Aidan:
Hello and welcome to the Ideal Nutrition Podcast. My name is Aidan Muir and I’m here with my co-host, Leah Higl and this is episode 204 where we’re going to be talking about gut health and mental health, the connection explained. There’s a few reasons why we’re going to be talking about this. One is that it matters. Two is that gut health is linked with everything and you could interpret it in a few ways. Like it literally is linked with pretty much everything.
The alternative way of viewing it is it’s linked with everything, but other stuff obviously still matters too. So we’re kind of acknowledging both aspects there. And then the third reason is because I’m a bit of an analytical person. I look at our stats and if we put gut health in the title, we get more downloads. So that’s another reason why we’re going to be talking about this as well.
Leah:
Starting pretty broad and just talking generally about the link between mental health and IBS. So the connection between the gut and the brain is pretty well established at this point. There’s a lot we don’t know about it, but we do know it is this very bi-directional strong link, especially when it comes to IBS and other functional gut disorders. Research shows that over one third of people with IBS actually also experience mental health conditions like anxiety or depression. So even from that perspective, we can see that there is a huge link or crossover in general.
We also know that stress can significantly influence gut symptoms, often making them much worse. So even working with my clients, we do a lot of nervous system regulation stuff, reducing stress type work if they are trying to manage ongoing gut symptoms, because this is often one of the main drivers in a lot of circumstances. But there is also a pretty complex interplay between chronic stress, gut health, and all that kind of stuff.
So we’re going to talk through mechanisms, but overall, focusing on how mental health affects the gut is a very interesting topic. But today we’re flipping the script and looking at how gut health and the microbiome can actually influence mental health in that direction.
Aidan:
So it’s almost a cliche at this point, but there’s a thing called the gut-brain access. And the gut and brain communicate with each other through nerves, hormones, neurotransmitters, and the immune system. Your gut produces neurotransmitters and one of many examples is one known as GABA, which is well known to have a calming effect, which would obviously play a role in mental health.
There’s another one. I don’t know how much you see of this on social media, Leah, but like you’ve seen people say how like 95% of the body’s serotonin is produced in the gut.
Leah:
I do see that. Yes.
Aidan:
Yeah. And it’s, it’s a really interesting one. like we know that serotonin plays a huge role in our mood. I don’t know how to describe this phenomenon that I’m looking at. It sounds like something that’s very smart. And there are many cases where it lacks a lot of nuance. The first piece of nuance is that serotonin from the gut does not cross the blood brain barrier. like, it’s not affecting the serotonin levels in the brain. But I’ve also people make that point and then also make the point that therefore it has no impact on our mood.
But then that also to a degree ignores the gut brain access where gut serotonin can still influence the brain indirectly through things like vagus nerve signalling, immune and inflammatory pathways, tryptophan and other microbiome effects. Like there’s a lot of like variables that go into it. And in both cases, people talking about both of those things are putting out content that makes them seem smart. In many cases, they would obviously believe what they’re saying and everything like that. And it’d be so hard just interpreting that as a consumer, like just reading that. So I don’t know, I just got on a tangent.
Like obviously like you got produces, neuro-transmitters that do play a role in all of this. We also know going further through mechanisms that poor gut health can contribute to inflammation, which we know is linked with anxiety and depression as well. And the gut can also produce short chain fatty acids like butyrate, which play a role once again in inflammation and also in mood.
You could go very deep with this list. I always struggle with this being like, I always want to give some mechanisms. So it’s not just like, I am very outcomes focused, like, it’s not just looking at outcomes without thinking about it deeper, but it is an endless list and to keep it good podcast content, we’re just gonna end the mechanisms there and we’ll go deeper with other stuff.
Leah:
I think it’s like often something that’s so oversimplified, but it’s like, once you start digging deep into it, there’s a lot that we don’t know and it just becomes very complicated, like this like very complex, like web of stuff and interactions. So I think that’s like a very good, simple summary at least.
So we’re going to tackle a bit of the research now. So looking at certain types of gut bacteria, things like, my God. Firmicutes, bacteroides.
Aidan:
I’ve stitched Leah up. I’ve written stuff out in advance and I’ve like phonetically spelled these out and it is still like even for me looking at the phonetic spelling. I struggled still. Anytime we do a gut health episode where we have to say the names of certain gut bacteria, I just look at them and like my brain cannot even phonetically spell. But so certain gut bacteria, so I’m going to attempt these the best that I can.
Firmicutes is I think the first one. See it written all the time, have never said it out loud. And then the other one is bacteroides.
Aidan:
So it’s bacteroides. And it’s the same thing where it’s like, I have read that hundreds and hundreds of times and I’ve read it slightly differently in my head. And when I like Googled how to phonetically like spell it, was like, ah, that’s how you say it. Bacteroidities.
Leah:
Bacteroides. Okay. Now that you’ve said it, I’ve got it. all right, bacteroides and firmicutes. So these are two, uh, bacteria that we find in the gut that have been shown to play a role in mental health in simple terms. These bacteria are generally considered beneficial, but it’s not just about having them in the gut, but also the balance between them matters. So having too much of one type compared to the other may lead to imbalances that could negatively impact mood and mental wellbeing. So again, it’s not just about having certain bacteria, but also how they exist in these ratios.
Aidan:
Podcasting is very one-sided and I’m always curious. I’m like, do you care if we know how to pronounce certain words? Cause like going deeper in that, like I’ve been to like conferences, I’ve been to talks, I’ve heard people say back to bacteroides before. And it’s just, I’ve just never connected the two in my head. But I wonder this on like certain other topics being like, Oh, if you can’t pronounce a certain word, have you gone really, really, really deep on this topic?
I’ve kind of mentioned I’ve been to conferences, I’ve like watched videos, I’ve listened to podcasts, I’ve read studies and I still struggle with the connection. I’m curious if people care. If you want to leave hate comments, DMs are open, please go for it.
Leah:
I’ve always struggled with words like that. I don’t think that’s going to change. But anyway, we move on regardless.
Aidan:
So there are other dietary components that can obviously play a role in all of this as well. So listing some of them, we’ve got probiotics, prebiotics, symbiotics, postbiotics, fermented dairy products, some herbs and spices, fruits and vegetables, noting that there is overlap there with like the prebiotics as well. It’s a really long list. And I’m also not even just like saying those words as if there is no connection, like stuff like postbiotics and symbiotics, you’re getting into pretty like weird territory there. But research has at least made a connection that these can play a role in gut health in a way that can affect mental health as well.
And as you can imagine, it’s really hard to go through individual research on that kind of the same as the mechanisms thing, just because it’s like obviously such a long list. Like I’m not going to talk about every single prebiotic study on this topic, but in our show notes, we’ve included a relevant review that does cover a lot of this stuff. So if you were interested in like reading more about that, you can go to that. I know it’s pretty science heavy, so not many people will go and do that, but if you are interested, feel free to.
Leah:
Yeah. And that review also included a list of studies finding different bacteria linked with an increased risk of mental health conditions and others with a decreased risk. So I just think like as a bit of a summary, there is a definite link there with different bacterias and increase and decrease risk of mental health conditions. But if you want more info above that, check out the actual study or the review itself.
Aidan:
And immediately that concept of that kind of bacteria stuff, should really make you think….Can I just take a probiotic based on that research? Like if it’s common for people to be either be lacking certain bacteria or having more of another type of bacteria that could be decreased by having more of the quote on quote like good bacteria, could that lead to benefits? And right now the research is mildly promising. I use the mildly because it’s like relatively consistently promising. It’s just a small to moderate beneficial effect. It’s obviously not a treatment by itself.
I personally have the belief that this stuff will get better and better over time. I’m not really big on like individual like microbiome testing right now in 2025 when we’re recording this, but I feel like it’s gonna get better and better to the point that we can individualise stuff a little bit more. My own two cents, I’m open to being wrong on that. But if we’re narrowing down to this kind of like broad advice, right now, a lot of lactobacillus ones in particular are the ones that have the most evidence for helping.
That’s not as helpful as you think it is because if you just go and grab a random probiotic off the shelf, look at the ingredients, there’s very likely to be lactobacillus in there if there’s like multi-strain type probiotic stuff. So make of this what you will, but there is mildly promising evidence at this stage.
Leah:
So whilst that review we spoke about did identify several probiotic strains that may support mental health, we want to touch on really like one surprising finding. And that was people who consumed more than two servings of fermented dairy like yoghurt per day actually showed an increase in depression symptoms.
So interesting. The researchers noted this doesn’t necessarily mean yoghurt is harmful. Factors like added sugar or sweeteners may have influenced the results. And that’s kind of what the researchers had kind of indicated in their conclusion. And lower sugar options might not have the same effect but more research is required to confirm this.
So I think it’s a very interesting result because we would think potentially the opposite would occur if there is like fermented dairy tends to have quite a positive impact on gut health. why is it, why is it having this particular outcome?
Aidan:
Yeah, I personally haven’t gone super, super deep on the probiotic benefits of yoghurt. It’s something that I think we all almost kind of assume is a positive thing. I have a general kind of guide for most people being like she probably includes some form of fermented foods in your diet at some stage relatively regularly. Like I do think it makes common sense.
But if somebody is like, Hey, I’ve got X problem. Should I have a probiotic or should I have yogurt occasionally? I’m usually going to point to a specific probiotic just because we have this like clinical evidence that’s like you do this thing and it helps this thing. And it’s very clear cut, whereas I haven’t seen as much of that with yoghurt.
The finding in that one is just interesting. I wouldn’t read too much into it, to be honest, but it’s…
Leah:
I wouldn’t stop having yogurt or anything like that. I think it’s interesting, but beyond that, I wouldn’t call like a conclusion from it.
Aidan:
If I was going to draw any conclusion, I would be like, it’s unlikely that this is going to massively help. Mental health is my own two cents. now I kind of touched on this at the start of this episode, but gut health affects everything but other stuff still matters. So I’m going to call this segment, what if we look beyond gut health?
And I recently saw somebody reference the SMILES trial for depression and anxiety and link all the benefits back to gut health changes. For those who don’t know, the SMILES trial was the first ever randomised controlled trial done on nutrition and depression. It was done in 2016. This doesn’t hit the same in 2025, but a point that I used to make back in 2016, note that I’ve been a dietician for that long.
The point I used to make was that that means up until 2016, there had not been randomised control trials done on nutrition and depression, which therefore meant everything that people had been saying up until that point was largely based on what you could call observational evidence.
Whether you take that definition quite literally or whether you just take it as like looking around the world and noticing certain patterns and stuff like that. However, you’d want to interpret that. We have a lot more randomised control trials now and there’s a lot of positive benefits or evidence out there now as well, but that small child one was a modified Mediterranean diet.
It was a bit of a combination of the Mediterranean diet and the Australian guide to healthy eating, but it was also done in a free living conditions where people could just eat whatever they wanted to. Like they were told to do this, but they didn’t have to strictly follow it. The authors of that study did point to the gut aspect, but they also pointed to so much more as well. This is an area where it’s really easy to hyper-focus on one thing, but clearly a lot matter.
For example, if somebody was looking at micronutrients and mental health, you would find that in depression and anxiety, it is quite common for certain micronutrients intake to either be inadequate or there to be more deficiencies as well. If you hyper-focus just on nutrients, you would potentially come to the conclusion that nutrients are what matter and we just need to address those things and those will help. Note that nobody would ever be out there being like, this will just solve everything. I’m just saying ‘help’.
You can do the same with gut health. If you just focus on gut health and just ignore everything else, you would come to a similar conclusion being like, there’s some deficiencies, so to speak, in certain beneficial bacteria. There’s too much of certain other things. If we just address that, that will help. A lot of things matter. In that specific study of SMILES trial, they pointed to gut health, but they also pointed to inflammation, which we kind of mentioned is linked to gut health to a degree as well. They pointed oxidative stress, they pointed to nutrients and a bunch more. All of the above can be true.
God health matters, other stuff matters. And then there could also be stuff that wasn’t pointed to as well. There was a massive overhaul in people’s diet in that study. And there was massive benefits across the board. 32% of people went into remission in that study over a 12 week period, noting that they all had to have severe depression at the baseline.
There can be so many explanations. We know that nutrition matters. It’s just harder to be like, is this 30 % gut health, 30% nutrients, so on and so forth. It’s hard to pinpoint like, what is the thing that matters the most?
Leah:
Yeah, I also think gut health as a term tends to be oversimplified. And we just think about the gut microbiome in regards to the makeup of the bacterial colonies, but it’s also things like intestinal permeability and like the inflammation and like there’s so much that is also gut health beyond just the microorganisms that live in our gut. And I think that is sometimes like gets glossed over in this context and is a lot more affected by those other factors you just spoke about.
So at the end of the day, what do we actually do with this information? Like what would be the actionable steps moving forward if we did somewhat want to focus on overall gut health to improve our mental health? So I would start by focusing on a nutrient rich balanced diet. So like very, very simplified. This is going to lay the foundation for both gut health and mental health. So not only is it going to help improve gut health, but it might fill gaps where there were nutrient insufficiencies previously and that could lead to better outcomes.
I’d also aim for a diversity of different fibre intakes. So from a variety of different plants to help support, again, a healthy gut microbiome. So we’re really looking for that plant diversity, ideally including some level of fermented foods like the yoghurt, kefir, kimchi, sauerkraut, all those kinds of like high probiotic options. Maybe this isn’t something that has necessarily a super strong link, but I think at this point we can say consuming these here and there, are probably beneficial to gut health overall.
There may also be value in taking some kind of probiotic supplement and potentially things like prebiotics and postbiotics too, but start with the options that are low risk and well tolerated would be just the general consensus there. And then I’d also recommend beginning with choices that overlap with the strategies outlined in the article in the show notes. So they are practical, evidence informed and unlikely to cause harm. So if it’s something you want to know more about, check that out.
Aidan:
This has been episode 204 of the Ideal Nutrition Podcast. If you haven’t already, as always, if you could please leave a rating and review, it means the absolute world to us. It really helps the show. It really helps get new listeners and everything like this, it makes a huge difference so we would massively appreciate it.