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We all know it’s generally healthier to have low blood pressure than high, and research has shown that high blood pressure in youth or middle age can lead to diminished cognitive abilities in old age.

But if your blood pressure drops in old age after having been high earlier on, you’re not necessarily in the clear, according to a new study published Wednesday in the journal Neurology. In fact, the study says, a person’s mid-life blood pressure is a better predictor of late-life dementia than blood pressure at the time of dementia.

“High blood pressure at a younger age damages vessels, so later you show low blood pressure because the vessels are stiff,” said Lenore Launer, a co-author of the report and head of the neuroepidemiology section of the Intramural Research Program at the National Institute on Aging, which is one of the study’s funders. Such patients are at risk for small vessel disease and perform more poorly on memory tests later in life, she said.

The study looked at the vascular health, cognitive function and brain structures of 4,057 men and women in their late 40s and early 50s and then again in their 70s and early 80s.

Chronic hypertension in midlife not only damages the vascular system, which can lead to reduced cerebral blood flow, but it also leaves brain tissue vulnerable to potentially harmful effects of low blood pressure such as inadequate delivery of blood to the brain, resulting in dizziness, Launer said.

“Just measuring blood pressure in older age is not enough to understand why people are complaining of brain problems – you need to know their history.”

The study also found that older participants who have high blood pressure are at risk for cerebrovascular lesions. An upcoming trial will look at whether lowering blood pressure in older age has any effect on dementia, even for people with damaged vessels, Launer said.

The takeaway? Keep your blood pressure low in middle age, so your low numbers in later years will be true indicators of good cardiovascular health and brain function.