ICYMI, your gastrointestinal system actually has its own "brain" known as the enteric nervous system (ENS) which communicates with the central nervous system (CNS)—of which the brain is a part—to influence mood, cognition, and mental health, according to research published in Integrative Medicine: A Clinician's Journal. The gut and noggin additionally communicate via hormones and the immune system.
All of the above are influenced by the gut microbiome, which may account for the growing body of research connecting poor microbiome health to mood and cognitive disorders. Patients with various psychiatric disorders including depression, bipolar disorder, and schizophrenia, for example, have been shown to have significantly different gut microbiomes than those without those conditions. And dysregulation of the gut microbiome has been linked to an increased risk for dementia, too.
Below, find more details on the science behind the gut-brain connection and the ways in which your microbiome seems to influence your mental well-being.
The gut-brain connection: How your microbiome influences your mood, cognition, and more
The growing body of research connecting microbiome composition to depression is compelling. Data published in the journal Nature Microbiology in 2019, for example, found preliminary evidence linking low levels of certain gut bacteria (specifically, Coprococcus and Dialister) to depression. Those enrolled in the study who claimed to enjoy good mental health, meanwhile, were found to have high levels of Coprococcus and another bug known as Faecalibacterium. A separate study published in the International Journal of Neuropsychopharmacology found lower levels of Bifidobacterium and Lactobacillus in individuals with depression, compared to those without.
The same researchers from the 2019 study also found that many forms of gut bacteria can produce mood-regulating/boosting neurotransmitters (or their precursors) such as serotonin and dopamine. So if you're deficient in those specific bacteria, you may be more likely to suffer from mood disorders—including depression—and benefit from taking prebiotics and probiotics.
Those same neurotransmitters influenced by the gut, serotonin and dopamine, play a role in anxiety, too, and some research supports the idea that microbiome composition can impact symptoms of the disorder. For example, a study published in the journal General Psychiatry published in 2019 found that regulating gut health could help individuals manage anxiety. The research evaluated 21 different experiments to draw this conclusion, and found that both probiotic supplementation and dietary changes (including low FODMAP diets, the addition of fermented foods, etc.) effectively reduced symptoms of anxiety by regulating gut microbiota.
Postpartum depression and anxiety
While the link between postpartum depression and microbiome composition remains largely speculative at this point, it's feasible given the microbiome goes through significant changes during and after pregnancy—and scientists have already established a connection between depression/anxiety and the microbiome more generally. Adding support to this theory, one study published in Frontiers in Cellular and Infection Microbiology specifically showed differences in the microbiomes of women who suffered from postpartum depression and those who did not.
Numerous animal studies have demonstrated the link between stress and the gut microbiome. In one study from the International Journal of Neuropsychopharmacology, germ-free mice, for example, exhibited elevated reactions to stress that were regulated by the introduction of probiotic species to their systems. This effect has been repeated in subsequent studies. And a 2018 study published in the journal Brain, Behavior, and Immunity linked injections of a particular bacterium, Mycobacterium vaccae, to reduced levels of stress in rats. Promisingly, probiotics have also been shown to make humans more resilient to stress, according to research published in the International Journal of Neuropsychopharmacology.
Attention-Deficit Hyperactivity Disorder (ADHD)
Western-style diets—high in sugar and saturated fat, and low in fiber—are not so great for the microbiome, and research published in the International Journal of Neuropsychopharmacology has shown a link between those diets and rates of ADHD. A reasonable conclusion, then, is that the microbiome may impact symptoms and development of ADHD. Some research backs up this theory, too; in one clinical cohort, altered microbiome composition was observed in adolescents and adults with ADHD. Another study found that children treated with probiotics had reduced rates of ADHD.
Sleep is intricately connected to mental health, and some studies have shown that probiotic supplementation might improve your Zzzs. This could be due in some part to the fact that gut microbes produce a form of melatonin along with other sleep-regulating hormones such as serotonin, dopamine, and Gamma aminobutyric acid (GABA). On the flip side, researchers have also discovered that certain specific bacterial strains correlate to poor sleep; however, it's unclear at this point if the bad sleep causes this specific gut composition, or if it's the other way around.
Cognition and memory
There is increasing scientific evidence that the gut influences cognitive health, too. Memory and learning are dependent upon proper functioning of the hypothalamic-pituitary-adrenal axis (HPA Axis), which refers to the interaction between the hypothalamus and pituitary gland just above the brainstem and the adrenal glands atop the kidneys). The HPA Axis is impacted by—what else?—the microbiome. Microbiota-produced serotonin (or lack thereof) shows up again here, too, as it also plays a role in learning and memory. These are just two of many connections between cognition and the microbiome.
Bolstering this connection is research published in Frontiers of Aging Neuroscience showing that the administration of microbiome-destroying antibiotics to mice caused cognitive impairment. Meanwhile, a different study published in the International Journal of Neuropsychopharmacology showed that administration of probiotics in an elderly cohort of humans improved their cognitive function. Both offer evidence of the effects of gut microbes on the way we (and mice) think.
While all of the above research remains in its relative infancy, it has exciting implications for some of the trickiest health issues we face today. After all, about one in four American adults suffers from a diagnosable mental disorder. While we await clarity and certainty around the myriad ways in which our gut microbiomes affect our brains and contribute to these issues, the best thing you can do for your health is to foster a healthy gastrointestinal ecosystem through proper diet and exercise.
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