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Michael Greger MD

Michael Greger MD

Posted January 7, 2014

Published in Health

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How Probiotics Affect Mental Health

Read More: anger, anxiety, bacteria, bowel movements, colectomy, colon, constipation, depression, emotional health, enteric nervous system, enteric neuroscience, fecal transplant, fibromyalgia, gut dysfunction, gut flora, intestinal flora, intestinal putrefaction, lobotomy, mastectomy, mental health, mental illness, mood, plant-based diet, probiotics, psychiatric illness, Thorazine, vegan, vegetarian

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 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 was the cause of mental illness. So the cure was to 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 perform 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 a “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 check out my 5-min video Gut Feelings: Probiotics and Mental Health, 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 sufferers 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? A study entitled “Assessment of the Psychotropic Properties of Probiotics” marked a turning point in 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 maybe not fiction. 

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.

Gut Feelings: Probiotics and Mental Health 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).

-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: Ana_Cotta / Flickr


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You should be eating a diet of clean organic veggies and probiotics should not be needed.

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