Have you ever been so stressed that your stomach hurt? Maybe you have been so worried or anxious that you vomited or had diarrhea. If so, you know firsthand the connection between the mind and the gut. What happens in one is inextricably linked to what happens in the other. Even the nervous butterfly feeling you sometimes get is related to gut microbiota activity.[1]
The term gut microbiota refers to the staggering 100 trillion microorganisms living in your gastrointestinal tract, both the good and the bad.[2] Good bacteria kill disease-producing cells, synthesize vitamins, affect hormone activities, and help break down food. But the number and diversity of these good bacteria in your gut are subject to many factors, and they can be negatively affected by stress, anxiety, and depression.[3] Researchers have known about this link for a long time.[4] But does it work the other way around? Can the state of your gut affect your mental health?
In a study published by the British Journal of Nutrition in 2011, subjects reported a decrease in anxiety and depression after only one month of treatment with probiotics—live organisms that promote the growth of good bacteria in the GI tract.[5] As the authors summarized, “These results provide further evidence that gut microflora play a role in stress, anxiety, and depression [. . .] Probiotics might offer a useful novel therapeutic approach to neuropathological disorders and/or as adjunct therapies in psychiatric disorders.”
It might surprise you to discover this thinking is not new. As discussed in a well-referenced historical analysis published in 2018, the so-called autointoxication theory generated much interest in the late 1800s.[6] This theory was promoted in France by Charles Bouchard, a physician who, in Lectures on Autointoxication in Disease (1887), articulated a vague notion about “lower organisms” in the digestive tract being the cause of numerous diseases. While the sophistication of this theory was very basic compared to our modern understanding of the gut-brain axis, the interest in autointoxication, which was at its greatest in France, Germany, and the US, does have many parallels with the present-day interest in gut health.
Unfortunately, “serious research on the topic was short-lived.” Because it was blamed for such a wide range of diseases, autointoxication was easily co-opted by many charlatans. This is a story as old as time:
Alongside the legitimate scientific interests in the effects of intestinal bacteria on health, alternative practitioners and charlatans were alert to the financial possibilities offered by the idea that cleaning out the colon could instantly improve wellbeing. Opportunistic entrepreneurs appropriated the theory in order to sell dubious therapies based on unfounded claims. Charles A Tyrrell’s syringe enema, ‘the Cascade’, for example, purported to cure a host of maladies, all of which Tyrrell attributed to so-called intestinal poisoning.
The autointoxication theory also became associated with a few physicians—most notably, Sir William Arbuthnot Lane, Henry Cotton, and John Draper—who performed radical colectomies, removing the colons of patients. These operations not only were risky and potentially fatal but also cast a negative light on the more plausible underlying ideas of autointoxication, such as the idea that there might be bacteria in the gut affecting health outcomes.
The interest in intestinal bacteria waned alongside these radical operations and the theory’s susceptibility to quackery. By the 1930s, the reputation of autointoxication was at rock bottom. It took a long time for the link between gut health and other aspects of physical well-being, including mental health, to recover a place of respect and prominence in the scientific and medical community.
A 2019 review focused on the gut-brain axis—the partnership between the central nervous system (the brain and spinal cord) and the enteric nervous system (the neurons and cells throughout the GI tract)—describes how the “‘sixth sense’ originating from visceral organs [. . . may] play an important role in regulation of the stress-related neural networks.”[7] As in the 2011 paper cited above, this article concludes with a note on the potential role of probiotics in treating mental health disorders and diseases (more on probiotics shortly).
Here is how a 2016 article describes the enteric nervous system:[8]
[It] is large, complex, and uniquely able to orchestrate gastrointestinal behaviour independently of the central nervous system (CNS). An intact ENS is essential for life and ENS dysfunction is often linked to digestive disorders. The part the ENS plays in neurological disorders, as a portal or participant, has also become increasingly evident.
The enteric nervous system is also often referred to as the second brain. It contains 400–600 million neurons, making it “the largest and most complex unit of the peripheral nervous system.”[9] It can communicate with the central nervous system but also function independently.
We should not underestimate the complexity of these systems and their interconnections. Without getting too far into the weeds, the major takeaway is that communication between the gut and the rest of the body goes in both directions and relies on a multitude of mechanisms.[10] It is this complexity that makes it possible for bacteria in the GI tract to modulate the release of cortisol, the primary stress hormone, and that allows an estimated 90 percent of the mood-regulating neurotransmitter serotonin to be made in the digestive tract.[10][11] But these are just two of countless examples.
After reading all this, you might feel tempted to rush to the nearest store to stock up on probiotic supplements. You would not be alone. The probiotic supplement industry is already sizeable, and forecasts suggest it will only continue growing.[12] This demand has been written about previously on the CNS website: “The global market size for digestive health products is forecasted to increase from 51.7 billion in 2022 to 98.4 billion by the end of the decade.” (Learn more.)
However, the evidence supporting the use of probiotic supplements is mixed at best. Because there are so many different products on the market featuring so many different organisms, we could not make a blanket statement even if certain products appeared beneficial. Some bacteria have been studied and others not. The quality of those studies varies. And because probiotics are classed as dietary supplements, not drugs, they are not regulated by the FDA, raising many potential concerns about quality control; one common concern is that “lower-quality products may not even contain the probiotic bacteria that are listed on the label.”[13]
For all these reasons and more, we recommend the easier, cheaper, and more wholistic alternative of eating for a healthier gut. Fermented foods like tempeh and kimchi naturally contain probiotics, and you can get prebiotics from consuming a whole food, plant-based (WFPB) diet. Prebiotics are not the live bacteria themselves; rather, they are what the bacteria in your gut feed on. The best way to ensure prebiotics is to consume adequate amounts of dietary fiber, the non-digestible part of plant foods. (Learn more about the importance of fiber; learn about the impact of high-fat diets on gut health.)
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