Probiotic studied to see if it lowers blood pressure in mice

Friday November 17 2017

"Eating yogurt high in 'good bacteria' could help lower high blood pressure," reports the Mail Online. Researchers found that mice fed a high-salt diet had lower levels of so-called "good" bacteria but that giving them supplements of these bacteria could counteract the effect of the salt.

Recent research has investigated the link between salt and autoimmune diseases, where the immune system mistakenly attacks healthy tissue. In this study, researchers looked at the effect of salt on bacteria that live in the gut.

They found that mice fed a high-salt diet had fewer lactobacillus bacteria in their guts and that a high-salt diet worsened a type of induced autoimmune disease (encephalomyelitis). Mice with fewer lactobacillus bacteria also produced more of a type of immune cell called TH17, which is associated with encephalomyelitis. But giving the mice lactobacillus supplements helped slow the disease.

Using 12 human volunteers, the researchers also discovered that eating a high-salt diet for 14 days raised blood pressure, decreased the number of lactobacillus bacteria in the guts of those people who had them at the start of the study and increased numbers of TH17 cells.

However, crucially, they didn't test the effect of lactobacillus supplements in humans. This study therefore can't tell us if eating probiotic yoghurt or taking probiotic supplements would make any difference to high blood pressure.

But we do know that eating less salt helps reduce and prevent high blood pressure. Read more advice about cutting down on salt.

Where did the story come from?

The research was carried out by teams from 24 research centres. These were mostly in Germany but also in Belgium, Switzerland and the US. It was funded by the German Centre for Cardiovascular Research, the Center for Microbiome Informatics and Therapeutics, and the MetaCardis research project. It was published in the peer-reviewed journal Nature.

The Mail Online jumped the gun with its suggestion that eating probiotic yoghurt (or other probiotic supplements) could reduce blood pressure. Its story didn't explain that the tests with probiotic supplements were carried out on mice, not humans.

It did include a helpful quote from the researchers warning that this was "not a licence for people to consume as much salt as they like, so long as they eat yoghurt". But despite that, its own headline read: "A daily helping of bio-live yogurt or sauerkraut could lower high blood pressure and cut the risk of a stroke or heart attack – even if you have a salty diet."

What kind of research was this?

The researchers carried out a series of experimental trials on mice and lab-grown bacteria. They followed up with an exploratory pilot study in 12 human volunteers.

These types of studies are useful to develop theories about how disease models work. They don't give definitive answers, but they can help us design bigger, more reliable studies to explore things further.

What did the research involve?

In a series of experiments, researchers fed mice either a standard diet, or the same food but with added salt.

The composition of bacteria in the mice's guts was checked through RNA analysis of their droppings. They then used more precise DNA analysis to pinpoint the most important differences between the bacteria found.

They also cultured the bacteria (which means growing them in a lab setting) that was found in the mice's guts to see what happened when different levels of salt were added to the lab-grown culture.

They then tested mice that had been given a form of autoimmune disease, encephalomyelitis, to see the effect of a high- or normal-salt diet on the disease, and on numbers of TH17 cells, which are known to be involved in this type of encephalomyelitis.

Some of the mice were fed lactobacillus bacteria to see whether this affected the disease. The experiment was also repeated on mice that were kept in a sterile environment and had no bacteria in their guts.

They monitored the blood pressure of mice on high- or normal-salt diets, with and without additional lactobacillus bacteria.

Finally, they carried out a study on 12 human male volunteers, who were fed a high-salt diet for 2 weeks. Before and after the study, they measured:

  • blood pressure
  • levels of lactobacillus bacteria in stool samples
  • levels of TH17 cells in participants' blood

What were the basic results?

In mice:

  • several types of bacteria, most importantly Lactobacillus murinus, were much less common after 14 days of a high-salt diet in comparison with a normal diet
  • a salty environment slowed the growth of bacteria, including L murinus and human strains of lactobacillus
  • a high-salt diet worsened induced encephalomyelitis and increased numbers of TH17 cells
  • mice on a high-salt diet that received L murinus supplements had fewer TH17 cells and slower disease progression than those that didn't get supplements
  • a high-salt diet made no difference to bacteria-free mice, suggesting that bacteria are an important link in the chain
  • blood pressure increased during a 3-week high-salt diet, but daily treatment with L murinus supplements reduced that increase

In the 12 men on a 14-day high-salt diet:

  • blood pressure increased
  • TH17 cells increased
  • of the 5 who had lactobacillus populations in their gut at the start of the study, most no longer did at the end

How did the researchers interpret the results?

The researchers said: "Our experimental data in mice suggest gut microbiota might serve as a potential target to counteract salt-sensitive conditions."

They also acknowledged that the human study was "small and limited in power", and that the results "need validation" before they can be taken further.

However, they said the identification of the possible role of lactobacillus bacteria in mice in compensating for the adverse effects of a high-salt diet "could serve as a basis for the development of novel prevention and treatment strategies".

Conclusion

It's tempting to think that something as simple as eating yoghurt or taking a probiotic supplement could undo the harm caused by eating a high-salt diet.

Unfortunately, there's nothing in this study to show that doing so would work – and the researchers made a point of saying this.

It's interesting to know that gut bacteria are affected by a high-salt diet, and that this might explain how both gut bacteria and a high-salt diet can affect blood pressure and the immune system, especially autoimmune disorders. This study should give researchers new avenues to explore when looking at potential disease models and targets for new treatments.

But we are only just starting to understand the way human gut bacteria work in and with our bodies. We don't know whether there's such a thing as a "desirable" or "ideal" population of bacteria for the human gut – the optimal mixture may well depend on the environment in which you live or the food you eat. Simply taking a probiotic supplement may make little or no difference to blood pressure – and because the study didn't investigate this, we don't know.

What we do know is that most people in the UK eat more salt than they need (no more than 6g a day), and that reducing salt, especially from packaged or processed foods, is likely to reduce blood pressure.

Find out more about salt and how you can reduce it in your diet.

Analysis by Bazian
Edited by NHS Choices