Gut Microbiota Blog – “Good” versus “Bad” and HOW it Talks to the Body
Dr Sean Wheatley, MSc, PhD. Science and Research Lead.
On X-PERT programmes, and in our handbooks, we talk about the importance of the bacteria in our gut – collectively known as the gut microbiota. Through necessity, this coverage is relatively basic, focusing on simple messages and practical advice.
Recently though we received some questions from an inquisitive participant, who wanted to know more about how the gut microbiota influences our health. I think this topic is fascinating, so I’ve adapted the answers for this month’s blog. Hopefully you’ll find them interesting too!
What do X-PERT programmes and resources currently cover?
Before going any further, let’s quickly cover the basics.
The key points, as included in our existing content, are:
– The gut microbiota can affect our health in a range of ways, including by helping us to digest food, affecting the absorption of nutrients and energy (calories), and influencing hunger and immune function.
– Our lifestyle choices affect the gut microbiota. Important factors include diet, smoking, stress, physical activity levels and medication usage. Somewhat helpfully, the recommendations are generally the same for promoting good general health as they are for supporting a healthy gut microbiota.
Good versus Bad – A necessary simplification
The first question from the X-PERT participant was about what makes bacteria “good” or “bad”, terminology that is widely used (including on our programmes) when discussing the microbiota.
The first thing to note is that this wording is an oversimplification, albeit a necessary one, to help convey the key messages in a simple way. In reality though, the truth is far more complex and nuanced. It often is! I’ve tried to make sense of some of the main points below though.
Good versus Bad – The role of individual species of bacteria
Amongst the trillions of bacteria in the human gut, there are lots of different species. Different bacteria will “eat” different things, and different bacteria will expel different things. Our health is affected by both what goes in, and by what comes out. As such, different bacteria can affect health in lots of different ways.
We are still learning about how different species of bacteria influence the body. It is also difficult to separate the impact of any given species from everything else that is going on around it (both within the gut, and elsewhere within the body). As a result, it is ultimately difficult to label individual bacterial species as “good” or “bad”.
Good versus Bad – Keeping it in the family
As it is difficult to unpick the impact of individual species, it is more helpful (and more practical) to think of the gut microbiota as a collective. In this vein, bacterial species that are related can be grouped into families.
Taking this approach, research has shown that having a higher or lower proportion of bacteria from certain families is associated with an increased or decreased risk of certain health outcomes, including decreased immune function, fatty liver disease, and type 2 diabetes(1). There is even evidence that certain patterns are linked with psychiatric disorders, including anxiety, depression and food addiction(2,3)!
This mean that, at the bacterial family level, we can start to think about “good” and “bad” bacteria. This still requires nuance though, as the role of any given family may be different within different contexts, based on things such as the make-up of the rest of the microbiota and the environment they exist in (within the gut, factoring in what is going on in other parts of the body, and considering factors that are external to the body).
Ultimately, there is no clear consensus on what a “good” microbiota would look like, at least not in specific terms. The consistent key factor though is that diversity within the gut microbiota is linked with better health; that is to say, there should be a wide variety of different species of bacteria living within the gut (something that has, for example, been linked with a decreased risk of type 2 diabetes)(4).
HOW the Gut Microbiota Influences Health – Introduction
The next part of the question was about how the gut microbiota affects the body. This is a fantastic question with a fascinating answer.
It is important to remember that the bacteria in our gut are independent creatures in their own right – they are not human cells. They are however able to communicate with the body as if they were.
The links between the gut and the brain are well established. This is often framed as the “gut-brain axis”, or, more recently, the “microbiota-gut-brain” axis (to reflect what has been learned about the causative role of the gut bacteria in this link). This link goes in both directions too; for example, as well as the gut sending messages that influence the body (as explored below) people often experience gastric symptoms when they are nervous or excited.
HOW the Gut Microbiota Influences Health – Pathways
As noted previously, the impact of the gut microbiota on health is affected both by what the bacteria “eat” and what they expel. Differences in what they “eat” affect how much energy (calories) and how much of certain nutrients we absorb. This affects weight and health management in multiple ways. Differences in what they expel determine what messages are sent on to other parts of the body, with pathways for this including through:
– The nervous system(5), mediated through the release of substances called neurotransmitters and neuromodulators (e.g., noradrenaline, dopamine and serotonin). These relay messages to the brain, affecting, amongst other things, the systems which regulate stress and sleep.
– The endocrine system(5), mediated through the release of hormones. This includes hormones related to the use and storage of nutrients (e.g., glucose and fat), and hormones that affect hunger/satiety.
– The immune system(5), mediated through the release of substances called cytokines. Amongst other things, cytokines impact inflammation. Some reduce inflammation, but some increase it. As many “modern” diseases – including type 2 diabetes, cardiovascular disease, dementia, and certain cancers – are linked to inflammation, the effect of the gut microbiota on inflammation can have significant consequences.
HOW the Gut Microbiota Influences Health – Even More Pathways
As if the above wasn’t impressive enough, there are multiple other ways through which the gut microbiota can affect health and wellbeing, both directly and indirectly.
This includes through other byproducts of metabolism within the gut bacteria that are subsequently released into the body. These include bile acids, short-chain fatty acids, amino acids and certain vitamins (including vitamin K and multiple b-vitamins).
They can also produce antimicrobial substances that help to protect the body from pathogens(1), so influence immune function via this route too.
There are even cells within the walls of the intestine that sense the release of substances from the gut microbiota, and secrete hormones in response to them(7).
HOW the Gut Microbiota Influences Health – Summary
The sheer number of ways the gut microbiota can (and do) affect the body is astounding. Interestingly, the make-up of many of the substances released by the gut microbiota is identical to that of substances released by the human body(6). This means the brain cannot differentiate between some of the signals sent by the gut microbiota and signals that derive from “actual” human cells!
On the other hand, some of the enzymes and biochemical pathways that occur within the gut microbiota do not occur in other human cells1. This means that the gut microbiota can also influence human health in ways other cells within the body cannot.
So, what’s the bottom line?
The gut microbiota is endlessly fascinating, if you are into that type of thing. Although we have to simplify messages on our programmes to try and provide practical help to our participants, the true picture is incredibly complex, with basic labels such as “good” and “bad” barely scratching the surface of what is really going.
The bacteria in our guts can talk to the body via multiple routes and can affect our physical and mental health in a wide range of ways as a result. Ultimately though, general advice around a healthy lifestyle is also likely to support a healthy gut, so trying to eat real foods, being physically active, avoiding smoking, and trying to minimise stress can go a long way to helping the bugs in your gut too.
References
1. Hou K, Wu ZX, Chen XY, et al. Microbiota in health and diseases. Signal Transduct Target Ther. Apr 23 2022;7(1):135. doi:10.1038/s41392-022-00974-4.
2. Dinan TG, Cryan JF. The impact of gut microbiota on brain and behaviour: implications for psychiatry. Curr Opin Clin Nutr Metab Care. Nov 2015;18(6):552-8. doi:10.1097/MCO.0000000000000221.
3. Dong TS, Mayer EA, Osadchiy V, et al. A Distinct Brain-Gut-Microbiome Profile Exists for Females with Obesity and Food Addiction. Obesity (Silver Spring). Aug 2020;28(8):1477-1486. doi:10.1002/oby.22870.
4. Menni C, Zhu J, Le Roy CI, et al. Serum metabolites reflecting gut microbiome alpha diversity predict type 2 diabetes. Gut Microbes. Nov 1 2020;11(6):1632-1642. doi:10.1080/19490976.2020.1778261.
5. Cryan JF, Dinan TG. Mind-altering microorganisms: the impact of the gut microbiota on brain and behaviour. Nat Rev Neurosci. Oct 2012;13(10):701-12. doi:10.1038/nrn3346
6. Galland L. The gut microbiome and the brain. J Med Food. Dec 2014;17(12):1261-72. doi:10.1089/jmf.2014.7000.
7. Hartstra AV, Nieuwdorp M, Herrema H. Interplay between gut microbiota, its metabolites and human metabolism: Dissecting cause from consequence. Trends in Food Science & Technology. 2016;57:233-243. doi:10.1016/j.tifs.2016.08.009.
