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Gut Feelings

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While Jad was on paternity leave, Carl Zimmer told Robert and producer Soren Wheeler about the ecosystem inside each and every one of us. According to Carl, when we're in the womb, we have no bacteria in us at all, but as soon as we're born we start gathering up the bugs. And by the time we're grown, we have a three-pound organ of others inside our guts.

With the help of some bacteria, swimming mice, and neuroscientist named John Cryan, we learn that this organ does much more than just help us digest food. And finally, Jonah Lehrer joins Carl to help us understand what the bugs in our gut have to say about the feelings in our heads.


Correction: An earlier version of this piece implied that subjects in the human probiotics study were given a larger dose of probiotics than they actually were. We also compared the results of the study to the effect of giving people anti-anxiety medication. The subjects did experience a significant positive mood change, but the researchers did not compare that result to the effect of anti-anxiety medication. The audio has been adjusted to reflect these facts.


John Cryan, Jonah Lehrer and Carl Zimmer

Comments [44]

Sam O from Los Angeles

I love this show!

Nov. 07 2017 03:03 PM
Stephen Hamm from Kempton, PA

Last August Ed Yong was all over NPR pushing his book "I Contain Multitudes". He insisted that scientific evidence proved taking probiotics was useless because they can't survive the trip through your stomach. Now John Cryan's work would imply the opposite. Which one is full of poop?

May. 02 2017 09:32 PM
Diane Bild

People interested in this topic might be interested in this recent paper, "The effects of probiotics on depressive symptoms in humans: a systematic review":

May. 01 2017 09:22 PM
Michael L McCann MD

Absolutely fascinating study. If a variation of this study can be reproduced in humans, namely that an increase in GABA receptors in the brain can be effected by L rhamnosus in the gut, by communication through the vagus nerve, it will be a boon to non-drug, natural therapy for anxiety and depression. I can think of many different blinded studies, both in rodents and primates, to prove or disprove this. Does anybody know of any such studies already done?. The argument that what happens in the mouse does not necessarily correlate with the human is obsolete, now that we know for sure that we are simply highly evolved mice. Floras may be different, but the principles are the same. Please tell us more!!

May. 01 2017 12:04 PM
Share from Kansas City, MO

You might have saved my life with this podcast today. Your presentation allowed me to finally make sense of years of severe gastro-intestinal issues and uncharacteristic fuzzy brain and depression that's been growing worse. I didn't know what to make of recent labs that showed many of my amino acids out of whack - until now! This information gives me important clues to help myself. If only all doctors were secure enough in themselves and trusting enough of their patients' intelligence to break things down for us this way! Thank you! I'm setting my calendar for Radio Lab from now on!

Apr. 29 2017 11:08 PM
Peleg from Israel

A few comments:
1. In it's weekly printed special, 'Calcalist' (Israel), made a great article on the subject, interviewing Giulia Enders, a young German scientist, who is delving into our guts and wrote a book about it. it's called 'Gut'.
2. I remember reading somewhere that there are 9 cells of bacteria inside our body for every cell of our own. so in terms of sheer numbers - we're in minority...
3. I also remember an article somewhere about the fact that maternal milk provides food for a specific bacteria, which makes up most of the baby's gut bugs for the first 6 months or so.
4. Baby elephants feed on adult elephant feces, both because it's easier for them to digest, but also as a means of getting good bacteria in them.
5. Eating Camel feces was used to treat severe cases of diarrhea in WW2 in Northern Africa
I could go on and on, but I think it's time for an update...

Nov. 16 2016 10:10 AM
Karlee Castillo from CO

What does this nerve do other than transport that bacteria and if you cut it what would be the consequences? Also this is silly but, CAAAAAARLLLLL, that kills people!

Sep. 30 2015 12:16 AM
al from boston

don't know if this is still active, but I just re-listened to this program. It was stated that "serotonin is the feel good" neurotransmitter, which is a major misconception. Ever heard of serotonin syndrome? it doesn't feel good.

Also SSRIs, while they work well for some, that is only 30-40% of people who try them, and no one knows why they work, or why they stop working. The balance of neurotransmitters, how they effect neurons, the growth of neurons, different parts of the brain, etc.-- these are all poorly understood, or not at all. The brain is very complex. The vagus nerve is like a second brain, and the same applies to it.

Jun. 25 2015 09:15 AM
Cindy Goldthwaite from Georgia

I'm not very pleased with how the connection between lacto acidophilus and stress was made (testing on mice), but I had to tell you about my cat, Pepper, and his yogurt medicine. For years, until I had to give it up when I gave up most dairy, I ate plain yogurt as part of my breakfast. Pepper loved slurping up the little bits I left him in my bowl. I put it down near his eating place every day, and he licked it clean. I had not given him any yogurt for months, but after I heard your show, I decided that it might help him with his stress levels after six months about two new kittens I had gotten. He had been hissing and growling even when the cats were nowhere near. He was getting very difficult to handle. I started him on a teaspoon of yogurt per day two weeks ago, and he is a changed cat--only one day of hissing and growling since then. I never dreamed that his yogurt "habit" was so critical to him! And to my sanity!

Feb. 09 2015 02:32 PM
James W Flower from United States

Does anyone know the names of the two types of bugs that were given the stressed out humans?

Jan. 27 2015 11:24 PM
Toni L. Gatsby from NY

This podcast is VERY interesting! The bacteria in your body is disgustingly cool, I have never heard of anything like this. The mice experiment is also fascinating, and the results amazed me. The mice that were fed the bacteria went crazy with panic, and this is so intriguing. I don't quite understand how the bacteria in their gut caused more stress, but the results were cool. The random chemical added into the mouses stomach from the bacteria is strange, and I like how it was described as "shhhhh." How the connection between the gut and the brain is described as a telephone line really helps me understand how the chemicals are all working together in the mouse.

Jan. 26 2015 09:31 PM


Nov. 21 2014 01:46 PM
Bill Jacobs from Maryland

Lactobacillus Rhamnosus was the specific strain that appeared to bestow calm upon the rats in the show. Other strains are mentioned in the research article titles so it may not be unique.

Lactobacillus Rhamnosus is included in multiple supplements but this one seem to be exclusively Lac. Rhamnosus:

Jul. 31 2014 11:31 AM
Sellers from North Carolina

These rats that were talked about in the podcast who were given a certain type of bacteria rather than just the typical bacterium found in rats sounded to me like were maybe super rats! They had been able to tread water for 4 minutes with regular bacteria in them and then when given the new bacteria they somehow are able to endure more physical work!

Apr. 22 2014 11:46 PM
David Mohr from nyc

Why doesn't NPR's left hand know what it's right hand is doing. Rob Stein's morning edition article on Gut feelings didn't reference anything in this radio lab episode. I feel you guys were robbed!

Nov. 18 2013 09:52 AM

@mynontmous Thanks so much for the links to the original articles! I love Radiolab but I wish they would use APA citations or at least links to the studies that they use. I guess they must get permission to talk about the studies but as a listener I often wish I could look up the studies for more information or for use in a school project afterwards. With the way they do things now, I often have a lot of trouble with this.

Nov. 17 2013 03:58 PM
mark acosta

idk if anybody still reads this but I too had anxiety for the past 2 years. I have a genius iq and great socia skills and after antibiotics two years ago I had insane anxiety and hours after taking a probiotic..boom. cured. I have a huge amount of literature and reasoning for these changes.

Nov. 15 2013 02:11 AM
cathy from Portland, Oregon

I am a small micro-farmer and a sauerkraut, kimchi, pickle maker. I also have PTSD and anxiety from childhood issues. I've done a lot of traditional therapy, but *everything* began to ease up for me when I began making kimchi and pickles with what I grew. Now I produce them commercially and have interacted with many customers who are using these foods for mental health issues. I don't push the health benefits much because I mostly want to push the flavor aspects of the foods I'm making, but the health benefits are profound for many of my customers, and it's been very affirming to witness. I am not alone in feeling these health benefits, and SO MUCH MORE could happen for people like me if we could understand more of what's happening on a physical level.

Oct. 19 2013 10:25 PM
tina from s.c.

What kind of probiotics..will acidophilus and lactobacillus (cfu)help..

Oct. 11 2012 12:22 AM

Having had a colonoscopy recently I wondered how the micro flora and fauna get back and is it a different species group/distribution. Doctors never said you should eat live cultured yogurt or anything else

Jun. 28 2012 02:54 PM
austin bardsley from fog city, california

great show.. it has inspired me to look into the field of neuroscience. can you do a radio lab just about bacteria and probiotics? then one about neurology? thank you.

Jun. 24 2012 05:58 PM
JH from Davis, CA

Thank you to all Responders; my questions were answered from reading your comments!!!

Jun. 19 2012 09:01 PM

I've had stomach issues all my life, and medical doctors were always stumped by my condition, mis-diagnosing me with everything from IBS to being HIV positive (due to thrush). I finally remembered that an Italian pharmacist had once put me on a regiment of probiotics, made by Bayer, the same company that makes aspirin. Since Bayer doesn't distribute probiotics here, I started looking around and found all different kinds at Whole Foods. I started trying ALL of the different brands at higher than recommended dosages and finally found one that worked. I stayed on my self-prescribed dosage for four months and then decided to stop cold. My stomach hasn't had major problems since and whenever I can feel my stomach turning for the worst (for example, after stressful weeks at work), I just pop a few probiotic pills. I wonder if there have been any "mice studies" of administering probiotics to help mice recover from a stressful event ...

Jun. 17 2012 08:24 PM

Such a wonderful show -- I love the blend of humor and scientific information.
thank you so much for explaining the link between digestion and emotion and how food deprivation affects mood. It once again spelled out the impact of nutrition on human potential.

I loved the story about the deranged post-surgical patient who plunged his hands into a cake.

During a research trip to rural Uganda, my colleagues and I became so frustrated and depressed from eating low calorie meals of steamed plantains three times a day -- we began fantasizing aloud to each other about pizza, turkey dinners and burritos, whipping ourselves up into a communal state of gastric lust. (in the privacy of our residence -- not in front of our hosts of course.)

It made me realize how spoiled so many of us are in America in being able to chose the type of food we want to eat and gave me true compassion for the millions of people around the world who live with chronic malnutrition every day of their lives.

Jun. 16 2012 05:44 PM
Sylvie from Canada's nation capitat region

How neet !! I believe I have emotional problems because of those bacteria building up since my birth 50 years ago. I also need more exercise (any kind of exercise... not as much alcohol or other bad things to consume. So amazing how we understand more and more What are we going to find out in the future!!!!
What a beautiful world and we can actually understand it!! somewhat. So not only the brain controls our mood our stomach and digestive system as well. We are all dependant on one another weither you accept it or not... all living creatures.

May. 21 2012 07:28 PM
Ryan D. Per. 1 from MA

1)- About 100,000 species of single-celled organisms live in the human gut.
- 80% of all the serotonin in your body comes from your gut.
- Your mood can be affected by the bacteria in your gut.
- The combined weight of all the bacteria in your body is 3 pounds.
- Your intestines are 25 feet long.

2)It was interesting to me that the bacteria in our guts are able to affect our mood. I do not understand how these little tiny bacteria that are living in our gut can affect how we feel.

3)The fact I found most interesting is that there are 25 feet of intestine in everybody. I've heard this fact before but i am still unable to understand how something 25 feet long can bunch up to fit into our bodies.

May. 18 2012 08:58 PM
Sam B. Period 1

1. The five things that I would like someone else to know are:
-We have 25 feet of intestines inside of each of us.
-We have trillions of micro organisms inside of us.
-We each have 0 microorganisms before we are born.
-80% of the serotonin in your body is found in you stomach
-Bacteria have been proven to alter your mood.
2.The thing that surprised me the most is that the net weight of the microorganisms in your stomach is 3 pounds. That is the same weight as your brain!
3.The most interesting thing in this podcast for me was that when the mice were feed certain bacteria they behaved differently. It is really weird to think that the saying "you are what you eat" is much more true than I ever thought.

May. 18 2012 07:48 PM
Krista M. period 1 from My Home

1. 5 facts I would like someone else to know.
- According to a study people with a lower stress level have been given large doses of probiotics.
- When combining all the bacteria in your body it weighs around three pounds
- The intestines are 25 feet long!!! And when zooming into your intestines there are about 2,000 different species there.
- In the embryonic sac you are completely sterile but as soon as you are born you are not.
- Your stomach acid (gastric juice) can digest even when it is outside of your body. (due to chemicals/ENZYMES)

2. What's new and surprising to me
Something that was new and surprising to me was the amount of bacteria we come in contact with everyday. How by the time we are in school we have about 100 trillion other kind of cells already in us and that when gathering up all the bacteria in our bodies it would equal about three pounds. Also, something that was very surprising to me was that our brian and our stomach work together. It was surprising to hear about the mice experiment that was done and the changes that went on with these mice.

3. What's most/least interesting
Well, of course, this was a very interesting podcast. For some reason I like these a lot more than I think I will. One thing I found the most interesting was when people took many probiotics pill they got less stressed. It strikes up a lot of questions wondering how the stomach can effect the brian. I don't understand how the bacteria in our stomach can control our mood but it is very interesting to learn. What was least interesting was any part where the people started to ramble on too long on one subject.


May. 18 2012 05:09 PM
Peirce C. Period 1

5 Facts that I'd like someone else to know
1. There is 25 feet of intestines inside of you
2. Average person has about 2000 species inside them.
3. Bacteria can affect your mood
4. All bacteria in your body weights three pounds
5. 80% of all the serotonin in the body is your gut

Things that were new and surprising to me:
-Mice with extra bacteria in their guts did not get despaired and changed hormones in the mouses body. This means foods can affect the ways you act.
Most interesting thing to me:
-The man who had terrible bacteria infection through his gut. It was strange how he had to eat to make himself healthy after the doctors said the only way he would be healthy is to not eat.
Least interesting thing to me:
-When the guy described himself eating a burger in New York. I don't want to hear about him enjoying a burger.

May. 18 2012 05:02 PM
Erica M. period 1

5 interesting facts:

1)Trillions of microorganisms enter your body every day. It's not that you're doing something wrong or being unhealthy, and these microorganisms may be good or bad, but just by carrying out every day life these organisms can find their way inside of you.

2)80% of all serotonin in your body (the neurotransmitter that controls sleep, depression, and many other neurological processes) is stored inside of your guts along with the bacteria in it.

3)Said bacteria within your guts can affect your mood. Scientists did an experiment with mice to find out if bacteria could control their mood, and they concluded it could. It is the same situation for humans.

4)“Good” bacteria in your body are directly linked to psychological problems you may have, such as depression or anxiety. According to scientific research and data, people taking probiotics had lower stress levels than people who were not taking probiotics.

5)A human’s most important decision used to be about the food they ate, therefore that is why there is such a strong connection between the bacteria living in us and our mood.

The most surprising thing that I learned from this podcast was that all the bacteria in your body combined weighs three pounds. This was surprising to me because it is crazy to think of how much one of those teeny tiny organisms must weigh, and how many of them there must be in your body in order for it to add up to three pounds of microorganisms.

The most interesting thing about the podcast was definitely just learning about what goes on in your intestines and that it has an effect on your mood. I had never thought of me being stressed as a result of bacteria inside of my body. Although learning this was interesting, the least interesting part for me was having to think about all the bacteria that is currently inside of me while listening to this podcast. It was really freaky and abnormal to think about all the things that find a home in our guts.

May. 18 2012 03:17 PM
Annie R. period 1

The five things I would like someone else to know:
- You have 25 feet of coiled intestines inside of you.
- People have a couple thousand species of bacteria and single celled organisms inside of them.
- If you feed mice different types of bacteria they change their personality.
- Bacteria in your gut can effect your mood.
- The mice who ate the bacteria and were dropped in the water reacted differently and continued to look for a way out much longer then the other mice. Just because the mice ate the bacteria it effected there performance in the water.

1. It was surprising to me that there are ecologists that only study the types of singled cell organisms in our bodies. I did not know that we even had them living in us before. They called it a complicated pattern that scientists have not figured out yet. It surprised me that we have thousands of species inside of us.

2. I found it very interesting that a hundred trillion micro organisms enter your body just by living. It was very fascinating that by just going to school and and doing normal actives micro organisms can get into your body.

May. 17 2012 10:24 PM
Natalie H. Period 1

• When you combine all of the bacteria in your body, it weighs about three pounds

• There are about 2,000 species in human intestines.

• A group of people that were given large doses of probiotics noticed a decrease in their stress levels.

•80% of all the serotonin in your body is in your gut.

•The reason for the connection between our guts and our brain may be because a humans most important decisions used to be about the food they ate.

2) It was surprising to me that a bacteria given to mice can control the way they act in certain situations. This bacteria that is located in the guts can make a mouse more relaxed and less stressed.

3) I found it interesting that bacteria in our guts affect our mood.

May. 17 2012 08:10 PM
Margot D. Per. 1

5 facts that I’d like someone else to know:
-About 100,000 species of single-celled organisms live in the human gut (digestive system).
-Mice have a large nerve that connects there brain to their gut.
-When put in water, mice that consumed a certain type of bacteria didn't give up swimming as easily as mice not given the bacteria.
-The results of the mouse experiment (that bacteria can make mice calmer) also proved true for humans.
-People given large doses of probiotics had lower stress levels than people not given the probiotics, according to a study.

Things that were new and suprising to me:
-People have a connection between their guts and their brains that can affect stress levels.
-Taking probiotics in large amounts can reduce stress.

The most interesting thing about the podcast:
-The topic covered (what goes on in the gut)
-Also, the history of the gut (Beaumont's discoveries)was very interesting.

The least interesting thing about the podcast:
-Hearing about reaching into cows' guts while waiting for the segment I was meant to listen to come on

May. 13 2012 05:46 PM
Carol from Miami

I learned so much usefully information. I have started taking probiotic regularly. I had them for after taking antibiotics, but I have stress in my life and like the idea of taking something with no bad side effects.

May. 11 2012 10:37 AM

I actually started to have anxiety problem about 2 years ago, and I about the same time I took antibiotics but didn't really put 2 and 2 together. After the second course of antibiotics I took these probiotics called Udo's Choice Super 5 Probiotic Lozenges, which, as it happens, also has Lactobacillus. I had noticed that my anxiety dropped after having one of the lozenges, but I just figured it had something to do with my digestion getting better.

Well, up until hearing this episode, I hadn't taken them again, but after hearing it, I decided to take the lozenges again. I have been taking the lozenges for the last few weeks, and it is now like I'm cured of my anxiety problems. I had seen psychiatrists and psychologist, and nothing has help as much as these simple probiotics have. Thanks for running this story Radiolab.

Apr. 24 2012 05:24 AM

Great show. The timing for my tuning in could not have been more helpful particularly per the Gut Feelings segment. Had an emergency laparoscopic appendectomy about a month ago. Ten days after seemed fine. Then woke up vomiting. Could not and still have not been able to eat without nausea, pain, heaving....New c-scans showed everything clean...then I ended up in ER with pseudo seizures. Diagnosed with "conversion disorder" - what once was considered "hysteria" - apparently convulsions coming from high anxiety and some deep psychological issue...hmm....I have always been very sensitive to antibiotics. I think perhaps rebuilding a legion of warriors in my gut might be an important strategy to feeling better!

All very interesting and helpful.

GREAT show! Thank you!!!!

Apr. 23 2012 01:06 PM
Julie from Brooklyn, NY

I think this is the French study that included humans that DJBoyle was asking about.

Messaoudia M, et al. "Assessment of psychotropic-like properties of a probiotic formulation (Lactobacillus helveticus R0052 and Bifidobacterium longum R0175) in rats and human subjects." Brit J of Nutr. 14 March 2011 105 : pp 755-764.

Apr. 17 2012 05:00 PM

Does anyone have a citation for the French clinical trial mentioned in this segment?


Apr. 17 2012 12:27 PM
Anthony from Amherst, Massachusetts

Fascinating show as always.

@ John from Vienna, Moanin' by Charles Mingus

Apr. 16 2012 12:17 PM
John from Vienna, Austria

What is the song that is playing right after this piece at 33 minutes and 40 seconds? I love it! Also love the podcast, thanks guys!

Apr. 15 2012 10:14 AM
Warren Harrison from Cincinnati, OH

I was quite amazed that there is a potential link between these "good" bacteria and decreased anxiety. The idea that pro-biotics might be used to treat anxiety is quite fascinating and potentially life-changing. Great story!

Apr. 10 2012 11:04 AM
Kirsten Nelson

Very, very cool! Worked for a pediatric GI clinic for a while and also worked in the psychiatric/mental health field. Very interesting to see the link between the two.

Apr. 08 2012 01:30 PM
Yvonne from Atlanta, GA

I really enjoyed this podcast. I just wanted to make a quick note about the study with the mice. When, Carl described some of the results of the mice that had an ingested the bacteria and had a reduced stress response to being dropped in a pool of water. Carl stated that these mice had an increased amount of a chemical, GABA. Technically what the study found was an increase in the level of expresion of GABA receptors. Presumably these receptors are binding to the GABA neurotransmitter. Just thought I'd point that out. Overall, a great show.

Apr. 07 2012 11:17 AM

The original PNAS article can be found at:

There are three relevant letters which can be found at:

Apr. 05 2012 11:16 PM

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