Hacking the gut-brain hotline

How is the "gut brain" connected to the "head brain"? And what have bacteria got to do with it? Katie Haylor asked Oxford University neuroscientist Katerina Johnson...

Katerina - We think that a strong link between our gut and our brain evolved because, ultimately, a lot of information about our environment comes from our gut. So if we eat something dodgy or something, our brain really needs to know about it and needs to respond to keep us safe. Our gut can affect our brain, but also how we feel and our emotional state can in turn affect our gut.

Adam - That's Oxford University neuroscientist Katerina Johnson.

Katerina - And one of the ways that this happens is that there's a vagus nerve, so this is a major nerve that travels between our gut and our brain. And interestingly actually 90 percent of nerve fibres in the vagus nerve communicate in the direction from our gut to our brain. And so this really suggests that our brain is actually more a receiver of information in terms of gut-brain communication.

Adam - One aspect of the gut not yet mentioned is the incredible abundance and diversity of microorganisms surviving and thriving down there in your intestines. They do vital jobs for us in exchange for taking up residence inside us, like helping to break down our food or to produce vitamins for us.

Katerina is interested in whether the different types of gut microbes can tap into this communication to affect the brain, and maybe even our mood. So how exactly do these microbes hack the gut-brain hotline? Katie asked Katerina.

Katerina - One of the messages is by neurotransmitters travelling along the vagus nerve. Also other messages as well. So for example we think that our immune system plays an important role, so the types of bacteria living in our got are very important for regulating our immune response and how we deal with infection. And it's good to have an active immune response. But if we have an overactive immune response, this in turn has been linked to low mood and depression.

So one of the interesting things at the moment we're looking into is that our gut bacteria can actually produce neurotransmitters - a chemical used to send signals between nerves. And we typically think of neurotransmitters as being in our brain, but really that's kind of a misnomer. Neurotransmitters are more just chemicals that help cells communicate with each other. So things like serotonin, dopamine, histamine, acetylcholine. And so one of the things we're interested in now is understanding whether these neurotransmitters produced in our gut can affect our brain. It may be that these neurotransmitters can trigger the vagus nerve and send signals to our brain that way, or perhaps the neurotransmitters or their precursors can travel through the blood and affect our brain.

Katie - It seems to be that there are multiple lines of communication. There's this vagus nerve so electrical impulses being fired up from the gut to the brain, but also there's the bloodstream, right, where perhaps particular metabolites are going up to the brain. Have I got that right?

Katerina - Yes. So for example in terms of metabolites, we know our gut bacteria produce fatty acids when they break down our food, and in particular sht chain fatty acids. These can actually travel through the blood and into our brain directly. But bacteria produce such a wide range of chemicals that we're only really starting to understand. And a lot of these chemicals may be able to influence our immune system, you know perhaps through our bloodstream. And, like I said, the neurotransmitters they produce, they might just stay locally in the gut. But that doesn't mean that they can't affect the brain. Because they may well trigger the vagus nerve. And in fact there's some new research at the moment looking at whether electrical stimulation of the vagus nerve may help with things like depression. That does suggest that maybe how much our vagus nerve is stimulated and in what way may affect how we feel and our mood.

Katie - It’s so interesting you said that because I actually quizzed the Naked Scientists office about occasions where they may have been “listening to their gut” or gone with a “gut feeling”. I mean in an emotional or psychological sense. But actually one person said they really associate the phrase gut feelings with anxiety. So how can this gut-brain connection have any impact on our mood as you say? What's going on?

Katerina - A lot of the studies at the moment are in animals because it's at early stages, but if you use a probiotic  - so this is a type of live bacteria, in this case lactobacillus - in animals it was actually found to reduce anxiety and the depressive like behaviours that the animals showed. But this was only the case when the vagus nerve wasn't damaged. We know that this probiotic actually seems to increase the firing rate of the vagus nerve.

There's a big link between anxiety and stress and how we feel in our gut. Most notably for example irritable bowel syndrome where people often suffer from gut conditions and kind of psychological symptoms at the same time. A study found that in two thirds of cases, people suffered from gut conditions before the onset of things like stress and anxiety, and in the other one third of cases they suffered from stress and anxiety prior to the onset of gut conditions. So this really underpins the fact that this is a 2-way communication between our gut and our brain.

Katie - Does that hint in any way as to the cause of someone's anxiety? Are we saying that what's going on in your gut may cause you to feel anxious in your brain?

Katerina - It's a bit tricky really because there's so many factors that might affect how we feel and our mood and our anxiety. And research in this whole field of the microbiome at the moment is starting to suggest that the types of bacteria in our gut may influence how we feel, but we don't know at the moment how strong this is. So do they have a really big impact on our brain? Or is it really just like one tiny piece of the puzzle?

But if you're stressed or you're worried about something, you often feel it in your gut. And if you’re stressed, particularly over long periods of time. There's some studies that suggest that stress can deplete the abundance of beneficial gut bacteria and actually it can affect our whole kind of gut environment and how a gut works in terms of things also like mucus secretion and are the motility of our gut.

So, yeah, it's an interesting question trying to understand how much our gut affects our brain. And there was one interesting study that found that people with a lower abundance of a certain type of bacteria, that we know is a prolific producer of the neurotransmitter GABA, actually also tended to show stronger kind of brain signatures of depression when their brains were scanned. And so this is just one study that we need to look more into, but it does suggest that maybe the types of chemicals and neurotransmitters produced by bacteria in our gut might be one factor influencing how we feel.

Katie - One of the functions that the gut has is to digest our food. So how does what we eat relate to this connection between the gut, and the microbes, and the brain?

Katerina - There's a lot of interest at the moment trying to understand whether what we eat has a big effect on the types of bacteria that live in our gut. So we know that, for example, people that eat more fruit and vegetables, they have a high abundance of gut bacteria that like to digest fibre. There have been some initial studies, for example, there was one looking at a particular type of fibre known as prebiotic fibre. And this is actually known to promote the growth of bacteria we tend to associate with good health. So bifidobacterium and lactobacillus that also have anti-inflammatory properties. And they actually found that after a few weeks, people that had been consuming this prebiotic fibre tended to have lower levels of cortisol, so their bodies’ stress response had reduced. So it does suggest that maybe changing the types of bacteria in our gut may influence our physiology and how we feel. And this prebiotic fibre is found in foods like banana, chicory, onion, garlic, pulses and grains, but really in terms of understanding how specific elements of our diet affect our gut, it's still in its infancy.

Katie - If there are bacteria that thrive on food that we may consider to be more healthy, like high in fibre for instance, I imagine there are gut bacteria that thrive on things you may consider to be less healthy. Does this have any relation to our (well, my) desire to consume foods which are high in fat and sugar?

Katerina - Yes. So when gut bacteria break down particular types of food, they can release metabolites that then potentially can travel to the brain and affect our feelings of appetite. But we're not sure at the moment, we can't say that, our gut bacteria make us crave a certain type of food because they want it. That's probably unlikely because it's very costly for bacteria to produce a signaling chemical and it would probably be out competed because our gut microbiomes are so diverse. But our gut bacteria are really important for affecting things like how hungry or full we feel. But yeah it's still kind of in its infancy at the moment.