High blood pressure: Could gut bacteria play a role?

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In this feature, we investigate whether the bacteria that live in our guts could influence our blood pressure. If so, could they guide future treatment?

Background

Scientists are growing increasingly interested in the role of gut bacteria. Each week, journals publish many study papers that examine how these microscopic visitors might play a role in health and disease.

As it stands, because the microbiome is a relatively new field of study, the full scope of gut bacteria’s role in health is still up for debate. However, it is becoming increasingly clear that the bacteria in our gut can open new avenues in our understanding of a wide range of conditions.

Bacteriophages, fungi, and parasites also find a home in the gut and influence both bacterial populations and our physiology.

Scientists have studied the role of gut bacteria in conditions as varied as obesityParkinson’s diseasedepression, and blood pressure. Hypertension is a risk factor for cardiovascular disease and affects almost 1 in 3 adults in the United States. Because of this, it is vital that medical scientists unearth the various mechanisms that underpin blood pressure regulation.

One study paper puts hypertension’s impact into sobering context: “Over 400,000 deaths in the United States are related to [hypertension] every year, more than all the Americans who died through all of World War II.”

Read more: Fourteen natural ways to lower your blood pressure

Beyond standard risk factors

Although researchers have established certain risk factors for hypertension — such as smoking, obesity, and drinking excessive amounts of alcohol — there appears to be more to the condition. More than 19% of the U.S. adults with hypertension have a treatment-resistant form of the condition, wherein medications do not bring blood pressure down to a healthful level. Also, lifestyle interventions do not work for everyone.

Some scientists are considering the dysfunction of the immune system and the autonomic nervous system. This is the branch of the nervous system that controls “automatic” functions, such as breathing, digestion, and blood pressure. A relatively new addition to this list of potential risk factors is gut dysbiosis, which refers to an imbalanced microbial community.

The microbiome is a relatively new field of study, the full scope of gut bacteria’s role in health is still up for debate.

A study in the journal Microbiome analyzed the gut bacteria of 41 people with ideal blood pressure levels, 99 individuals with hypertension, and 56 people with prehypertension. Prehypertension refers to high blood pressure that is not yet high enough for a person to receive a diagnosis of hypertension.

People in this range have an increased risk of developing hypertension in the future. They found that in the participants with prehypertension or hypertension, there was a reduction in the diversity of gut bacteria. In particular, species such as Prevotelle and Klebsiella tended to be overgrown.

Read more: How some fruits can lower blood pressure

Next, the scientists transplanted fecal matter from the participants into germ-free mice, which are animals that lack gut bacteria. The mice that received fecal matter from people with hypertension also developed hypertension A study in the journal Microbiome analyzed the gut bacteria of 41 people with ideal blood pressure levels, 99 individuals with hypertension, and 56 people with prehypertension.

The scientists transplanted fecal matter from the participants into germ-free mice, which are animals that lack gut bacteria.

Prehypertension refers to high blood pressure that is not yet high enough for a person to receive a diagnosis of hypertension. People in this range have an increased risk of developing hypertension in the future.

They found that in the participants with prehypertension or hypertension, there was a reduction in the diversity of gut bacteria. In particular, species such as Prevotella and Klebsiella tended to be overgrown.

Next, the scientists transplanted fecal matter from the participants into germ-free mice, which are animals that lack gut bacteria. The mice that received fecal matter from people with hypertension also developed hypertension.

Conversely, the authors of a 2019 study in the journal Frontiers in Physiology transplanted feces from mice without hypertension into mice with hypertension. This resulted in a reduction in blood pressure in mice with hypertension.

Prehypertension refers to high blood pressure that is not yet high enough for a person to receive a diagnosis of hypertension.

Another study investigated the bacterial residents of pregnant women with obesity and overweight pregnant women, both of whom are at increased risk of hypertension. They found that in both sets of participants, bacteria of the genus Odoribacter were significantly rarer.

Those with the lowest levels of Odoribacter had the highest blood pressure readings.

Read more: Lack of sleep may cause stroke & blood pressure problems

How do gut bacteria affect blood pressure?

Many of the factors that increase the risk of hypertension — such as the consumption of alcohol and salty food — enter the body through the digestive system. Nutrients, along with certain chemicals that bacteria produce, have the opportunity to enter the blood supply; once in circulation, the body is their oyster.

Also, the gastrointestinal tract hosts a number of processes that have the potential to play a role in hypertension, including metabolism, the production of hormones, and a direct connection with the nervous system.

The authors of a 2019 study in the journal Frontiers in Physiology transplanted feces from mice without hypertension into mice with hypertension.

Bacteria living in the gut can produce chemicals as part of their normal metabolism after they’re exposed to the food we eat.

When those chemicals are then absorbed into the bloodstream, it’s thought that they activate receptors in the blood vessels to lower blood pressure. In studies on mice, these blood pressure changes are significant, especially when considering the potential impact over the span of a lifetime.

Most likely, if gut bacteria truly do have the power to produce hypertension, it is likely to be via a number of interlinked routes. Scientists have several theories. For instance, some experts see a role for the autonomic nervous system.

Read more: Can diets and supplements really protect the heart?

A probiotic for hypertension?

Eating food that contains probiotics—consumable live bacteria—has been linked to healthier blood pressure in previous studies.

Bacteria living in the gut can produce chemicals as part of their normal metabolism, after they’re exposed to the food we eat.

“Yogurt is the clearest example of a probiotic,” says Pluznick. “People might not be as aware of prebiotics.” Prebiotics are things that you eat that contain the precursors bacteria need to make the special chemicals that are then absorbed by our bodies, potentially lowering blood pressure.

Read more: Almost a third of Scots are now obese – and at risk of more cancers than smokers

“Fiber can be prebiotic for a lot of bacteria, so when you eat fiber, the bacteria break it down to make those chemicals,” says Pluznick. You can find prebiotics in fiber-containing foods such as garlic, onions, asparagus, whole wheat pasta and sweet potatoes.

Designing a probiotic that reliably reduces high blood pressure will take some time, but some researchers are looking at this option.

A 2013 meta-analysis examined the effect of probiotic fermented milk on blood pressure. In all, they took data from 14 studies, which included 702 participants. Although the authors write that “[s]ome evidence of publication bias was present,” they concluded that:

“[P]robiotic fermented milk has blood-pressure-lowering effects in prehypertensive and hypertensive [people].” A 2014 systematic review and meta-analysis investigated probiotics more generally. Its authors only included randomized controlled trials, and their search only turned up nine papers that fit their criteria.

Read more: Can very low levels of ‘bad’ cholesterol raise stroke risk?

Overall, they concluded, “The present meta-analysis suggests that consuming probiotics may improve [blood pressure] by a modest degree.” With that in mind, it is likely to be a long time before a probiotic will bring blood pressure down.

For the future

Science is relatively new to the question of gut bacteria’s impact on blood pressure, so plenty more work will be needed. Although some evidence now supports the interaction between gut bacteria and hypertension, it is a complex beast to dissect.

Our diet, the drugs we take (particularly antibiotics), other health conditions we might have, and many more variables can all influence our gut bacteria. Bacteriophages (viruses that attack bacteria), fungi, and parasites also find a home in the gut and influence both bacterial populations and our physiology.

Read more: Can bacteria be used to treat metabolic syndrome?

This mystery will only unravel slowly, but at least the wheels of research are now in motion. As one reviewer writes:

“Evidence is rapidly accumulating implicating gut dysbiosis in hypertension. However, we are far from understanding whether this is a cause or consequence of [hypertension], and how to best translate this fundamental knowledge to advance the management of [hypertension].”

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