Last updated February 19, 2018 at 3:11 pm
Salty diets may be reducing bacteria in the gut microbiome and triggering high blood pressure.
High sodium intake and high blood pressure go together like French fries and salt, but scientists have just discovered even more about how the connection works – and it’s something to do with your gut bacteria. In fact, their findings propose the gut microbiome as a potential therapeutic target.
Lactobacillus murinus under the microscope. Credit: Lisa Maier
The gut microbiome is a mystery that’s been inside us all along.
In recent years, scientists are revealing more and more about the symbiotic microbes that live in our gut (and other places such as our skin, mouth, vagina, and other nooks and crannies).
A study has been published in Nature which found that a high-salt diet reduced Lactobacillus bacteria in mice. The best way to determine this is to analyse the tiny little faecal samples from mice.
After 14 days of high salt consumption, several microbial species were decreased in the mice that had a high salt diet compared to the normal salt diet. The diet also caused the increased production of T helper 17 (TH17) immune cells, which are linked to high blood pressure.
Using gene sequencing, they found that the decrease of the bacteria Lactobacillus murinus from the Lactobacillus genus was most strongly associated with the high salt diet.
The researchers took the next step to even demonstrate replenishing the guts of the salty mice with L. murinus reversed the effects. TH17 cells were reduced, it prevented the trigger of an experimental mouse model of brain inflammation, and salt-sensitive hypertension.
To top it all off, they did a small pilot study with healthy humans who increased their salt intake and found the same high blood pressure side effects as the mice.
Your taste buds might appreciate the salt but your gut microbiome might disagree with that side of extra fries. The scientists suggest further trials with probiotics to see if they can prevent or treat salt-related conditions like high blood pressure.
Perhaps one day you’ll be asked if you’d like a side of probiotics with your fries.
The amount of salt in a person’s diet affects populations of cells in the gut. High levels of salt increase the numbers of Th17 immune cells (RED) but lower the number of lactobacilli (BLUE). Credit: Russ Hodge
Read these expert reactions from Australian scientists about this salty research:
Brian J. Morris is Professor Emeritus of the School of Medical Sciences and Bosch Institute at the University of Sydney
“The role of our gut bacteria in diseases ranging from cardiovascular to cancer has come to the fore in recent years.
Earlier this year a study from the Baker Institute in Melbourne published in the top cardiovascular journal, Circulation, showed that a diet high in fibre increased the abundance of bacteria that produce acetate. This was able to lower blood pressure and heart disease in mice.
Now, in a new study published today in Nature, researchers in Germany showed that a diet high in salt, typical of Western countries and long implicated in hypertension, also affects the spectrum of bacterial species in the gut – the gut microbiome. A high salt diet was able to virtually wipe out the healthy Lactobaccilus species in the gut.
This led to a rise in the immune system mediator interleukin A, which raised levels of key inflammatory cells known as TH17 cells, increased inflammation in blood vessels and led to blood pressure going up.
People with a healthy traditional diet have high Lactobaccilus in their gut, but the high salt diet of people in more affluent countries probably explains why they have low Lactobaccilus levels and an epidemic of hypertension, as well as various autoimmune diseases.
Studies have shown that taking probiotics rich in Lactobaccilus can lower blood pressure in people with hypertension.
Large, well-controlled randomized clinical trials are now warranted to test whether boosting Lactobacillus can help stem the tide of rampant hypertension in our society.
But of course, if governments took the advice of health authorities and legislated against high salt and junk food generally, e.g., by massive taxes as proven effective in reducing tobacco consumption in Australia, we would see a reduction in not only hypertension but the scourge of obesity, diabetes, many cancers and cardiovascular diseases generally.”
Dr Francine Marques is a National Heart Foundation Future Leader Fellow and Baker Fellow at Baker Heart & Diabetes Institute
“This is an exciting and fast moving field, and as I said in the talk I gave this afternoon at the American Heart Association scientific sessions, we are still in our baby steps to fully understand the role of the gut microbiota in complex diseases such as hypertension.
The study by Wilck and colleagues adds to this important puzzle by characterising the changes in the gut microbiome after the intake of a high salt diet, which is known to increase blood pressure.
These findings, however, are not surprising considering we have known for a while that the proximal colon is an important site for dietary sodium absorption (as I wrote recently in our review in Nature Reviews Cardiology).
In this study, the authors determined that a high salt diet changed several types of microbes in the gut, in particular the levels of the bacteria Lactobacillus murinus (a rodent-specific species), which was also inhibited in the presence of salt in vitro.
They also observed that a high salt diet increased the numbers of a pro-inflammatory immune cell type, called T helper 17, in the spleen, and daily treatment with L. murinus was able to lower it.
Similarly to high salt intake, these same cells had been previously implicated in the development of hypertension.
In terms of blood pressure, as expected, the high salt diet increased blood pressure, but the authors were able to produce a very small (but significant) reduction in systolic and diastolic blood pressure when the mice were treated with L. murinus.
But how about in humans? Similarly to mice, in a small group of 8 subjects that received salt tablets on top of their normal diet for 14 days, there was an increase in blood pressure (as expected) and a decrease in T helper 17 cells.
They also looked into the amount of Lactobacillus species (since L. murinus is not found in humans) in faecal samples of these subjects before and after the added salt. Important here is that this genus is not highly abundant in the human gut, and not present at all in the gut of some people (in their study only ~40% had this genus). In this study, most of the species had been lost at the end of the salt challenge in the subjects that originally had Lactobacillus.
Lactobacillus can be found in several foodstuff including yogurt and some types of cheese, and some strains help us digest lactose present in milk.
This study adds to the puzzle of the gut microbes in hypertension and highlights another way blood pressure could be regulated by gut microbes – adding to our recent findings related to fibre and its metabolites.
There was, however, no molecular mechanism proposed between salt, Lactobacillus and levels of T helper 17 cells, which was a bit disappointing, but I am sure other studies will address this limitation.
In future studies, it would be interesting to determine, in the hypertensive community, whether the lack of Lactobacillus species is generally low or absent, as this may have implications for treatment.”
Hannah Wardill is a PhD student in the Cancer Treatment Toxicities Group at the University of Adelaide.
“Your gut is home to trillions of bacteria. In fact, there are more bacteria in your gut than there are cells in your body, meaning we are all more bug than human!
As we begin to scratch the surface of this fascinating ecosystem, the biggest question on everyone’s mind is why are they there, and what are they doing.
One of the biggest roles these gut bacteria play is regulation of a person’s immune system. Higher levels of good bacteria are associated with a more tolerant immune system, whilst bad bacteria are thought to drive a more aggressive immune response associated with a process called inflammation.
This interaction between the gut and immune system is now at the centre of new findings, published in Nature, linking gut bacteria with cardiovascular health.
Authors reported that salt intake, a major risk factor for cardiovascular disease, changed the types of the gut bacteria in both mice and humans. They showed a decrease in a particular species of “good bacteria” called Lactobacillus following salt consumption.
This exciting research is the first to suggest that gut bacteria might act as the middle-man between salt and heart health, and provides a new therapeutic target to counteract salt-sensitive diseases.”
Expert comments gathered by the Australian Science Media Centre (AusSMC).
Follow us on Facebook, Twitter and Instagram to get all the latest science.
