Imagine you're a contestant on "Family Feud" and this prompt pops up: Things Your Gut Does. You ring in and shout "digests food!" Ding! That's correct — but there are several other answers still on the board. What could they possibly be?
It turns out, your gut does a whole lot more than manage your meals on their way through your body (a mighty task on its own).
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Here's a look at just a few aspects of your body affected by your gut.
Meet the Gut Microbiome
Before we dive into the gut's many spheres of influence, a quick review: Your gut — also referred to as GI tract or gastrointestinal system — is made up of several organs, including the esophagus, stomach, pancreas, liver and intestines, according to NYU Langone Health.
It's home to a microbiome consisting of trillions of bacteria, fungi, parasites and other microorganisms, per the Harvard T.H. Chan School of Public Health.
Try not to think of these bacteria and microorganisms as "bad guys" that get you sick. The bacteria in your gut are more complicated than that — both potentially pathogenic and friendly microorganisms exist in the gut of a healthy person, according to the Harvard T.H. Chan School of Public Health.
The precise mix of microorganisms in your gut is unique and determined by several factors, including diet, habits and where you live, per the Mayo Clinic. Taking antibiotics, for instance, will change your microbiome's makeup, as can eating yogurt or other foods with probiotics (aka good-for-you bacteria).
Your gut microbiome is ever-changing. "The microbiome is so dynamic that even in 10 minutes, it could be different [from before]," says Ashkan Farhadi, MD, a gastroenterologist at MemorialCare Orange Coast Medical Center in Fountain Valley, California.
So, what exactly does that gut microbiome do? Lots of things, including:
1. Regulates Immune Function
An estimated 70 percent of the body's immune cells are in the gut, according to a September 2008 article in Clinical and Experimental Immunology.
"A very big part of immune function is regulated by the gut," Dr. Farhadi says. The gut, he notes, appears to be able to both activate and regulate your immune system.
The gut's role in immune health can be protective. For example, acids in your stomach help to destroy pathogens in food, per the Harvard T.H. Chan School of Public Health. And the lining of your gut is a barrier, preventing pathogens inside it from traveling to other parts of the body, according to a 2017 review in Experimental and Molecular Medicine.
"The gut wall barrier separates our bodies from the healthy organisms in our guts (microbiome) and everything else we put into our mouths — which can include healthy foods but also chemicals, bacteria, viruses and other potentially dangerous organisms," Dr. Ivanina says.
The gut's function is not simply to isolate and neutralize pathogens. It also allows those immune cells within the gut to get familiar with germs, Dr. Farhadi says. This builds your immune function, and creates a tolerance due to this exposure.
The Effects of Diet and Stress
What you eat matters when it comes to your immune health. Diets low in nutrients and variety can negatively affect a healthy immune system, per the Harvard T.H. Chan School of Public Health.
"The problem happens when diversity [of gut bacteria] is gone, for any reason," Dr. Farhadi says.
Dysbiosis occurs when the microorganisms in your gut aren't in balance. And it has been connected to several health conditions, such as autoimmune disorders, obesity and cardiometabolic conditions (that is, heart attack, type 2 diabetes and stroke), per a June 2018 article in The BMJ.
Stress also affects the makeup of the bacteria in your gut, according to the American Psychological Association. And when you're feeling stressed out, your gut's lining can become more permeable, allowing bacteria and pathogens to cross, Dr. Farhadi says. (You may have heard this referred to as "leaky gut.")
Gut barrier permeability increases with both everyday, short-term stress (aka acute stress — think: a spat with your partner or a traffic jam) and chronic, long-term stress, per an August 2018 review in Current Opinion in Behavioral Sciences. Couples who don't get along with each other have more permeable guts than those who do, according to the review.
2. Plays a Role in Your Mood
Nearly all — 95 percent — of the mood-regulating hormone serotonin in your body is found in your gut, per the GI Society. So it's perhaps not surprising that the microbiome can influence your mood. The reverse is also true: Your brain can also influence your gut.
The gut-brain connection, or the communication that occurs between your gut and your brain, is well established, Dr. Farhadi says.
The gut is home to a nervous system, known as the enteric nervous system (ENS), per the Cleveland Clinic. The ENS is known as the "second brain."
Communication between the central nervous system and the ENS is constant, complicated and involves the microbiome, Dr. Ivanina says. "The connection goes both ways — gut conditions affect the brain, and nervous system conditions such as anxiety and depression affect the gut as well," she says.
People with depression, for example, consistently have lower levels of two gut microorganisms, according to an February 2019 study in Nature Microbiology. And people with irritable bowel syndrome (IBS), a disorder affecting the large intestine, have higher levels of depression and anxiety than healthy controls, according to an April 2014 meta-analysis in the European Archives of Psychiatry and Clinical Neuroscience.
"In many conditions, like IBS, the pattern of the microbiome is totally different" for someone with the condition than in a person who doesn't have it, Dr. Farhadi notes.
And psychological interventions — such as antidepressants and therapy — may be helpful for people with IBS, because helping the brain may influence the gut microbiome in a helpful way, per Johns Hopkins Medicine.
And that might work in the reverse direction, too. Researchers found that regulating the gut microbiome through dietary changes and/or taking probiotics may ease anxiety symptoms, according to a May 2018 systematic review in General Psychiatry.
In fact, growing knowledge about the brain-gut axis points to the possibility that a wide range of conditions — including psychiatric, neurological and GI disorders — might "respond to personalized therapy using dietary, prebiotic or probiotic interventions," per an April 2018 review in Cellular and Molecular Gastroenterology and Hepatology. However, as the review points out, far more research is needed to expand our understanding of the brain-gut-microbiome connection.
3. Might Be Connected to Skin Diseases
There's also a skin microbiome teeming with microorganisms, and it's connected to the gut through the gut-skin axis, Dr. Ivanina says.
And the gut microbiome may be involved in the development of some skin diseases. For example, it may play a role in developing eczema, according to a November 2019 article in Microorganisms.
In the same study above, the authors also note that other studies have shown a link between gut bacteria and psoriasis — specifically, that people with psoriasis tend to have less diversity of bacteria in their gut and may have lower levels of specific bacteria.
But while there may be a gut-skin axis, "the exact mechanism is poorly understood," according to the Microorganisms article.
"Although still being researched, the mechanism by which the gut microbiome exerts its influence on skin appears to be related to the microbiome's ability to affect the systemic immune system," Dr. Ivanina notes.
Much Remains to Be Discovered
Skin, mood and immune function are just a few of the ways the gut plays a role in the body. There's also a connection between obesity and the gut, per the June 2018 article in The BMJ, which notes the gut influences appetite and energy metabolism as well.
While our understanding of the gut is far richer now than a decade ago, it remains somewhat crude, Dr. Farhadi says.
He has an analogy at the ready to describe our knowledge about what's going on in the gut: Imagine looking at Earth from another planet and wanting to know the role of an individual or seeking to understand how society functions, he says. That's what we're trying to do with the gut and its trillions of microorganisms and thousands of species. We can see the trends, but we don't yet know the nitty-gritty details.
"The more we know, the more we may be able to modulate things by food or probiotics or something," he says. The science is not yet there to say that changing the microbiome in one way will lead to results elsewhere, he adds.
But we're learning more each day, Dr. Ivanina notes. "Stay tuned — one day we will be manipulating our microbiome to treat all sorts of medical conditions," she says.
- NYU Langone Health: "Your Gut Feeling: A Healthier Digestive System Means a Healthier You"
- Harvard T.H. Chan School of Public Health: "The Microbiome"
- Mayo Clinic: "Mayo Clinic Minute: What is the ‘microbiome,’ and how does it affect your weight?"
- Clinical and Experimental Immunology: "Allergy and the gastrointestinal system"
- Harvard T.H. Chan School of Public Health: "Nutrition and Immunity"
- Experimental and Molecular Medicine: "Roles of intestinal epithelial cells in the maintenance of gut homeostasis"
- The BMJ: "Role of the gut microbiota in nutrition and health"
- American Psychological Association: "Stress Effects on the Body"
- Current Opinion in Behavioral Sciences: "Stress, depression, diet, and the gut microbiota: human–bacteria interactions at the core of psychoneuroimmunology and nutrition"
- GI Society: "Irritable Bowel Syndrome (IBS) and Serotonin"
- Nature Microbiology: "The neuroactive potential of the human gut microbiota in quality of life and depression"
- European Archives of Psychiatry and Clinical Neuroscience: "Anxiety and depression comorbidities in irritable bowel syndrome (IBS): a systematic review and meta-analysis"
- Johns Hopkins Medicine: "The Brain-Gut Connection"
- General Psychiatry: "Effects of regulating intestinal microbiota on anxiety symptoms: A systematic review"
- Cellular and Molecular Gastroenterology and Hepatology: "The Brain-Gut-Microbiome Axis"
- Microorganisms: "The Skin and Gut Microbiome and Its Role in Common Dermatologic Conditions"
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