Here’s a question for you. What do screeching tyres, bleeding gums, and chomping down chickpeas all have in common? Hint: they’re not the elements of some avant-garde music video.
When we think about the things that send our blood pressure soaring, we tend to think of factors such as stress, physical inactivity, smoking, and consuming too much salt and alcohol.
It’s true that these factors are all important, as they can contribute to high blood pressure or help you to lower it if you make the right changes to them.
But in recent years, scientists have begun to realise that high blood pressure is not as simple as once thought. Key causes have been overlooked, including traffic pollution, gum health and the amount of fibre we eat.
The old-fashioned view of high blood pressure as a problem of ‘pipes and plumbing’ is being tossed aside and a new model is emerging that could change everything, leading to new strategies to deal with the problem.
Critically, these discoveries are empowering because they highlight things that we can do now to help keep our blood pressure in check.
Around the world, high blood pressure, also known as hypertension, affects one in three adults – an estimated 1.5 billion people – and is the leading factor for premature death around the world.
It occurs when the pressure in the blood vessels becomes dangerously high and is diagnosed when someone has a persistent blood pressure reading of more than 140/90 mmHg (millimetres of mercury).
Here, the first (systolic) number represents the pressure in the vessels when the heart contracts or beats, and the second (diastolic) number represents the pressure in the vessels when the heart rests between beats.
High blood pressure is a silent killer
High blood pressure increases the risk of developing chronic health conditions, such as cardiovascular disease, stroke and kidney failure, and contributes to more than 10 million deaths every year.
It’s known as a ‘silent killer’ because around half of those who have high blood pressure don’t know that they do, as most people with hypertension don’t have any symptoms. And equally alarming – of those who do have a diagnosis, only one in five have their blood pressure under control.
This is despite the availability of good medications and the proven ability of lifestyle changes, such as exercising more, losing weight and eating less salt, to help bring blood pressure down.
“This tells us that we don’t know everything about the disease,” says Prof Tomasz Guzik, chair of cardiovascular medicine at the University of Edinburgh, “otherwise we would have solved the problem by now.”
According to the classic view of hypertension, formulated over 70 years ago, the disease emerges from a perfect storm of interacting elements.
This includes environmental factors (like stress and diet) and biological factors (such as genetics and problems with the heart, kidneys, nervous system and blood vessels).
This model isn’t wrong, but it downplays the complexity of high blood pressure. For years, researchers who studied hypertension overlooked a crucial piece of the jigsaw.
The role of the immune system
In 2007, Guzik showed that mice lacking a working immune system were protected from developing severe hypertension.
Then, when the missing immune cells – known as T lymphocytes – were returned to the animals, this protection was lost. The mice developed high blood pressure.
“The study showed us there is another player here,” says Guzik. Specifically, Guzik’s work hinted that the immune system is involved in the development of high blood pressure.
The idea, however, was met with some resistance. Critics countered that the study, which was done in mice, might not have much relevance for humans.
But then studies on people, from Guzik and others, showed not just that the immune system is involved in hypertension, but that it can actually cause it.
The immune system is now seen as the missing link that helps to explain how genetics and lifestyle factors lead to high blood pressure – as well as the trail of organ damage that can follow in its wake.
When immune cells, such as T lymphocytes, infiltrate the walls of blood vessels and other organs, they release molecules that trigger inflammation. This causes the blood vessels to stiffen, making it more difficult for blood to pass through.
Pressure increases, raising the risk of stroke. The heart has to work harder to pump blood around the body, which causes the main pumping chamber to thicken and become more rigid.
This makes it more difficult for the heart to do its job, raising the risk of abnormal heart rhythms and heart failure. Meanwhile, the kidneys retain more salt, which leads to an increase in blood volume, and forces the already beleaguered heart to pump more blood with each beat.
A chronic, low grade inflammation seeps through the body like a malaise. The immune system triggers inflammation, which leads to high blood pressure and chronic health problems.
But what causes the immune system, usually so helpful, to go rogue in this way?
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Oral health clues
Guzik’s wife, Dr Marta Czesnikiewicz-Guzik, is a dentist and lecturer at the University of Glasgow. She sees signs of inflammation every day when she looks inside the mouths of her patients.
When gums become red, or brushing causes bleeding, it’s a sign of inflammation. Up to 90 per cent of adults have mild gum disease – something that’s easy to manage with good oral hygiene, but if it progresses can lead to advanced gum disease or ‘periodontitis.’
Bacteria-laden pockets form in the gums and teeth can become wobbly and fall out. Around the globe, severe periodontitis affects around a billion people.
Using genetic data from the UK Biobank (a huge biomedical dataset containing information from about half a million people), the Guziks were able to show not just that periodontitis and high blood pressure occur together sometimes, but that the former can cause the latter.
Excited by this discovery, they followed up with a clinical trial. If periodontitis can cause high blood pressure, the Guziks wanted to know whether treating periodontitis could lower it.
One hundred people with both hypertension and periodontitis were split into two groups. One group had intensive treatment for their gum disease, which included deep cleaning below the gumline.
The other group had a control treatment where plaque and tartar were removed from above the gumline only.
Two months later, those in the first group had healthier gums and lower blood pressure than those in the control group.
“If you treat the mouth intensively, you lower blood pressure,” says Tomasz Guzik.
People who received intensive periodontal treatment also had lower levels of the pro-inflammatory molecules interferon-gamma and interleukin-6. Intensive treatment calmed the immune system, reduced inflammation and eased hypertension.
This suggests that – alongside existing blood pressure drugs – gum disease treatments could be used to help control blood pressure.
The air we breathe
Gum disease, however, is just one of many issues known to trigger inflammation, with the potential to impact blood pressure.
Prof Douglas Brugge is chair of the department of public health sciences at the University of Connecticut, in the US.
For the last 15 years, he has been working with residents from Somerville, Massachusetts, who are worried about the effect that traffic pollution is having on their health.
Somerville has three busy highways, including the Interstate 93, which is used by more than 150,000 vehicles a day. The city is densely packed and many people live close to these roads.
Cars brake, tyres screech and exhaust pipes belch out fumes, generating ultrafine particles that seep into their homes.
Studies have shown that when people breathe in these ultrafine particles, it can lead to systemic inflammation and high blood pressure. So the question is: what can be done?
Brugge set up an experiment involving 154 Somerville residents who live within 200m (656ft) of a busy road. Everyone was given a high-efficiency particulate air purifier to plug into their home.
For the first month, half of the participants used a real purifier, and the other half used a sham purifier that had had the filter removed. Then, after a four-week break, the devices were swapped around.
None of the participants knew which purifier they were using, and all of them had regular blood pressure measurements.
Brugge found that when participants with pre-existing hypertension had the genuine air purifiers in their homes, their blood pressure was, on average, 3mmHG lower than when they had the sham units plugged in.
“This is clinically significant and potentially meaningful for their health,” says Brugge.
What it means is that plugging in a particulate air purifier today could help to lower blood pressure and “stave off a heart attack or stroke decades down the line,” says Brugge.
That said, work is needed to make these admittedly costly devices more affordable to those who need them the most.
Feeding the gut microbiome
Another way to help look after our blood pressure is to look after the microbes that live inside our guts. Gut microbes feast on fibre, and we’ve known for decades that eating fibre is good for us.
People who eat high-fibre diets tend to have lower blood pressure, reduced risk of heart disease and stroke, and longer life expectancy compared to those who eat fibre-poor diets. But for a long time, no one knew why.
Then, in 2017, Prof Francine Marques, who leads the hypertension research laboratory at Monash University in Melbourne, Australia, showed that when mice prone to hypertension were fed a high-fibre diet, their blood pressure decreased and their heart health improved – all because of changes to their microbiome.
“That was a really crucial piece of information,” she says. “Our study ended up being cited over a thousand times.”
Marques went on to thrash out the link in detail. When gut bacteria break down fibre, they release molecules called short-chain fatty acids (SCFAs). These molecules bind to receptors on the host’s immune cells and dial down pro-inflammatory signals.
The gut bacteria themselves interact with the immune system, dampening inflammation and helping to protect the blood vessels and organs from damage.
The takeaway? Eating a diet rich in fibre could help to bring down blood pressure. And in 2023, Marques tested this idea out.
She ran a randomised, double-blind clinical trial, involving 20 people with untreated hypertension. For three weeks, some of them ate a diet containing fibre that was chemically modified to contain high levels of SCFAs.
Then they took a break and switched to a control diet. And further group of them ate the control diet for three weeks, before taking a break and switching to the SCFA-enriched diet.
The SCFA-enriched fibre was hidden inside burgers, frittatas and muffins, specially prepared for the experiment, which tasted and looked like the regular alternative.
This meant that during the trial, participants couldn’t tell which of the two food types they were eating. (A research chef developed the recipes, Marques says.)
Marques found that when people ate the SCFA-enriched diet, their blood pressure decreased by around 6mmHG. “This is equivalent to taking one standard dose of an antihypertensive drug,” she says.
Their microbiome changed too – there were more of the bacteria that make SCFAs.
Food for thought
Collectively, these findings from the trial suggest that when you look after the gut microbiome – and stoke it with fibre and the substances that ‘good bacteria’ make – blood pressure can benefit.
Marques points out that this has to be a long-term commitment, however. Eating the fibre-rich diet may have boosted levels of SCFA-producing bacteria during the trial, but then their numbers declined when the diet was abandoned.
This is potentially problematic because most adults already don’t consume their daily recommended amount of fibre (25g for women and 30g for men), and because not all fibre is equally beneficial.
“Fibre is quite diverse,” says Marques. At present, no one is sure which specific type or source of fibre is best for our blood pressure.
Marques is now working with chemical engineers to develop a supplement that contains the SCFA-enriched fibre she used in her trial, but in the meantime there are already lots of things we can do using the knowledge generated from this and other research to help control our blood pressure.
The first is, simply, to find out what your blood pressure is. Most pharmacies offer a free blood pressure check, so take advantage of this.
If you find that you have high blood pressure, see a doctor. There are good medications that can help.
Then look at all the modifiable risk factors there are for high blood pressure – from exercise to caffeine intake, salt consumption to sleep quality – and see what you can change.
But also – in line with the latest science – consider buying a high-efficiency particulate air purifier if you live by a busy road, look after your oral health, and eat lots and lots of fibre.
There are some things you can’t change, like your genetics or the busy road next to your house, but there are also plenty of ways to take action to lower your blood pressure.
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