ZOE Science & Nutrition
Most replayed moment: Fix Your Gut: Improve Your Mind | Dr Will Bulsiewicz
28 Apr 2026
Chapter 1: What is the brain-gut axis and why is it important?
Hello and welcome to Zoe Recap, where each week we find the best bits from one of our podcast episodes to help you improve your health. We've all heard phrases like gut instinct or trust your gut. And these sayings hint towards a sort of mind in your midriff. And it turns out there's actually some truth to it.
Emerging science reveals that the brain and the gut are closely connected, constantly exchanging signals through an intricate network of nerves. So the question is, if they're so tightly intertwined, can treating one help heal the other? I'm joined by gastroenterologist Dr. Will Balsawish to unravel the mystery of this connection and find out how we can use it to our advantage.
There is tremendous overlap between digestive health, and mood. And we see this represented in a powerful way in this particular condition, irritable bowel syndrome, because yes, the criteria are not based upon your mood. The criteria are based upon your gut, based upon your gut symptoms. But the issue is that a huge percentage of these people simultaneously are dealing with mood issues.
So it's roughly 50% of people that have irritable bowel syndrome that actually can be diagnosed with major depression or generalized anxiety disorder or both.
50% of people with IBS can be diagnosed with serious depression or anxiety.
Up to 50%. At a minimum, 50% are suffering in a way where if you were to measure their mood, compare that to normal people, at a minimum, they're in a depressed mood relative to other people. But it is a very large percentage of people that are potentially... able to be diagnosed with one of these two conditions or both, which are mood disorders.
So there's this overlap between irritable bowel syndrome and these disorders of our mood. And the question that has come up is like sort of a chicken or egg thing, right? What causes what? And in the past, like in the very beginning, Jonathan, when they first started studying this, they thought that actually started in the brain and that these people had mood issues. They were depressed.
And because they were depressed, they were feeling it in their gut. But then we actually lined up studies where we took a group of people that don't have any of these symptoms and we track them. You know, you take 5,000 people, track them and see what comes first. And it's actually quite fascinating.
And it teaches us quite a bit where they discovered that actually most of the time it starts with gut symptoms. By the way, this is the exact same thing that happens in Parkinson's disease. where there's now research that Parkinson's disease starts with constipation and changes in the gut and then subsequently manifests with a neurologic health condition.
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Chapter 2: How does irritable bowel syndrome relate to mood disorders?
There's multiple ways. that your gut is talking to your brain right now. As you and I sit here and speak to one another, your gut is talking to your brain. And some of the ways are with the production of chemicals, which are what we call bioactive. So that means they can actually impact the physiology in your body. These chemicals, by the way, are often produced by your gut microbes.
70% of your immune system exists in your gut. And the immune system is able to activate things that can affect the entirety of your body and create inflammation. Also your hormones and your neurotransmitters that are produced in your gut. So it's interesting when I finished medical school in 2006, I had learned about serotonin and things like this.
And I didn't realize that there's over 30 neurotransmitters. I didn't realize there's that many. I thought there were like six. There's over 30 neurotransmitters. They are produced in the gut. 90 to 95% of serotonin is produced in the gut and 50% of dopamine is produced in the gut. And so now these things can have influence in the gut, but also throughout the body.
And particularly with serotonin, I just want to be totally clear because there's going to be someone who's a scientist who's going to want to make sure that I'm clear on this. The serotonin that's produced in your gut does not actually cross into the brain, but there's a precursor to serotonin. called 5-hydroxytryptophan.
And this 5-HTP actually has the ability to cross into the brain and signal and change how your brain works. And then the last but not least, perhaps the biggest and most important, is a pair of nerves, which are probably my favorite nerves in the entire body, Jonathan. called the vagus nerves.
All right, now the vagus nerves start in our brain, pass through our skull, and descends down to our gut. And they're collecting information from your gut. There are in your gut 500 million nerves, feeling and sensing. by the microsecond. And all that information that gets collected by 500 million nerves, by the way, I should add that 500 million is a ridiculous number.
So, I mean, I know that sounds like a lot. I mean, it sounds ridiculous. But just to compare this to your spinal cord, this is five times what you will find in your spinal cord.
Hang on, so I've got five times more nerves in my gut, which as far as I'm concerned, I can't have any control over, versus my spinal cord, which ends up controlling my legs and other things. I know I'm moving around all the time.
That's right. So yeah, so you have five times more nerves in your gut. This is why we call it the enteric nervous system, or some people will call it the second brain. I actually would challenge that. I would call it the first brain. Serious. Okay, so here's why. From an evolutionary perspective, for you and I to be here, it required us to evolve and mature our brain, right?
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Chapter 3: What evidence supports the gut influencing mental health?
That's the answer. And simultaneously, we have discussed how our mood is affected. Research is now showing, Jonathan, that the common tie between major depression and Parkinson's disease, and by the way, also Alzheimer's disease, is inflammation inside the brain. So our immune system is getting activated in both of these places, both in the gut and in the brain.
And if you want to reduce these things, the most anti-inflammatory chemical that exists are the short-chain fatty acids, butyrate, acetate, and propionate. And so it's kind of interesting to take that into consideration when we think about people that have irritable bowel syndrome. And if you zoom in on their gut, what you would see is the gut microbiome is damaged. There's a loss of diversity.
They've lost the anti-inflammatory microbes like bifidobacteria and lactobacilli. They're not as well represented as they used to be. And there's an increase in the pathogenic microbes, the inflammatory ones. All right, so this is what we see in irritable bowel syndrome. Well, actually, it kind of is in parallel to the same changes that we would see in a person who has a major depression.
similar changes in terms of loss of diversity and more inflammatory microbes and less of the protective ones. So when we look at this whole picture, we start to see that the changes are occurring within the gut and the manifestation can affect gut rhythm, gut nerves, sensitivity, and simultaneously can affect our mood.
So it feels like the perfect time to switch to, so what can you do about it? And I think you know, Will, we always like to make sure we cover really actionable advice. So if we understand that the mechanisms that end up leading to IBS start back with this damage to the gut microbiome, What can you do about it? Right.
So the advice that you received was to cut fiber. Correct. I understand why they said that. On some level, if the only thing we cared about was improving your symptoms, fiber is hard for the gut microbes to digest. So in theory, if you cut fiber, you may feel better. There's research with a dietary approach called the low FODMAP diet, which is something that we've discussed before on the show.
So FODMAP is an acronym. It's super nerdy. I'm going to put it out there, but you don't have to memorize this.
No test afterwards.
Yeah. So FODMAP, F-O-D-M-A-P, fermentable oligosaccharides, disaccharides, monosaccharides, and polyols. Basically what this is saying is these are carbohydrate-based foods. that are fermented by our gut, by our gut microbes. And in the process of fermenting them, they produce gas. And you know what gas can do? Gas can distend. Gas can activate those nerves that are very sensitive.
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Chapter 4: How can gut health affect neurological diseases like Parkinson's?
Thanks for tuning in and see you next time.