"Fifty-year-olds with slightly raised blood pressure are at an increased risk of getting dementia in later life," The Independent reports.
A long-running study of 8,639 British civil servants found that people who had blood pressure above the ideal level – but below that used to diagnose high blood pressure – were more than a third more likely to get dementia.
The link between high blood pressure and dementia has been known for some time. It's thought to be because high blood pressure can cause bleeding and damage to the brain.
Previous studies haven't agreed on the level of blood pressure that creates this risk, or the age at which this risk begins.
Most guidelines recommend treating people for high blood pressure once it reaches 140mmHg systolic pressure (the pressure when the heart beats and pushes blood around the body).
But this study found the risk of dementia rose from about 130mmHg systolic pressure for people aged 50.
High blood pressure when people were older wasn't linked to dementia risk, perhaps because the damage to the brain is done over decades of high blood pressure.
If you're concerned about the findings of this study, the best first step is to get your blood pressure tested. Blood pressure testing is available from GP surgeries and some pharmacies.
Find out more about blood pressure and how to keep it healthy.
The researchers who carried out the study are from University College London and the French National Institute of Health and Medical Research.
The study was funded by the US National Institutes of Health, the UK Medical Research Council, and The British Heart Foundation.
Most of the reports in the UK media were broadly accurate, but reported a higher risk figure of 45% (raised to 50% in the Mail Online).
The 45% raised risk figure appears in the study, but only before researchers took account of all potential confounding factors.
After adjustments for other health behaviours and conditions, the researchers estimated that the risk figure was 38%.
This adjusted result is likely to give a truer picture of the risk from blood pressure alone.
This cohort study used data from the long-running 30-year Whitehall II study of civil servants.
This type of study is good for spotting patterns, such as the link between one factor (blood pressure) and another (dementia). But it can't prove that one factor directly causes the other.
Researchers used data from 8,639 people (32% women) who'd had their blood pressure measured in 1985, 1991, 1997 and 2003.
They used electronic health records to find out whether people had developed dementia by the end of March 2017.
They then analysed the figures to see whether there was a link between blood pressure at different ages and people's chances of getting dementia.
They took account of the following potential confounding factors:
They carried out separate analyses to find out whether the length of time people had high blood pressure for was important, as well as the age when they first had it.
They also looked at the effect of cardiovascular disease (such as heart attack and stroke) to see whether these conditions explained the link between high blood pressure and dementia.
Blood pressure was measured by taking the average of 2 seated readings after a 5-minute rest.
Of the 8,639 people in the study, 385 (4.5%) got dementia, with the average age at diagnosis being 75.
People who had systolic blood pressure of 130mmHg or over at the age of 50 had a 38% increased risk of getting dementia, compared with people aged 50 with blood pressure below that level (hazard ratio [HR] 1.38, 95% confidence interval [CI] 1.11 to 1.70).
But having systolic blood pressure of 130mmHg or more at age 60 or 70 didn't increase the risk of dementia.
And raised diastolic blood pressure (measured between heart beats) wasn't linked to dementia at any age.
The length of time people had high blood pressure for was important.
People who'd had 3 high blood pressure readings over a 16-year period between the ages of 45 and 61 were 29% more likely to get dementia than those who had high blood pressure only on later readings or healthy blood pressure throughout (HR 1.29, 95% CI 1.00 to 1.66).
Cardiovascular disease explained some of the raised risk of dementia, but not all of it.
People who had systolic blood pressure over 130mmHg at the age of 50, but who were never diagnosed with cardiovascular disease, still had a 47% increased risk of dementia compared with those with lower blood pressure (HR 1.47 95% CI 1.15 to 1.87).
The researchers say their results "support the hypothesis that hypertension [high blood pressure] in mid-life but not later life is associated with increased risk of dementia".
They noted that they saw an increase in dementia risk at blood pressure levels "much under the conventional 140mmHg threshold used to define hypertension".
They say their results also support the theory that mid-life high blood pressure is important because "those with hypertension at age 50 are likely to be 'exposed' for longer" to the damage that it can cause to the brain.
We've known for some time that high blood pressure is bad news. It puts strain on all the blood vessels in the body, which can raise your chances of kidney damage, stroke, heart attack and heart disease.
It makes sense that it could also damage the brain, and that the longer you have high blood pressure, the more damage it can do.
This study found people aged 50 with systolic blood pressure raised above the recommended level of 120mmHg, even if it's not as high as the 140mmHg used to diagnose high blood pressure, are at an increased risk of dementia.
The study was carefully carried out, but has some limitations to be aware of:
In addition, this type of study can't prove that high blood pressure was the cause of the increased risk of dementia.
One important point to note is that we don't know whether lowering people's systolic blood pressure below 130mmHg at the age of 50 would actually lower their risk of dementia.
We need studies to investigate this, but they're likely to take a long time because dementia tends to develop in people aged over 70.
Studies to lower blood pressure in older people didn't seem to reduce dementia risk, but this may be because the damage had already been done.
The study adds to the reasons why you might want to avoid developing high blood pressure.
These are some of the steps you can take:
These steps may also have the added benefit of lowering your risk of dementia.