Elevated blood pressure is a risk factor for cardiovascular disease, and it is estimated to affect 1 in 3 adults within the USA alone; due to this it is important the the various mechanisms that underpin blood pressure regulation be revealed.
To put the impact of hypertension into context one study stated that, “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.”
Certain risk factors have been established for hypertension such as obesity, smoking, and excessive alcohol consumption, but there is more to the condition. Over 19% of American adults with the condition have a treatment resistant form of hypertension, wherein medications don’t lower blood pressure to a more healthy level, and lifestyle interventions also do not work for everyone.
Relatively new to the list of potential risk factors is dysfunction of the immune system and autonomic nervous system, and gut dysbiosis which refers to an imbalanced microbial community. A study published in Microbiome analyzed gut bacteria of 99 participants with hypertension, 56 with prehypertension, and 41 with ideal blood pressure levels. Fecal matter was transplanted from the participants into germ free mice; animals that received transplants from those with hypertension were observed to also develop hypertension.
Frontiers in Physiology published a study describing transplanting fecal matter from mice without hypertension into mice with hypertension which resulted in a reduction of blood pressure in the animals with hypertension.
Another study investigated gut bacteria of pregnant women who were overweight or obese who are increased risk of hypertension, and found that in both groups bacteria of the genus Odoribacter were significantly rarer; those with the lowest levels had the highest blood pressure readings.
Evidence of gut bacteria influencing hypertension is mounting, however most of the studies have been observational, meaning it is not possible to determine whether changes in gut bacteria influence blood pressure or whether hypertension alters gut bacteria.
Blood pressure and the gut may not appear to be companions but a connection may not be so surprising as many of the factors that increase the risk of hypertension enter the body through the digestive system such as nutrients, salty foods and alcohol. The GI tract hosts a number of processes that also potentially play roles in hypertension including production of hormones, metabolism, and a direct connection with the nervous system.
One of the links between the gut and hypertension may be short chain fatty acids as some gut bacteria produce these molecules as they digest fiber; after the bacteria produces SCFAs the blood supply absorbs them, and they affect a range of physiological processes one of them appears to be blood pressure.
One study has found there are differences in gut bacterial populations between those with and without hypertension; those with hypertension had lower levels of certain species that produce short chain fatty acids.
Hypertension published a study investigating the role of gut bacteria in sleep apnea induced hypertension in rats. Rats were either fed a standard diet as control or a high fat diet, those in the high fat diet developed hypertension and had a significant reduction in number of bacteria responsible for producing SCFAs. Bacteria from animals with hypertension was transplanted into those who displayed normal blood pressure from the group eating a standard diet which resulted in the previously healthy animals developing hypertension.
If gut bacteria can produce hypertension it is likely to be via a variety of interlinked routes, such as the autonomic nervous system. Hypertension has been shown to increase sympathetic nerve activity which increases gut permeability, and the change impacts gut environments and alters the microbiome while allowing bacterial products to pass more easily into the blood. Other factors such as stress and smoking also alter the sympathetic system which may help to explain why these factors also lead to cardiovascular changes.
Some research is looking at designing a probiotic that can reduce blood pressure. A meta-analysis examined the effect of probiotics on blood pressure using 14 studies involving 702 participants and concluded that, “Some evidence of publication bias was present. Probiotic fermented milk has blood pressure-lowering effects in prehypertensive and hypertensive people.”
Another systematic review and meta-analysis investigating probiotics only including randomized controlled trials using 9 studies concluded that, “The present meta-analysis suggests that consuming probiotics may improve blood pressure by a modest degree.”
While the public appears to have a substantial appetite for probiotics outside of a small number of specific conditions there is little evidence to suggest that they can reliably or substantially be of benefit to human health.
The question of gut bacteria’s impact on blood pressure is still relatively new, although some evidence does support the interactions much more research is required. Diet, drugs taken, and other health conditions as well as other variables can all influence gut bacteria. Other factors such as bacteriophages, fungi, and parasites also make their way to the gut and can influence bacterial populations and our physiology. These mysteries will be revealed slowly as the wheels of research are 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.”
