The Association Between High Blood Pressure and Changes in Human Gut Microbial Metabolic Pathways

Michael Nakai

Michael’s paper on the association between hypertension and gut microbial metabolism is available online: https://www.ahajournals.org/doi/10.1161/HYPERTENSIONAHA.121.17288.

When I originally graduated from Unimelb, I expected that I would go into infection disease research. After all, my undergraduate major was in pathology, and my honors year focused on trying to specifically deliver therapeutic molecules to cancer cells. It would be a surprise to my freshly-graduated self, then, to discover that I’d be authoring a paper focused on the effects of gut bacteria on the human body three years later.

For many people, the gut is a simple organ (at least, it was for me). You eat, it digests, and anything left over comes out the other end. In reality though, every time your stomach dissolves food into a liquid-y paste (called chyme) and passes it to your gut, an extremely complex series of reactions occur in tandem. First, your gut starts undulating, causing the muscles surrounding the gut to pulsate. This undulation pushes the chyme further and further down your small and large intestine, and keeps it in motion. A series of hormones are released by your brain and travel around your body through the bloodstream, causing various changes that both allow digestion to easily occur, and prime your body to receive any nutrients extracted from the chyme. And lastly, but most importantly, your own little army of bacteria start going to work.

We often think of bacteria in the context of disease, from everyday sniffles to life-threatening complications. However, the bacteria in your gut serve a wholly different purpose. They are the bread and butter of your digestive system and your gut immune system, among others. They break down fibrous and otherwise hardto-digest molecules in your food, freeing up amino acids and energy that we use to maintain a healthy everyday life. They protect you against many food and waterborne diseases by killing the bacteria or viruses that get past your stomach acid. Your gut immune system even uses them to train itself, and makes sure that it doesn’t attack them. Taken as a total, these bacteria in your gut, which collectively make up your gut microbiome, are amazingly important to your survival.

But can these small organisms, perhaps, play a role in non-infectious disease? At first, it seems like there is no link here: how could a small collection of bacteria in your gut influence your whole body? Yet, year by year, the amount of data confirming the importance of the gut microbiome increases. Dysfunctional gut microbiomes have been linked to everything from asthma (Hufnagl, Pali-Schöll et al. 2020) to cancer biology (Zitvogel, Galluzzi et al. 2015). One of the fields where the gut microbiome is implicated to play a large role is in cardiovascular biology, and here’s where my lab enters the story.

I currently work as a research assistant and bioinformatician in the Marques lab at Monash University in addition to tutoring at St. Hilda’s College. The Marques lab focuses on the link between the gut microbiome and hypertension (high blood pressure), and looks at how introducing dietary fiber in a person’s diet can affect this hypertension.

Certain types of dietary fiber are broken down into short chain fatty acids (SCFAs) which then can bind onto many different receptors on many of your cells. This binding has a multitude of effects on the body, including tempering the gut immune system, protecting your gut against unnecessary inflammation, and the maintenance of gut barrier integrity (to put it simply: less diarrhea). This digestion of dietary fiber into SCFAs is actually done by, you guessed it, your gut bacteria, although a range of efficiencies exist. Certain species of gut bacteria are extremely quick at this digestion, while others are either extremely inefficient at this digestion, or don’t have the digestive capabilities in the first place.

Here’s where my research enters the picture. Our lab previously found that high fiber diets, along with SCFA supplementation, prevent the development of hypertension and heart failure in mice (Marques, Nelson et al. 2017), while low fiber diets tended to lead to cardiovascular disease in mice (Kaye, Shihata et al. 2020). The next step was then to perform an observational study in humans, looking at whether participants with hypertension have altered gut microbiomes compared to participants with normal blood pressure. To achieve this, we recruited 65 participants (45 without hypertension, 20 with) across two clinics (Nakai, Ribeiro et al. 2021). We then recorded all possible demographical data about the participant (age, sex, exercise per week, smoking status, etc.) as well as dietary information via a food frequency questionnaire. We also collected fecal samples from each participant, which then had all the bacterial DNA extracted (yuck) and sequenced, allowing us to figure out exactly which species of bacteria were found in each participant’s gut microbiome.

To analyze whether a variable had an effect on the gut microbiome, or vice versa, we created Principal Coordinates Analysis (PCoA) plots that show how similar each participant’s gut microbiome is to each other, and then labelled the participants by category. To put it simply, if two samples are similar, then they are located close together on a PCoA plot, meaning that samples further from each other have quite different gut microbiomes. If hypertension had a clear effect on the gut microbiome, we’d expect to see the colors separating out into groups, since hypertensive people would have similar gut microbiomes to each other, and quite different microbiomes to healthy controls. However, that wasn’t seen at all when I first analyzed our data. So can we say that hypertension really doesn’t have an effect on the gut microbiome in humans from that plot? Here’s where the story gets interesting. Human data is inherently more “noisy” than cell or mouse data for the simple fact that you can’t keep a person in a cage, feed them the exact same thing for months, and make sure that they aren’t exposed to any complicating variables without having a quite stern talking to by the police. Humans are complicated: we grow up in our own environments, with different family dynamics, different diets, and have different motivations and goals in life. What this means is that the variance in gut microbiomes is inherently much larger in humans than in mice, so teasing out correlations can be extremely hard.

In that case, how do we even start to try and reduce this noise? The approach we settled on was computational: I created a script in the R programming language that created an error model for the data (via a machine learning function), then applied the model to our data and de-noised it to the best of its ability. Simply, the error model only looks for species of bacteria that significantly change in abundance within each group (for example, hypertensives vs non-hypertensives) or that are highly abundant and throw the rest out (to varying degrees). This approach worked surprisingly well, with the postdenoising PCoA plot showing a clear grouping.

This, along with other analyses (which would add an additional 10 pages to this already-long piece), supported the idea that the digestive capabilities in the gut microbiome were significantly different between hypertensives and healthy participants, which is an exciting find, and opens up the door to more therapeutic studies.