.6 percent decline in mental reasoning in both men and women aged 45-49 at the start of the study, while the decline for men aged 65-70 was 9.6 percent and 7.4 percent for women.
Since the youngest individuals at the start of the study were 45, it is possible that the decline in cognition might have commenced even earlier.
Singh-Manoux said the results may also have underestimated the cognitive decline in the broader population, since the office workers in the study enjoyed a relatively privileged and healthy lifestyle.
Factors affecting cardiovascular function -- such as obesity, high blood pressure, high cholesterol and smoking -- are believed to impact the development of Alzheimer's and vascular dementia through effects on brain blood vessels and brain cells.
The research findings were published in the British Medical Journal, alongside an editorial by Francine Grodstein of the Brigham and Women's Hospital in Boston, who described the results as convincing.
Most research into dementia has focused on people aged 65 and over. In future, scientists will need to devise long-term clinical studies that include much younger age groups and may have to enroll tens of thousands of participants, she said.
One way to deal with this "major challenge" might be to use computerised cognitive assessment tests, rather than face-to-face interviews, although more research is still needed on this approach, she added.