Why the gut makes most of your body's serotonin

HostWe often hear about serotonin as the happy chemical in our heads, the thing that keeps our moods steady and helps us feel good. But it turns out the brain is actually a pretty small player when it comes to where this stuff is actually made and kept. Most of the serotonin in your whole body is actually sitting way down in your gut. It feels a bit backwards since we talk about it like a brain thing, but your stomach and your pipes are just loaded with it. Why on earth is all that mood-lifting chemical hanging out in our digestive system instead of our heads?

GuestIt's a massive amount, too. If you looked at all the serotonin in a person, about ninety-five percent of it's in the gut. Only about five percent is actually in the brain. And the reason for that's mostly about movement. Think of your gut as a very long, very smart tube. To get food from one end to the other, those muscles have to squeeze in a very specific rhythm, like a wave. Serotonin is the main messenger that tells those muscles when to squeeze and how hard. Without it, everything would just sit there. The gut uses it to manage the whole flow of your lunch.

HostSo it's less of a happy chemical and more of a housekeeping chemical down there? That's a bit of a letdown. I always thought if I had a lot of it in my system, I would just be in a better mood. But it sounds like it's just doing the heavy lifting for my dinner.

GuestWell, it does a lot more than just move things. It's also a huge part of your early warning system. Your gut is one of the main ways the outside world gets inside you. If you eat something that's bad for you, or some kind of nasty bug gets down there, your gut detects it and releases a giant flood of serotonin all at once. That flood hits the nerves and tells the gut to work double time to get that stuff out of you. That's why you get that sudden, rumbly, sick feeling. It's also why some drugs that raise serotonin in the brain can make people feel a bit sick to their stomachs as a side effect. The gut senses the extra chemical and thinks something is wrong.

HostThat makes sense. I have definitely felt that jittery stomach before. But here is what I don't get. If we have this huge supply of serotonin in our bellies, why can we not just send some up to the brain when we're feeling low? It seems like a waste to have all that good stuff sitting in the gut if the brain is running short.

GuestYou would think we could just ship it up there, but the body is very strict about its borders. There's a wall between your blood and your brain that acts like a high-security gate. Serotonin is a pretty big molecule, and it simply can't fit through that wall. So the serotonin your gut makes is stuck in your body, and the brain has to make every bit of its own from scratch. They're like two separate pools of the same liquid that can never touch. Even if your gut is overflowing with it, your brain could still be running on empty.

HostThat sounds like a design flaw. Why have the same chemical doing two different jobs if they can't help each other out? It's like having two different kinds of money in the same house that you can't swap.

GuestIt does seem strange, but it keeps the systems from getting mixed up. If every time your gut moved your breakfast it also changed your mood, you would be an emotional wreck just from eating a sandwich. By keeping them separate, the gut can do its job and the brain can do its job without stepping on each other. But there's a catch. While the chemical itself can't cross that wall, the nerves in your gut are constantly talking to the brain. Your gut has its own massive network of nerves, so many that people call it the second brain. It uses serotonin to talk to itself and then sends messages up a giant nerve cord to your head. So your gut can still tell your brain how it feels, even if it can't send the serotonin itself.

HostWait, so the gut is actually talking to the brain? I thought the brain was the one giving all the orders. It sounds like the gut has a lot more say in how we feel than we think.

GuestIt really does. It's a two-way street, but about ninety percent of the chatter on that nerve cord is going from the gut up to the brain, not the other way around. The gut is constantly reporting back on what's happening down there. And here is where it gets even weirder. A lot of that serotonin making isn't just up to you. There are trillions of tiny bugs, bacteria, living in your gut. We're finding out that these bugs actually talk to the cells that make serotonin. They can give the signal to ramp up how much is being made.

HostSo the tiny bugs in my stomach are basically pulling the strings on my nerve messengers? That's a bit creepy. Why would a bunch of bacteria care about how much serotonin I have?

GuestThey use it for their own reasons. Some of them use it to grow, or to make the environment in your gut better for them. But because they trigger your cells to make it, they end up affecting the whole system. If your gut bugs are out of balance, your serotonin levels can get wonky. This can change how fast food moves through you, but it can also change the signals being sent up to your brain. We're starting to see links between the state of those gut bugs and how people feel mentally. It's not just about a happy chemical in your head, it's a whole conversation happening in your pipes.

HostIt's wild to think that the gut is so independent. It's making its own chemicals, listening to bacteria, and then sending reports up to the head. It makes the brain seem almost like it's just the person watching the news, while the gut is out there doing all the actual reporting.

GuestThe gut really is the only organ that can run itself without the brain telling it what to do. You could cut the connection between the brain and the gut, and the gut would still keep moving food and managing itself. It needs that huge supply of serotonin to act as its own local language. It's its way of keeping the peace and making sure everything runs on time, completely separate from whatever you're thinking about or how your day is going.

HostThe gut has its own secret life with its own massive stash of chemicals just to keep the internal gears turning. Those butterflies we feel in our stomach are probably just a tiny part of a much bigger conversation our nerves are having while they move our lunch along.

GuestThose bugs in your belly are the ones actually setting the pace for that whole hidden system.

HostThe stomach isn't just a bag for food, it's a busy control center that has to build its own wall to keep its secrets from the brain.

GuestAnd that wall is the only thing keeping your last meal from deciding your whole mood.

HostThe next time a gut feeling hits, I'll be thinking about that huge wave of chemicals and the tiny bugs that started it all.