You’ve probably had “a gut feeling.” Or maybe you’ve felt like you felt “butterflies in your stomach.” These feelings may be more than figures of speech. Science says that your gut – or, to be exact, the trillions of bacteria that call it home – can also influence your brain.
Scientists and the public alike are fascinated by how these tiny bugs can have a major impact on our health. Research has found associations between gut bacteria, known collectively as the gut microbiome, and conditions ranging from allergies to heart disease. Even more intriguingly, these bacteria can also affect mental health. Studies have linked them to depression, anxiety and even Alzheimer’s disease.
What could explain this gut-brain connection? As it turns out, the brain and gut microbiome constantly “talk” to each other. The brain can alter the composition of gut bacteria by triggering changes in the digestive and immune systems. In turn, gut bacteria secrete neurotransmitters, immune molecules, and other chemical messengers that affect brain function. Much of this communication happens through the fibers of the vagus nerve, the “freeway” linking the gut and the brain.
Studies suggest that disrupting the delicate balance of the gut’s bacterial ecosystem – such as through diet or antibiotics (which can wipe out the good bacteria along with the bad) – could have consequences for mental health. One hypothesis is that these bacteria regulate the gut lining. Disrupting the gut microbiome may allow harmful substances to leak into the blood and reach the brain, linking the gut with certain psychiatric disorders.
Read on to learn more about how your gut can affect your brain – and steps you can take to keep both healthy.
What the Science Says
Research suggests that the gut microbiome could shape several aspects of brain health, including:
Mood. A 2016 study suggests that the makeup of our gut microbial ecosystem might shape our day-to-day moods. Three adult participants provided stool samples and took mood questionnaires every two weeks for 105 days. Researchers then correlated certain types of bacteria in their stool with positive or negative moods. Since it’s such a small study, though, more research is needed to establish a link.
Cravings. Are you scarfing down those cookies because you want to – or because your gut bacteria do? Research suggests our gut bacteria can influence our eating choices, triggering cravings for foods containing nutrients they need to survive. “Chocolate desiring” people, for instance, have different bacterial byproducts in their urine than those who are “chocolate indifferent,” suggesting they have different gut microbiomes.
Animal studies have begun teasing apart how this happens, suggesting that gut bacteria release signaling molecules that affect our appetite and even alter our taste receptors. In one study published earlier this year, researchers found that fruit flies deprived of essential amino acids – the building blocks of proteins that the flies can’t make on their own – showed a strong hankering for protein-filled yeast. But after researchers increased certain bacteria that live in the flies’ digestive tract, their cravings disappeared.
Stress and anxiety. One line of evidence of the gut microbiome’s role in stress and anxiety comes from a squirm-inducing procedure in which researchers transfer bacteria from the fecal matter (aka poop) of anxious mice to calm mice. When McMaster University researchers performed this fecal transplant, it increased anxiety-related behavior. But when they performed the transplant in reverse – transferring gut bacteria from calm to anxious mice – it lowered anxiety-related behavior.
Other studies have also shown that treating rodents with probiotics, or “helpful” bacteria, can lower stress. Probiotics might do the same in humans, too. A 2016 study showed that a probiotic strain lowered stress and anxiety in healthy human volunteers.
Depression. Fecal transplants suggest that the gut microbiome plays a role in depression, too. University College Cork and University of Groningen researchers found that transplanting gut bacteria from depressed patients to rats whose gut microbiome had been depleted lowered their ability to feel pleasure and boosted anxiety-like behavior.
As with anxiety, probiotics seem to help. In a March study, researchers found that after mice underwent stress, they showed depressive-like behavior, and their gut no longer contained the probiotic bacteria Lactobacillus. After the researchers fed them Lactobacillus, their behavior improved. A recent review of 10 clinical studies suggests that probiotics might improve depression and anxiety symptoms in humans, too.
Alzheimer’s disease. Studies hint that the gut microbiome might contribute to Alzheimer’s by affecting the formation of clusters of misfolded proteins in the brain, a hallmark of the disease. In a 2016 study, researchers transferred mouse embryos into germ-free female mice raised in an extremely clean lab environment to create mice without a gut microbiome. These mice received fecal transplants from either healthy mice or those genetically engineered to model Alzheimer’s disease. The brains of mice that received gut bacteria from diseased mice had more of a type of protein cluster found in Alzheimer’s.
Parkinson’s disease. Research has also linked the gut microbiome to Parkinson’s disease. A study of data from more than 386,000 individuals found that those who had undergone surgery to remove the main trunk of the vagus nerve at least five years earlier were less likely to develop Parkinson’s than those who didn’t undergo the surgery and were tracked for at least five years. As with Alzheimer’s disease, some scientists believe misfolded proteins in the gut may lead to the formation of similarly misfolded proteins in the brain.
How to Keep Your Gut Healthy
It’s important to note that most studies on the brain-gut connection have been done in animals, so their findings might not directly translate to humans. Still, they point to a few general rules of thumb when it comes to keeping your gut and brain healthy:
Eat a plant-based diet, which studies have linked to improved gut microbiome health. That’s largely because plants are chock full of antioxidants, shown to increase probiotic bacteria while reducing disease-causing bacteria. Plants also pack lots of fiber – a so-called prebiotic, essentially food for probiotics – associated with increased gut bacteria abundance and diversity. Eating different types of fiber recruits a greater diversity of bacteria to break them down, says Emeran Mayer, a UCLA professor and author of “The Mind-Gut Connection.” On the other hand, diets heavy in animal products, such as red meat and cheese are linked to the growth of bacteria associated with inflammation.
Studies have already correlated a plant-based Mediterranean diet with a lower risk of depression and Alzheimer’s disease. This suggests that the gut microbiome may be “the missing link” that explains the effects of nutrition on brain health, says John Cryan of University College Cork and co-author of the upcoming book, "The Psychobiotic Revolution," who led the study in which rats were transplanted with gut bacteria from depressed patients.
Probiotics might help – but they’re not a cure-all. For starters, “we still don’t know who are the good guys, and who are the bad guys,” says Brittany Mason, an assistant professor of psychiatry at UT Southwestern Medical Center. What’s more, “we do have evidence that shows if you give someone probiotics, you can see an effect, but [it] tends to disappear if the person is not taking the probiotic.” Eating probiotic-rich foods rather than supplements may improve the chances that the probiotics will take hold, since they’ll be in their natural form, which may make them more likely to survive their journey to the gut, Mason says. You can find probiotics in fermented foods, like yogurt, kimchi, tempeh, and kombucha tea.
Exercise. There’s plenty of evidence that exercise keeps our brain healthy. Could this have something do with its benefits to the gut microbiome? A 2015 study found that juvenile rats that exercised daily had a healthier gut microbiome composition than sedentary rats and adult rats that had exercised daily, bolstering research that the gut microbiome is more sensitive to changes earlier in life. A study published a year earlier also found that, compared with the general public, professional rugby players had a richer diversity of gut bacteria, plus lower markers of inflammation.
“Go outside, and get dirty,” Mason says. According to what’s known as the hygiene hypothesis, growing up around more germs – living on a farm, or spending lots of time outside, for instance – teaches kids’ immune systems to recognize the good guys and bad guys, resulting in a lower risk of asthma, allergies and other conditions. Studies suggest that this could extend to the gut microbiome, as well, with some showing an association between low diversity in gut bacteria in infancy and allergies. While evidence suggests that exposure to germs early in life seems to strongly shape the composition of our gut microbiome, Mason suspects that adults could still benefit to some extent from being outside and “letting the wider bacterial community cross-pollinate.” Plus, research also suggests that spending time in nature could improve mood and other aspects of mental health.
Selected References
Jane A. Foster, et al. “Stress & the Gut-Brain Axis: Regulation by the Microbiome.” Neurobiology of Stress. 19 March 2017.
Schmidt, Charles. “Mental Health May Depend on Creatures in the Gut.” Scientific American. 1 March 2015.
