What's happening in your gut could have an impact on your mind.
Trust your gut. Go with your gut. What does your gut say?
Kind of like all that business about following your heart, usually "gut check" talk is figurative, not literal. It's in your head, after all, where you're working things out – in matters of love and logic.
But what if, in a way, what's happening in your gut actually does plays a role in what's going on in your brain? As it turns out, that's precisely the case (and vice versa). There's even a name for it: the gut-brain axis. As described in research, this goes both ways, involving the bidirectional interaction between the nervous system, including the brain, and intestinal function. Increasingly, as more research is done on the gut microbiome – the typically diverse mix of microorganisms living in our guts – and how it affects our health, the importance of these microbiota in influencing the gut-brain interaction is becoming clearer. Building on this foundation, researchers are now developing a better understanding of how the gut microbiome – which may be affected by everything from our genes to what we eat – could also be linked to mental health.
The many, various bacterial species in your gut play different roles. "And every single human being has a unique complement of species," says Jack Gilbert, a professor and faculty director of the Microbiome Center at the University of Chicago. "What we've started to see is that the bacteria in our gut can predict our likelihood of developing depression," he says. "They can predict how we might respond to depression treatment – so for example, how we might respond to antidepressants."
That's not to say there's a way to test an individual's microbiome, determine that person is at higher risk for depression – or that the person won't be as responsive to medication – and alter prevention or treatment strategies accordingly. Big questions about how genetic predisposition influences the gut microbiota and the microbiome's potential link with psychological concerns such as depression, anxiety and post-traumatic stress disorder still need to be answered. So while human and animal research is ongoing, it's not yet possible using the emerging data to, say, determine if a person is more likely to develop depression or PTSD based on analyzing that individual's gut microbiota. "I don't think it's diagnostic at the moment," says Christopher Lowry, an associate professor of integrative physiology at the University of Colorado Boulder.
But scientists have pieced together ways the gut microbiome influences health, including impacting immune function and inflammation in the body, which may shed light on how it could affect mental health. Overall, what's becoming clear is that there are health benefits to having a highly diverse microbiome. "This is particularly well-established in the context of allergy and asthma," Lowry notes. "Allergy specifically is related to an overactive immune response. And what we're learning in psychiatry is that [with] both affective disorders like depression and trauma and stressor disorders like PTSD, an increase in inflammation appears to be a risk factor for development of these disorders."
Adds Gilbert: "Think of the immune system and the microbiome as two sides of the same coin. So if you have an imbalance in the microbiome, you have an imbalance in the immune system." Since all things are connected in the body, that can have far-reaching effects. "If you have an imbalance in the microbiome, you're also going to have an imbalance in the kind of chemicals being produced," he says. This can alter the neurotransmitters, or so-called chemical messengers, produced by nerve cells, that communicate with other cells, such as other nerve cells and muscle cells. When neurotransmitters are altered, that "will change how your body senses those neurotransmitters, and hence will alter brain development and brain function," Gilbert says.
Underlying it all is the human genome – our genetic makeup. Our genes may predispose us to a microbiome imbalance affecting our mental health. "So you need genetic predisposition, then you need the microbiome to get out of whack and then you might have the onset – or exacerbation – of a mental health problem like depression," he explains.
In short, it's complex – and just as trying to puzzle out precisely how it's all connected is stymying, experts say, it follows that it's not clear exactly what – if anything – a person can do about it. Still, some approaches have shown promise in restoring or maintaining a diverse and healthy gut microbiome.
"One of the strongest predictors of diversity in the gut microbiome is the number of different plants that you eat," Lowry says. A diverse diet of many different plants, he says, is perhaps one of the best strategies to ensure a healthy gut microbiome. "A three- to four-leaf spinach plant has over 800 different species of living bacteria inside the plant," he says, adding that those can't be washed off (and that's a good thing for your gut).
Specifically consuming prebiotics – food for the microorganisms in your gut – is important as well. These differ from probiotics, or live microorganisms thought to confer some health benefit to the host sold in supplements and found in some foods like yogurt, which are also sometimes recommended to restore gut microbiome balance (like when a person is taking antibiotics). While prebiotics are also sold in supplement form, health and nutrition experts note that they occur naturally in a range of foods from asparagus, garlic and bananas to inulin, or chicory root fiber, Jerusalem artichokes, leeks and onions. "One reason that diversity of plants is so helpful is because not only are you consuming all those bacteria, but you're consuming many different kinds of prebiotics that are made by different plants," Lowry says.
In addition to a proper diet that includes consuming prebiotics, getting adequate sleep and exercise – two other pillars of good health – are recommended, and Lowry says, relevant to increasing diversity in the gut microbiome.
That's not to say, as scientific understanding of the gut microbiome's influence on health continues to evolve, it's guaranteed to influence that relationship. But experts say since these health behaviors are already shown to have benefits in many other ways, there's certainly no harm in trying.
Acknowledging the gut-brain connection also undercuts flawed historical thinking of the brain as somehow separate from other organs and body systems. "When you start thinking about the microbiome and you understand the microbiome impacts mental health, then you're really forced to consider how the body is impacting the brain through these different mechanisms," Lowry says. "It's less of a brain on a stick kind of perspective of how mental health is maintained."
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