The Way to Your Brain Is Through Your Stomach

Can what you eat actually change your mind? Spend any time in the mental health and wellness corners of social media, and the answer seems to be a resounding yes: influencers all over the internet preach the gospel of using probiotics and diet to treat everything from depression and anxiety to PTSD.

For once, this social media hype is actually echoed by many scientists: In the past decade, researchers have begun teasing out exactly how the microbes in our guts affect our brain chemistry. On the latest episode of Gastropod, “Gut Feeling,” co-hosts Cynthia Graber and Nicola Twilley tackle the science and history behind this exciting new field of medicine.

One of its leading lights is John Cryan at the University of Cork, and, in 2011, he and his colleagues coined a new word to describe this new science: psychobiotics. “A psychobiotic is an intervention that targets the microbiome for mental health benefit,” he told Gastropod; that encompasses probiotics, or the kind of beneficial bacteria you might find in your yogurt, as well as prebiotics, which are foods like leafy greens and whole grains that the beneficial bacteria already living in your gut like to eat.

Curiously, although this kind of food-based mental health therapy seems brand new — in the early 2000s, the idea that gut bacteria might play a role in psychological issues was a pretty radical and speculative suggestion — the idea behind it isn’t at all. In fact, in the late 1700s and 1800s, medical historian Ian Miller says that most doctors thought the stomach was “the great nervous center of the body, or great sensorium” — in other words, that the stomach was more important than every other organ, including your brain, in determining how you felt. But it wasn’t until the 1840s, when scientists realized that bacteria (then still a relatively new discovery) could be found in the human gut that they also started to look into why things that affected digestion also might affect the mind.

No sooner had they been discovered than these microscopic gut microbiota started taking the blame for a variety of mental and physical ills. In the late 1800s, a handful of doctors developed a theory that the consumption of certain foods and their subsequent breakdown by microbes in the gut could create toxic substances that, if absorbed, could induce melancholia, stress, and general mental deterioration, among other disastrous consequences. They called this condition “autointoxication,” and, for a while, it was all the rage.

In 1914, the fabulously named Dr. Bond Stow, a pathologist from New York, framed the era’s focus on autointoxication as a “battle royale,” writing that doctors must “strike at the root of the trouble by dispelling a malevolent intestinal flora with a beneficent one.” A few years earlier, in 1909, British doctor Hubert Norman reported that soured milk had proven an effective cure for “melancholia” in one of his patients. Around the same time at the Bethlem Royal Hospital (better known by the term it inspired, Bedlam), doctor George Porter Phillips alleged he could treat mental illness with a sort of prescription kefir, made up of a microbe commonly found in yogurt, Lactobacillus bulgaricus, combined with whey. Phillips’ drink has been called the world’s first psychobiotic.

So far, these treatments had ranged from being harmless to even scientifically promising by modern standards. But things took a turn in the 1920s. A handful of doctors, including an American, Henry Cotton, began promoting the idea of “surgical bacteriology.” Cotton began removing all or part of the colons of patients in the New Jersey State Lunatic Asylum — the most direct way, he alleged, to get rid of all those bad gut bacteria. (He also recommended removing all of patients’ teeth, to ensure any oral bacteria were eliminated, too.) Cotton claimed that 80 percent of patients thus treated were cured of their mental illness, yet the procedure had a staggering 30 percent mortality rate from the surgery alone — and, of course, those who were “cured” then faced the significant challenge of managing without a major part of their digestive system.

However, just as Cotton and his cohort were wielding their scalpels in an attempt to cure mental illness through gut modification, new theories were emerging that signaled the end for this early psychobiotic enthusiasm. By the 1930s, Sigmund Freud’s theories of psychoanalytics and his emphasis on talk therapy — so much less messy than colon surgery! — had become the new mental health orthodoxy. Around the same time, scientists were teasing out the chemicals that brain cells — neurons — use to signal to each other within the brain. In 1951, the very first neurochemical antidepressant, iproniazid, was discovered by accident among some very happy tuberculosis patients. Drugs like Prozac, Paxil, and Zoloft, which all seem to work by tweaking the levels of different neurotransmitters in the brain, soon followed.

By the end of the 20th century, the standard for treating mental health disorders consisted of various flavors of talk therapy, often administered in combination with drugs that targeted brain chemistry. Guts were no longer part of the conversation. That is, until today.

So, were those early pioneers onto something? Can you really improve your mental health by tweaking the bacterial inhabitants of your gut? What do scientists know works — and what are they still figuring out? Follow and subscribe to Gastropod for the scoop on the science behind the hype — it’s news we can all use, combined with the story of some sad mice, stressed students, and a brand-new U.S. military experiment.