Gut, Nerves and the Immune System: An Intro

Put your hands on your belly. Inside, under layers of skin and tissue, a complex tangle of body systems come together—in particular, the gastrointestinal (GI) system, the nervous system and the immune system. And surrounding all of them is our resident microbiome, which some scientists think of as our most recently discovered human organ. In this and future posts, I will explore how these systems intersect and what it means for our health.

We’re used to thinking about our bodies in pieces. If something goes wrong, we seek out the appropriate health practitioner to help us. Digestive woes? Find GI doc. Depression and anxiety? Psychiatrist or therapist. Getting sick a lot or dealing with infections? Immunologist.  

But increasingly, studies of the gut and microbiome show that the division lines between these systems are not that clear.

For example:

·      The gastrointestinal tract is home to 70-80% of the immune cells in our entire bodies.

·      The gastrointestinal tracts houses over 100 million neurons, which surround it from esophagus to anus.

·      There are at least as many microbial cells as human cells in our bodies, and perhaps a great deal more.

This diagram shows how physically close the gut, nerve and immune cells are. Tiny nerve fibers directly connect to gut and immune cells. These cells “talk” to each other through neuro-chemicals such as serotonin, adrenalin and acetylcholine. Bacteria and other microbes are in on this conversation, too.

OK, pause.

That’s kind of a radical statement, so re-read it. We think of these chemicals – serotonin, adrenalin and acetylcholine (Ach)– as communication signals within the nervous system. We may think of serotonin as a “mental health hormone”, adrenalin as the “fight or flight” chemical and Ach as responsible for the “rest and digest” function. In rough terms, that’s all true. But what’s also true is that these chemicals don’t just talk to the nervous system. They also cross boundaries and create four-way communication between nerve, gut and immune cells, and the microbes in our intestines. Gut, immune and microbe cells can receive and/or transmit these neuro-chemicals. And not just these three chemicals, but many more.

In a series of posts, I will address pieces of the new science that’s emerging on gastro-neuro-immunology (as I will refer to it from now on). This is brand new science and it’s messy. But it’s also a lot more accurate and interesting than thinking of these systems as separate entities. In practice, what it means is that inflammation, mental health and digestive function should not be thought of as separate processes that can be “fixed” by going to separate doctors, at least not with complex conditions like IBS. It also means that the mind/body divide is… well, not a thing. Not at all.

Here’s a preview of some things I’ll talk about in future posts.

Inflammation 101: New science shows that inflammation underlies every chronic disease that impacts physical and mental health. It is especially impactful to gut health and mental health conditions such as depression and anxiety. But what is inflammation? Can I feel it? How do I know if I have it? And what do I do about it?

Leaky Gut and Foggy Brains: The enteric (gut) nervous system directly influences immune activity. This means that mental health plays a role in gut inflammation and leaky gut. And vice versa. Inflammation and leaky gut directly impact your ability to think and feel good. This is not simplistic, and it’s NOT meant to imply that you can “just be happy” and spontaneously heal your gut.

Serotonin: The Gut Hormone: Over 90% of serotonin is not produced in the brain, but in the gut! We think of it as a mental health hormone, but it also directly modulates immune activity. In addition, it promotes peristalsis, the action that moves food along the gut. This is a critical function and when things go awry, we feel the consequence. Too-quick peristalsis can lead to diarrhea and too-slow can lead to constipation—familiar symptoms to IBS sufferers.

 The Mighty Microbiome: The resident microbes in our body are not just hanging out. Like an organ, they work for us. They produce necessary vitamins, regulate our immune system, talk with our endocrine system and even make hormones (including helping to produce serotonin!), impact our mood and mental health, and so much more. One study of toddlers showed that the type and quantity of their microbiota was associated with their temperaments!

Stay tuned as I dive into this paradigm-shifting science that helps us better understand who we are and how we tick.