How a Healthy Gut May Be the Key for Reducing Stress and Anxiety
Should you take probiotics for mood issues? Scientists have discovered a link between the bacteria that populate your gut microbiome and mental health. Here's what to know—and what to feed those bugs.
At this very moment, your body is playing host to trillions of bacteria. You are, in fact, just as much microbe as you are human—carrying around roughly the same number of bacterial cells as human ones, with nearly all of the bacteria living in your gastrointestinal (GI) tract.
This microbial menagerie is called, of course, the microbiome. You pick up the bacteria that colonize it early in life (through everything from traveling through the birth canal and being breastfed to not following the five-second rule) and by about age 5 it has fully populated itself. And you've probably heard that a diverse mix of the right bacteria results in good overall health. Researchers have found connections between gut microbes and conditions ranging from allergies and heart disease to obesity and type 2 diabetes. More recently, however, scientists have identified some fascinating links between the gut microbiome and mental well-being.
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We already know that feeling anxious affects our bellies. It makes us run to the bathroom, makes us queasy and generally makes our insides do flips like an Olympic gymnast. In fact, the gut has its own nervous system—the enteric nervous system—and it contains as many nerve cells as the spinal cord. This nervous system of the GI tract is directly connected to the brain through the vagus nerve, which acts as a sort of information superhighway.
"The vagus nerve connects the two biggest systems of our body," says Emeran A. Mayer, M.D., Ph.D., executive director of the Oppenheimer Center for Neurobiology of Stress and Resilience at UCLA, and one of the pioneers of gut-brain research. "When you feel stressed, that's your mental state affecting your gut. The stress hormones your body secretes impact the microbes and change the way they function."
While it may seem intuitive that anxiety and depression can influence the gut, the surprising thing is that the reverse is also true: the bugs in our intestinal tract also affect our mood. It turns out that our gut bacteria—including strains you may have seen on your yogurt container or supplement label, like Lactobacillus acidophilus—secrete and modulate a huge number of compounds that send signals to the brain. In other words, that superhighway runs in both directions.
Most of the neurotransmitters that regulate our mood—including 50% of the dopamine and 95% of the serotonin in our bodies—are produced by microbes in the intestine, where they also influence appetite and feelings of fullness and digestion. Mayer says that the GI tract and brain are so intimately connected that they should actually be viewed as one system: "The gut has been called your second brain."
The Links between Bacteria and Mood
There's evidence, for example, that people who suffer from GI disorders, like irritable bowel syndrome, also have higher rates of depression or anxiety. And researchers have noticed that certain species of bacteria (ones that seem to be able to make us melancholy) are more likely to be found in the guts of depressed patients, while those linked to better mood are lacking. In one study, formerly carefree rodents inoculated with bacteria from depressed people developed signs of the condition.
And recent research published in the journal Nature Microbiology that looked at 2,100 adults with and without depression found that two specific types of gut bacteria (Coprococcus and Dialister) were depleted in people with the disorder, even after controlling for the effects of antidepressants. However, participants with high levels of Coprococcus and another bacteria called Faecalibacterium reported greater feelings of happiness. It's not yet known whether the specific type and balance of bugs are causing the depression or if people with the condition just have a different composition of microbes, but there is a clear link between the gut and mental health.
Also clear: this connection impacts us all. A growing body of research suggests that a well-balanced microbiome can improve mood even if you don't have a clinical case of depression or anxiety—helping to ease everyday stresses and keep the blues at bay. One study of healthy adults published in the journal Brain, Behavior, and Immunity found that a positive shift in gut bacteria populations resulted in significantly fewer reports of sad mood and negative thoughts.
Researchers are continuing to tease out the good and bad guys of the microbe world. (There's a lot more to learn; in 2019 alone, scientists discovered more than 100 new species of gut bacteria.) But there is a consensus that what you eat has a major role in determining the diversity and health of your gut bacteria.
The two main players are probiotics and prebiotics: Probiotic foods help populate your microbiome with beneficial bacteria, while prebiotics feed the good bugs you already have in your gut. Together, they give you a healthier overall balance of microbes. And—equally important—they help crowd out the ones you don't want, so getting a regular dose of both is key to a happy microbiome. Research shows that changes in your diet can produce shifts in gut bacteria within just 24 hours. You need to maintain those eating habits, however, for the shift to last.
Pictured Recipe: Homemade Kimchi
The Pros of Probiotics
So what, exactly, does a gut-friendly diet look like? Many fermented foods, such as sauerkraut, yogurt, kombucha and kimchi, naturally contain probiotics. Look for terms like "live and active cultures" or "unpasteurized" on food labels to ensure the product has live bacteria—not all do. Members of the Lactobacillus and Bifidobacterium groups—commonly found in yogurt and kefir—produce GABA, a neurotransmitter that research has shown can reduce anxiety. (The most common anti-anxiety drugs, like Valium and Xanax, work by targeting receptors that increase GABA levels.)
In one study, Mayer and his colleagues at UCLA gave healthy women yogurt twice a day for a month, then conducted brain scans as the women were shown pictures of actors with frightened or angry facial expressions. Normally, such images would trigger increased activity in emotion-processing areas of the brain linked to a state of heightened alert. (Back when we lived in caves, this fight-or-flight response was advantageous for escaping mastodons or wolves. Today it just makes us feel on edge.) But the women on the yogurt diet exhibited a calmer response. In another study, men who consumed probiotics for a month reported feeling less stress than when they were given a placebo. They also had lower levels of the stress hormone cortisol.
Neuroscientist John Cryan, Ph.D., chair of the department of anatomy and neuroscience at University College Cork in Ireland, and his team have produced some of the best-known work linking gut microbes and mental health. In a series of stress tests with mice, they found that a broth infused with Lactobacillus rhamnosus had the same effect as Prozac in creating relaxed animals. Cryan hopes that one day research like this will lead to what he calls psychobiotics—drugs made from live bacteria that can improve mental health.
What About Probiotic Supplements?
You can also buy supplements, as well as all kinds of foods—cereals, fruit juices, sausage, even pet food—that tout being fortified with probiotics. But because probiotics are categorized by the Food and Drug Administration as supplements (so they're not regulated like drugs), there's no assurance of the effectiveness, safety or quality of a product, which means what you see may not be what you get.
A study led by researchers at the University of California, Davis, for example, evaluated 16 probiotic products and found that only one matched its label claims. Nine samples contained a strain not listed on the label, 12 were missing strains they claimed to have, and four had unidentified species. Also worth noting: there's no way to know whether the strains a product does have are ones that you are lacking—and need—or if they'll be effective for the condition you're taking them for. So even if a product's label is spot-on, it still may not do you a whole lot of good.
What's more, scientists at the University of Copenhagen published a 2017 review of seven randomized, placebo-controlled trials (the most rigorous type of research) to see whether probiotics, including capsules and fortified foods, changed the diversity of people's bacteria. Only one study found a statistically significant shift.
Stanford microbiologist Erica Sonnenburg, Ph.D., who co-wrote The Good Gut with her fellow microbiologist husband, Justin, notes that there are very few well-researched bacterial strains approved for sale. Plus, our GI tract is host to hundreds of bacterial species and thousands of strains; just 20 or so strains are available in supplements. The most common include strains of Lactobacillus and Bifidobacterium—in part because those are also the most studied and have shown a link to reduced incidence of anxiety, depression and other mood disorders. And they are mostly the ones you'd find in yogurt and kefir. The Sonnenburgs prefer to dose themselves with fermented foods rather than a pill.
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Prebiotics: Feeding Your Flora
Your gut bugs, like all creatures, need to eat to survive. And what the good guys crave most are prebiotics—a variety of fibers, namely oligosaccharides and cellulose. Mayer notes that prebiotics may be even more important than probiotics for microbiome health. You'll find them in everything from asparagus to yams. But the best sources of oligosaccharides are Jerusalem artichokes, chicory root (which is often ground and made into a coffee substitute), leeks, onions, raspberries, artichokes, beans and asparagus. Cellulose-rich foods include celery, broccoli stalks and unpeeled carrots.
The Sonnenburgs suggest aiming for at least 33 to 39 grams of dietary fiber a day though foods like these. (The standard recommendation is around 28 grams.) In fact, there's evidence that people benefit more from probiotics if they get them in conjunction with prebiotics, although experts aren't sure exactly why.
Prebiotics are also available as supplements. A study published in the journal Psychopharmacology found that people who took a supplement containing 5.5 grams of the prebiotic galacto-oligosaccharide had lower levels of the stress hormone cortisol—and psychological tests showed that they focused more on positive information and less on the negative stuff. The results were similar to the effects of antidepressant or anti-anxiety medications. Another study on mice using the same prebiotic found that it increased amounts of Bifidobacteria and Lactobacilli, which, in turn, upped levels of several neurotransmitters that decrease anxiety.
Protect Your Gut
It's also important to minimize your intake of foods that are known to have a negative impact on good gut bacteria. Added sugar (namely fructose and glucose) halts the production of proteins that foster the growth of those bugs, according to recent research. And a 2017 British study found that men who ate 67-plus grams of added sugar a day were more likely to develop anxiety or depression compared to those who ate less than 40 grams a day (still a lot!). A diet high in salt may cause changes in your microbiome, as well. In mice, it decreased levels of microbes that provide anti-inflammatory protection and have been linked to reduced anxiety and depression.
No one is suggesting that anyone throw out their antidepressants just yet. But pairing them with probiotics and prebiotics can't hurt. "And focus on a plant-based diet that is as diverse as possible," says Mayer. So mix it up: get your fix of fiber-rich beans, fermented pickles, low-sugar yogurt and loads of bug-friendly veggies and fruits. The wider the variety of healthy foods you eat, the greater the diversity of bacteria you'll have. And that could mean a happier microbiome—and a happier you.
Changes in the microbiome—courtesy of the superhighway of chemicals that runs between the gut and brain—appear to also play a role in a variety of neurological diseases. The research is preliminary, but offers hints that influencing the community of microbes in the gut could improve the following conditions.
This condition is mainly a disorder of the brain that affects social behavior and communication, but research suggests that many with the condition also suffer from gastrointestinal problems, such as inflammatory bowel disease. The gut microbes of children with autism typically have fewer kinds of intestinal bacteria than those without the condition. A 2019 study from Baylor College of Medicine found that introducing Lactobacillus reuteri into mice with autistic symptoms—such as repetitive behavior and an unwillingness to communicate and socialize (for mice that would be ultrasonic squeaking)—made those symptoms disappear. Interestingly, L. reuteri produces the hormone oxytocin, which plays a part in how mammals form social bonds and promotes social interactions.
The Baylor researchers also discovered that the offspring of mouse mothers that were fed a high-fat diet during pregnancy exhibited autistic characteristics. The baby mice were found to have no L. reuteri populations in their guts, but giving them supplements reversed their symptoms. Another study found similar effects—though not as profound—with Bacteroides fragilis administration. The researchers realize that mice studies are no guarantee that the same will hold true for humans, but note that it's nonetheless a promising way of affecting the brain through the gut.
Mounting research (also in mice) suggests microbiome composition may also influence the onset of Alzheimer's. An international team of researchers, for example, looked at both healthy and diseased rodents and found that those with Alzheimer's had a different composition of gut bacteria. And when bacteria-free mice were colonized with microbes from rodents with Alzheimer's, they began developing beta-amyloid brain plaques—a hallmark of the disease. The reverse also held true: bacteria-free mice populated with gut bacteria from healthy rodents showed significantly fewer brain plaques. Researchers are now looking at ways of using intestinal flora to prevent Alzheimer's and alleviate symptoms in those who already have it.
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Most patients with this neurodegenerative disorder experience constipation, nausea and other GI troubles. And these symptoms may occur years before the onset of motor (movement) issues and before a diagnosis. Some research suggests that the disease may start in the gut, with motor symptoms being a late development. When California Institute of Technology researchers essentially sterilized the intestines of mice with Parkinson's to make them microbe-free, motor symptoms eased. And after repopulating the rodents' microbiomes with gut bacteria from human Parkinson's patients, symptoms returned.
The lead researcher, microbiologist Sarkis Mazmanian, Ph.D., notes that for many neurological conditions, the conventional treatment approach is to get a drug into the brain. "However, if Parkinson's disease is not solely caused by changes in the brain, but instead by changes in the microbiome, then you may just have to get drugs into the gut to help patients, which is much easier to do," he says. "This new concept may lead to safer therapies with fewer side effects." In fact, Mazmanian has identified a single strain of gut bacteria that triggers symptoms of Parkinson's in mice. He recently started a company that's testing a compound to block the effects of this microbe.
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