Torturers were long aware that our digestive track is loaded with nerves, and I recall talking to one American solider captured by the Germans during World War II who was subjected to ice water enemas in order to get him to talk. He said he’d never experienced anything so painful before or since, including burns.
Beyond the direct link the SS torturers employed, there’s another, more subtle link between gut and brain, namely the role the microbes harbor in our viscera play in shaping emotions, extroversion and introversion, and, quite possibly, the development of addictions, obsessive behaviors, and even autism and Parkinson’s disease.
From a 5 September 2013 New York Times report:
The trillions of bacteria that live in the gut — helping digest foods, making some vitamins, making amino acids — may help determine if a person is fat or thin.
The evidence is from a novel experiment involving mice and humans that is part of a growing fascination with gut bacteria and their role in health and diseases like irritable bowel syndrome and Crohn’s disease. In this case, the focus was on obesity. Researchers found pairs of human twins in which one was obese and the other lean. They transferred gut bacteria from these twins into mice and watched what happened. The mice with bacteria from fat twins grew fat; those that got bacteria from lean twins stayed lean.
The study, published online Thursday by the journal Science, is “pretty striking,” said Dr. Jeffrey S. Flier, an obesity researcher and the dean of the Harvard Medical School, who was not involved with the study. “It’s a very powerful set of experiments.”
The enteric nervous system harbors more neurons than the spinal cord, and is constantly signalling the brain.
And, it turns out, the nature of those signals is shaped by the billions of critters that dwell within us, the microbes that play vital roles in the digestion of out food.
And, it turns out, they may also make us happy or sad.
Consider the following from a 23 June 2015 New York Times report:
Since 2007, when scientists announced plans for a Human Microbiome Project to catalog the micro-organisms living in our body, the profound appreciation for the influence of such organisms has grown rapidly with each passing year. Bacteria in the gut produce vitamins and break down our food; their presence or absence has been linked to obesity, inflammatory bowel disease and the toxic side effects of prescription drugs. Biologists now believe that much of what makes us human depends on microbial activity. The two million unique bacterial genes found in each human microbiome can make the 23,000 genes in our cells seem paltry, almost negligible, by comparison. ‘‘It has enormous implications for the sense of self,’’ Tom Insel, the director of the National Institute of Mental Health, told me. ‘‘We are, at least from the standpoint of DNA, more microbial than human. That’s a phenomenal insight and one that we have to take seriously when we think about human development.’’
Given the extent to which bacteria are now understood to influence human physiology, it is hardly surprising that scientists have turned their attention to how bacteria might affect the brain. Micro-organisms in our gut secrete a profound number of chemicals, and researchers like Lyte have found that among those chemicals are the same substances used by our neurons to communicate and regulate mood, like dopamine, serotonin and gamma-aminobutyric acid (GABA). These, in turn, appear to play a function in intestinal disorders, which coincide with high levels of major depression and anxiety. Last year, for example, a group in Norway examined feces from 55 people and found certain bacteria were more likely to be associated with depressive patients.
So critical is the nervous system in our guts that many scientists now call it our “second brain.”
From a 12 February 2010 Scientific American report
“The system is way too complicated to have evolved only to make sure things move out of your colon,” says Emeran Mayer, professor of physiology, psychiatry and biobehavioral sciences at the David Geffen School of Medicine at the University of California, Los Angeles (U.C.L.A.). For example, scientists were shocked to learn that about 90 percent of the fibers in the primary visceral nerve, the vagus, carry information from the gut to the brain and not the other way around. “Some of that info is decidedly unpleasant,” Gershon says.
The second brain informs our state of mind in other more obscure ways, as well. “A big part of our emotions are probably influenced by the nerves in our gut,” Mayer says. Butterflies in the stomach—signaling in the gut as part of our physiological stress response, Gershon says—is but one example. Although gastrointestinal (GI) turmoil can sour one’s moods, everyday emotional well-being may rely on messages from the brain below to the brain above. For example, electrical stimulation of the vagus nerve—a useful treatment for depression—may mimic these signals, Gershon says.
Given the two brains’ commonalities, other depression treatments that target the mind can unintentionally impact the gut. The enteric nervous system uses more than 30 neurotransmitters, just like the brain, and in fact 95 percent of the body’s serotonin is found in the bowels. Because antidepressant medications called selective serotonin reuptake inhibitors (SSRIs) increase serotonin levels, it’s little wonder that meds meant to cause chemical changes in the mind often provoke GI issues as a side effect. Irritable bowel syndrome—which afflicts more than two million Americans—also arises in part from too much serotonin in our entrails, and could perhaps be regarded as a “mental illness” of the second brain.
And with that by way of preface, here’s a talk given by Dr. Emeran at UC San Francisco Medical School in December and just posted to the Web by University of California Television [and yes, it’s a bit on the technical side, but there’s enough there that’s accessible to a lay viewer that we though it worthwhile to post]:
The Microbiome Mind and Brain Interactions
Dr. Emeran Mayer, an expert on the clinical and neurobiological aspects of the gut-brain axis, is a Professor in the Department of Medicine, Physiology and Psychiatry at the David Geffen School of Medicine at UCLA. He is also the Executive Director of the Oppenheimer Family Center for Neurobiology of Stress, and Co-director of the CURE: Digestive Diseases Research Center. Recorded on 12/10/2015.