People who are prone to psychological distress are more likely to develop Alzheimer's disease, research suggests.
Distress may cause changes in the brain
Rush University in Chicago found people plagued by negative emotions like depression and anxiety were at double the risk of more laid-back individuals.
The researchers focused on 797 Catholic nuns, priests and brothers with an average age of 75.
The research, published in Neurology, suggests that anti-depressants may cut the risk of Alzheimer's.
Lead researcher Dr Robert Wilson said levels of stress varied widely from person to person - but tended to stay constant throughout an individual's life.
He said chronic stress had been associated with changes in the hippocampal area of the brain, which plays a role in learning and memory.
Possible drug treatments
He said the findings of the latest research were important because evidence has shown that many of the adverse effects of stress on the brain can be blocked by drugs, including antidepressants.
However, he said: "Much more research is needed before we can determine whether the use of antidepressants could help reduce the risk of Alzheimer's disease."
The researchers examined the brains of 141 people - including 57 with Alzheimer's disease - who died during the course of the study.
They found that those people who were vulnerable to stress did not always show signs of the plagues and tangles of protein in the brain that have been closely linked to Alzheimer's.
This enabled them to rule out the theory that high stress levels were simply an early symptom of Alzheimer's.
Professor Clive Ballard, director of research at the Alzheimer's Society, said previous research had also linked Alzheimer's to depression and stress.
However, he said methodological problems had made it difficult to draw firm conclusions.
He said: "This work takes our understanding forward substantially as it highlights stress and mood symptoms as an important trigger of subsequent dementia in a very rigorous long-term follow-up study.
"There are several possible explanations. Most likely the stress itself results in an increase in certain hormones, such as cortisol which may be damaging to the brain.
"Alternatively, an underlying disease process such as damage to the small blood vessels triggers both the depression and the dementia.
"Unravelling these issues offers the exciting potential to prevent or delay dementia in many at-risk people."
Rebecca Wood, chief executive of the Alzheimer's Research Trust, said it was still not clear exactly what was the relationship between stress, depression and Alzheimer's disease.
However, she said there was evidence that stress and depression could damage brain structures which were important for memory and learning.