How Probiotics Affect Mental Health

Before Thorazine was invented in 1950, mental illness was often treated surgically. In fact, in 1949 the inventor of the lobotomy was awarded the Nobel Prize. Before tens of thousands were lobotomized, however, colectomy was all the rage. There was a theory that bad bacteria in the gut­—”intestinal putrefaction”—was the cause of mental illness. So the cure was to just surgically remove the colon. Yes, the surgery killed about one in three–but when they didn’t die, surgeons claimed positive results. Some, for example, bragged that when they resected the colons of schoolchildren as a preventive measure there was a cessation of “abnormal sex practices” like masturbation (which was viewed at the time as a precursor for mental illness later in life). Reminds me of the mastectomies they used to do for menstrual breast pain (Plant-Based Diets For Breast Pain).

Others, though, suggested a less drastic approach, proposing that one could instead treat this intestinal putrefaction by changing the intestinal flora. Indeed, over a century ago there were reports of successfully treating psychiatric illnesses like depression with a dietary regimen that included probiotics. Doctors perceived a connection between depression and “feces deficient in quantity and moisture and very offensive in odor.” Reportedly, after the probiotic regimen not only did people feel better psychologically, but they had their “feces increase in quantity, become softer, and of regular consistency, and the offensive smell diminish….” Concurrent with the probiotics, however, all patients were started on a vegetarian diet—so it may not have been the probiotics at all.

Why might the vegetarian diet alone have improved mood? Check out my videos Plant-Based Diet & Mood and the follow-up Improving Mood Through Diet as well as my serotonin series starting with Human Neurotransmitters in Plants.

This entire field of inquiry remained dormant for about a hundred years, but a new discipline has recently emerged known as enteric (meaning intestinal) neuroscience. Our enteric nervous system—the collection of nerves in our gut—has been referred to as our “second brain” given its size, complexity, and similarity. We have as many nerves in our gut as we do in our spinal cord! The size and complexity of our gut brain is not surprising when considering the challenges posed by the interface. We have a hundred times more contact with the outside world through our gut than through our skin. We also have to deal with our 100 trillion little friends down there. That takes a lot of processing power.

Anyone who’s had butterflies in their stomach knows that our mental state can affect our gut. Studies show that every day stresses can actually affect gut flora populations. An innovative study out of Australia looked at feces scraped from toilet paper used by undergrads during exam week. If you click the above video, you can see how many bacteria the undergrads had in their feces before and after the exam. You’ll notice the effect lasted the whole week. Their findings show that our mental state can indeed affect our gut, but can our gut affect our mental state? We didn’t know until recently.

Many suffering from chronic fatigue syndrome complain of gut dysfunction, so researchers tried giving people probiotics to see if their mental and emotional state could be improved and it did appear to help. You can learn more about treating chronic fatigue syndrome in:

What about for healthy people, though? The study entitled “Assessment of the Psychotropic Properties of Probiotics” was the one that most altered our thinking. Researchers found that one month of probiotics appeared to significantly decrease symptoms of anxiety, depression, anger and hostility. Until that study was published, the idea that probiotic bacteria administered to the intestine could influence the brain seemed almost surreal–like science-fiction. Well, science yes, but apparently fiction? No. So might people suffering from certain forms of mental health problems benefit from a fecal transplant from someone with more happy-go-lucky bacteria? We don’t know, but this apparent ability of probiotics to affect brain processes is one of the most exciting recent developments in probiotic research.

The video above closes out my 4-part video series on the latest in probiotic science. I started with the two most established indications for their use in Preventing and Treating Diarrhea with Probiotics, then moved onto a more speculative use in Preventing the Common Cold with Probiotics?, and then offered practical advice on how to best take probiotic supplements in  Should Probiotics Be Taken Before, During, or After Meals?

What else might our good bacteria be doing for us? They may help with weight control (Fawning Over Flora and Gut Flora & Obesity) and serve up anti-cancer compounds! (Flax and Fecal Flora and Sometimes the Enzyme Myth Is True.)

In health,
Michael Greger, M.D.

PS: If you haven’t yet, you can subscribe to my free videos here and watch my live year-in-review presentations Uprooting the Leading Causes of Death and More Than an Apple a Day.

Image credit: KayVee.INCPčre Ubu and via Flickr and

Improving Mood Through Diet
Natural Treatment for Chronic Fatigue Syndrome
Saffron vs. Prozac for Depression


Karen Everton
Karen E3 years ago

interesting, thank you

Mike W.
Mike Wilson4 years ago

Thanks for all the information.Being a vegetarian is what we humans beings were always suppose to be,but somewhere down the line we adapted to the non vegetarian diet,forgetting its disadvantages. I really do feel, it is better to take precautions than look for care as the interconnected systems of our body are regulated by what we eat,as food is the fuel on which our body runs.

Ro H.
Ro H4 years ago


Joy S.
Joy s4 years ago

I'll take being a vegetarian over a lobotomy every time!

Val M.
Val M4 years ago


Jeanne Rogers
Jeanne R4 years ago


Lin M
Lin M4 years ago

B etter than other treatments for sure.

sarah s.
sarah s4 years ago


Sue Holtz
Sue H4 years ago


Fi T.
Past Member 4 years ago

One's mind and body are interconnected