Everyone talks about the importance of treating high blood pressure, the "silent killer." And everybody knows that untreated high blood pressure can lead to heart attacks and strokes. But can treating high blood pressure reduce your risk of cognitive impairment and dementia?
High blood pressure is a risk factor for cognitive impairment and dementia
Cognition encompasses thinking, memory, language, attention, and other mental abilities. Researchers have known for many years that if you have high blood pressure, you have a higher risk of developing cognitive impairment and dementia. However, just because high blood pressure is a risk factor, it does not necessarily mean that lowering high blood pressure will lower your risk. Many things in health and science correlate without one causing the other (my favorite is the correlation between the drop in birth rate and the decline in the stork population). Thus, randomized, double-blind, controlled studies are needed to answer this question.
Prior studies have not provided clear answers
There have, in fact, been a lot of these studies. The most recent relevant study is the SPRINT-MIND study, designed to measure the effects of lowering high blood pressure on dementia and/or mild cognitive impairment. This study was so successful at reducing the risk of mild cognitive impairment by lowering high blood pressure that it ended early, because the data and safety monitoring board felt that it was unethical to continue the control group. However, the dementia endpoint had not yet reached statistical significance — likely because of this early termination. Thus, while the study succeeded in one sense, it ultimately concluded that treating systolic blood pressure to below 120 mmHg (versus lower than 140 mmHg) did not reduce risk of dementia.
A new analysis of many studies
Because SPRINT-MIND and many other prior studies have not clearly shown whether lowering our high blood pressure can reduce our risk of cognitive impairment and dementia, meta-analyses are needed to answer this question. Researchers in Ireland looked at data from 14 studies comprising almost 100,000 participants, followed over an average of more than four years. They found that older individuals (average age 69) who lowered their blood pressure are slightly less likely to develop dementia or cognitive impairment (7.0% versus 7.5%). Thus, the answer is: Yes! Lowering high blood pressure will lower our risk of dementia and cognitive impairment.
The relationship between high blood pressure and dementia
So, how does lowering high blood pressure reduce our risk of cognitive impairment and dementia? Most people who have dementia don't have just a single cause. Two or even three different problems in the brain cause their cognitive impairment and lead to their decline in function. One study estimates that the fraction of dementia risk attributable to cerebrovascular disease — that is, strokes — was nearly 25% in people who developed significant memory loss late in life. These researchers also found that the dementia risk attributable to Alzheimer's disease was considerably higher, nearly 40%.
My reading of the literature is that lowering blood pressure reduces dementia risk because it reduces the risk of stroke. It's the strokes — not high blood pressure by itself — that cause cognitive impairment. Note, however, that the strokes may be so tiny that one doesn't even know that they have them. But developing a lot of these tiny strokes (or a few big ones) will greatly increase our risk of dementia.
Optimal blood pressure for optimal brain health
Okay, but what's considered a healthy blood pressure from the perspective of the brain? The SPRINT-MIND study answers that question: people are less likely to develop mild cognitive impairment if their systolic blood pressure is lower than 120 mm Hg compared to the control condition of between 120 and 140 mm Hg. Thus, for optimal brain health, it's best to keep your systolic blood pressure below 120 mm Hg — at least according to the SPRINT-MIND study.
The bottom line
The take-home message is clear: You can reduce your risk of cognitive impairment and dementia by lowering your systolic blood pressure to less than 120 mm Hg, preferably with aerobic exercise, a Mediterranean diet, and a healthy weight, and by adding medications if those lifestyle changes alone are not sufficient.