The study found that women who developed the condition, also known as hypertension, in their forties were 73 per cent more likely to develop debilitating brain decay than women who had stable, normal blood pressure throughout their thirties and forties.
Doctor Rachel Whitmer, who led the American research said: “High blood pressure in midlife is a known risk factor for dementia.
“But these results may help us better understand when this association starts, how changes in blood pressure affect the risk of dementia and what the differences are between men and women.“
The study involved 7,238 people who were part of the Kaiser Permanente Northern California health care system.
They all had blood pressure checks and other tests from 1964 to 1973 when they were an average age of 33, then again when they were an average age of 44.
Around 22 per cent of the participants had high blood pressure in their thirties 31 per cent of men and 14 per cent of women. In their forties, 22 per cent overall had high blood pressure, but the make-up was 25 per cent of men and 18 per cent of women.
The researchers then identified the 5,646 participants who were still alive and part of the Kaiser Permanente system in 1996 and followed them for an average of 15 years to see who developed dementia.
During that time, 532 people were diagnosed with dementia.Having high blood pressure in early adulthood, or in their thirties, was not associated with any increased risk of dementia.
But having high blood pressure in mid-adulthood, or in their forties, was associated with a 65 per cent increased risk of dementia for women.
Women who developed high blood pressure in their forties were 73 per cent more likely to develop dementia than women who had stable, normal blood pressure throughout their thirties and forties, according to the findings.The results, published online by the journal Neurology, were the same when researchers adjusted for other factors that could affect risk of dementia, such as smoking, diabetes and body mass index (BMI).
Dr Whitmer added: “Even though high blood pressure was more common in men, there was no evidence that having high blood pressure in one’s thirties or forties increased the risk of dementia for men.
“More research is needed to identify the possible sex-specific pathways through which the elevated blood pressure accelerates brain aging.
”She said for women who made it to age 60 without dementia, the cumulative 25-year risk of dementia was 21 per cent for those with high blood pressure in their thirties compared to 18 per cent for those who had normal blood pressure in their thirties.