Can your gut bacteria cause high blood pressure?

Doctors have cautioned us for decades that increased salt intake leads to high blood pressure, and there are lots of studies that support the association. But why this happens no one really knows. Could it be though, Dominik Mueller from the Max-Delbruck Centre for Molecular Medicine in Berlin, wonders, that dietary salt affects our gut bacteria, which in turn affect the immune system and this causes the high blood pressure? He doesn’t know for sure, but it does look like low doses of microbes might reduce the risk of hypertension, because mice fed on high salt diets - or chow - and probiotics don’t get high blood pressure. Plus, cardiologists Sharon Wilson and Tian Zhao explain other risks to high blood pressure...

Dominik - We know that salt promotes cardiovascular disease and a rise in blood pressure, and whether this is somehow related to any interaction with the bacteria was the aim of the study.

Chris - What did you do?

Dominik - We started in mice and gave mice a standard control with normal salt and one with high salt and we wanted to find out whether we get changes in these bacteria.

Chris - When you say changes, do  you mean as in the types of bacteria that live there or do you mean the way those bacteria behave, or both?

Dominik - Both. The first thing is that you screen the abundance of bacteria, the composition of the different bacteria to identify which bacteria reacted and we came up with a short list of eight bacteria, and then we could focus on the top listed candidate, which was a lactobacillus. And then we focused more specifically if we would use this one for a kind of treatment whether we could improve cardiovascular health and affect the blood pressure regulation.

Chris - When you give the salt diet, and then you see these changes in bacteria, is this reflected in a change in the blood pressure of the animals though?

Dominik - This is a tough question because we cannot definitely say that if you decrease the lactobacillus that this increases the blood pressure. It could also be that both happen in parallel. We did the other way round. We supplemented with lactobacillus and we could prevent the increase in blood pressure.

Chris - What about if you take animals that don’t have any bacteria in their intestine, the so called “germ free” mice? They’re born, they’re never allowed to be colonised by bacteria. Do you seen any influence if you do the experiments there?

Dominik - We could not, at the moment, measure their blood pressure because you have to do surgery to implant the blood pressure measurement device, and as soon as you do a surgery you open the possibility for contamination with bacteria and, therefore, this experiment was not done yet.

Chris - Putting all this together then, would your idea be that high salt in the diet changes the spectrum of bacteria that live in the intestine this, in turn, in some way, signals to the cardiovascular system and translates into changes which include an increase in blood pressure? Is that a reasonable summary of where you think you are?

Dominik - Again, we have to be careful about causality. We know that the chain you have mentioned exists. Whether one depends on the other is not clear yet. However, if you turn it around and if you think of strategies to supplement the missing bacteria by taking a probiotic you can prevent the increase in blood pressure. And we are currently setting up a clinical study where we want to test this in humans.

Chris - Do you have any theories as to how this is protecting the mice when they have the right spectrum of bacteria or the right levels of bacteria so that in these animals, at least, they don’t appear to get the high blood pressure?

Dominik - We learn more and more that cardiovascular disease and hypertension depends on the immune system. In the gut we have a big number of immune cells and it is known that the microbes affect the immune system in the gut. And new research in the cardiovascular field also demonstrates that the immune system plays an important role in the regulation of blood pressure. So we speculate what we eat might have an influence on the gut, on the gut immune cells and, therefore, on the development of high blood pressure.

Chris - And that’s a trial you’re doing, is it? You’re actually taking adults and supplementing them to see if you can get a change in their blood pressure in response to giving them more probiotics?

Dominik - Yes. We are currently in the preparation phase of this clinical study, and this will be exactly the question we will want to ask.

Chris - Be interesting to see what that shows won’t it? Dominik Muller; he’s from the the Max-Delbruck Centre for Molecular Medicine in Berlin.

Now, in the studio with me this week are Cambridge cardiologists Tian Zhao and Sharon Wilson:

Sharon, we were talking about high blood pressure there, but why does high blood pressure damage arteries and increase your risk of heart disease?

Sharon - Well, if you’ve managed to think about your blood pressure, the harder your heart has to work the more strain you’re putting on your heart. You’ve got basically changes that are based on the vessel wall, so the wall is getting stiffer and everything is sort of getting a little bit harder for it to distend or to change in shape. You also have the effect directly on the actual heart muscle where if it has to maintain a higher blood pressure, it gets thicker. As it gets thicker, the heart doesn’t relax particularly well so it has to work harder again to relax and contract and fill with blood in order to get blood to go forward. So there’s a couple of effects of why we prefer your blood pressure to be within the target range.

Chris - Also, it intuitively feels to me as though blood rushing through blood vessels which are tighter at higher pressure is more likely to damage the blood vessel than blood which is flowing in a more sedate way? Is that a reasonable synopsis?

Sharon - We more say with valvular disease from a heart rather than actual blood pressure changes. It’s more the distensibility of your vessel is the main issue with having a higher blood pressure.

Chris - And it’s not just your heart that’s at risk from hypertension is it. Tian? You could get other bits of your body which are going to get damaged by blood pressure that’s too high?

Tian - Indeed, yeah. The organs that mainly are affected are the brain, in which case we get strokes. The heart, that we’ve discussed and also the kidneys. We know that every 2 mm of mercury increase in your blood pressure you get a 10 percent increase in your stroke risk, a 7 percent increase in your heart risk. So the closest correlation actually is with stroke.

Chris - And normal blood pressure is 120 over 80 give or take?

Tian - Well, give or take, yeah. It’s what we aim to achieve. Often patients don’t achieve that, but the closer we get to that the lower your risk.

Chris - Now, Dominik was talking about salt in the diet and his theory is that this, or at least is partly attributable to its effect on microorganisms in the gut which might make the immune system make you have higher blood pressure. What other theories are there for why salt intake puts your blood pressure up?

Tian - We know quite well that the more salt you eat the higher the blood pressure, but the reason of that is not well understood. When that’s the case in medicine, often the reason is it’s that there’s a number of reasons all coming together. We know that, as you eat more salt, the more salt in your body, the body retains water to balance the concentration of salt in the body. And, as more water is retained, there’s more volume and therefore the blood pressure goes up is one idea.

Chris - The thing that I find wrong with that is that actually it appears to be how long you’ve been exposed to salt over your lifetime that determines your blood pressure, doesn’t it? Because I could eat shovelfuls of salt tomorrow, I might get a short term increase in my blood pressure but then it would go back to normal. But if I carried on doing that day after day after day relentlessly, I’m not going to swell up like a balloon so I can’t continually be taking on board water, there must be something else going on?

Tian - Well, the idea is that there’s a thermostat in the body which controls how much salt one should take in. With a prolonged exposure of salt over time that thermostat is adjusted to retain more and more salt, and the result of a number of years the blood pressure goes up. That’s probably why we see it as a disease of ageing as well. As people get older, people get high blood pressure.

Chris - And linking that to what Dominik Muller was saying, is it possible then that that thermostat for salt could be being influenced by what the dietary conditions are, or what the microbes in the gut are saying to the brain and the cardiovascular system?

Tian - Exactly. There’s several factors which could be influencing the thermostat and gut bacteria could certainly be one of those.