Mid-life stress can increase the risk of women developing Alzheimer's disease, a study has shown.
Women who reported repeated episodes of stress and anxiety in middle age were up to twice as likely to develop dementia than those who did not, a team of Swedish scientists found.
The majority of those affected were diagnosed with Alzheimer's, the most common form of dementia.
Researchers followed the progress of 1,415 women between 1968 and 2000.
Three surveys in 1968, 1974 and 1980 were carried out to assess levels of psychological stress experienced by the women, who were aged between 38 and 60 at the start of the study.
Stress was defined as a "sense of irritation, tension, nervousness, anxiety, fear or sleeping problems" lasting a month or more.
During the course of the study, 161 of the women taking part developed dementia, mainly in the form of Alzheimer's disease.
Dementia risk was 65% higher in women who suffered frequent stress in middle age.
The chances of developing dementia increased as women responded to more than one survey by saying they were frequently stressed.
The risk increased by 73% when women reported frequent or constant stress on two occasions, and more than doubled when all three surveys showed they were stressed.
Researcher Lena Johansson, from the University of Gothenburg, said: "This is the first study to show that stress in middle age can lead to dementia in old age, and confirms similar findings from studies of animals.
"Stress has previously been shown to increase the risk of cardiovascular diseases such as stroke, heart attack and hypertension.
"This study could result in a better understanding of the risk factors for dementia, but our results need to be confirmed by other studies, and further research is needed in the area.
"Most of those who said that they were stressed did not develop dementia, so it's not currently possible to advise people to be less stressed or warn about the dangers of high stress levels due to an increased risk of developing dementia."
The findings were published in the journal Brain.