Just one stressful event early on in life could cause the brain to age by as much as four years, scientists say, as fresh research points to such stress contributing to the risk of dementia.
The new research, presented at a conference in London on Sunday, looks at how stress and dementia are related, with the results helping account for higher incidents of such degenerative diseases among African Americans in the US, who are almost twice as likely to suffer from the disease over the age of 65.
"These studies were done with US data, but they add weight to the global body of evidence around disadvantage and dementia risk, which is an issue governments around the world grapple with, and one that requires coordinated action," said Alzheimer's Association chief scientist Maria Carrillo.
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"For a racially diverse nation like the United States, and to address Alzheimer's and dementia on a global scale, these findings support the need for targeted interventions, whether preventive or service-driven, to help address the gaps we know exist - and for more research."
One study of more than 1,000 adults by the University of Wisconsin looked at stressful experiences such as being fired from a job, the death of a child, or growing up with a parent who abused alcohol or drugs and how they impacted cognitive abilities such as problem solving and memory.
Rates of stress were found to be 60 per cent higher among African Americans and these events predicted cognitive abilities even more than traditional risk factors such as age, education and genetics.
"Adversity is a clear contributor to racial disparities in cognitive ageing, and further study is imperative," said researcher on the study Megan Zuelsdorff.
A second study found that rates of dementia were higher among African Americans in states with a high infant mortality rate when compared with white counterparts, suggesting the long term impact on the brain of early events.
"African Americans born around 1928 were likely exposed to harsher early life conditions that may have increased their risk of dementia later in life," said University of California researcher Paola Gilsanz. "Our findings suggest that differences in early life conditions may contribute to racial inequalities in dementia rate, and they point to growing evidence that early life conditions contribute to dementia risk in late life."
And a third study of nearly 1,500 people by another group of researchers at the University of Wisconsin found "markedly worse" cognitive performance, based on tasks such as verbal learning, immediate memory and speed and flexibility of cognition, in people from the most disadvantages neighbourhoods.
"This study provides evidence to suggest that living in a neighborhood challenged by poverty, low education, unemployment, and/or substandard housing may increase risk of Alzheimer's disease, and may account for some of the observed differences in Alzheimer's disease risk among people of different racial backgrounds and income levels," said lead researcher Dr Amy Kind.
A fourth study by Kaiser Permanente found that the disparity between African Americans and other ethnicities continued in late life, with a 30 per cent greater risk of dementia.
"Given the increasing ethnic diversity in the coming decades, it is imperative to identify what factors contribute to the differences in rates, whether genetic, social, or lifestyle, as some of these factors may be modifiable," said Rachel Whitmer, senior scientist at the US health firm's research division.