Over 2500 years ago, Hippocrates declared, “All diseases begin in the gut.” All? Doubtful. Some? For sure. Mental illness? Maybe.
Modern science suggests that the gut and the brain are more intimately connected than we ever imagined. And rapidly advancing research on the gut microbiome suggests that the little creatures in our intestines may directly affect our mental health. Here are some highlights.
1. Gut microbiome is the body’s rain forest. At least 160 species of bacteria live in our guts – making our intestines home to the most diverse flora in a single area of our bodies. Our microbiome starts colonizing bacteria at birth (maybe even before). It is acquired from our mothers, our environment, and our diet. Each person has a unique mix of microbes in the intestines. These bugs become like our native species in our intestinal ecosystem, protecting us and making it difficult for other bacterial species to move in. By age three our gut biome has nearly the profile it will have when we are adults. But it isn’t static. The microbiome can become enhanced or depleted based on things like age, diet, environment and medications.
2. The brain-gut axis. So what do these intestinal bugs have to do with mental disorders? It turns out that the brain and the gut talk to each other in important ways. Researchers are still trying to figure out exactly how this happens, but one idea is that gut bacteria control the metabolism of neurotransmitters like serotonin.
3. Stress and gut bugs. Part of the link between the intestinal microbiome and mental disorders may have to do with the development of the body’s stress signaling pathways. Proper gut colonization must occur for normal development of the hypothalamic pituitary adrenal (HPA) axis. While much more research is needed, the concept of the brain-gut axis suggests that modifying gut microbes could be a new strategy to treat stress-related diseases, including anxiety and trauma-related disorders.
4. Can intestinal bacteria cause depression? The genetic and environmental factors that lead to depression are, of course, complex. But gut microbiota might have a role to play. Some of the early evidence for a causal role of the microbiome in depression comes from animal models: When gut material from humans with and without major depression was transplanted into mice engineered to have no gut bugs of their own, the mice who got the depressive gut material displayed depressive-like behaviors compared to the mice who got the healthy gut bugs.
5. Leaky gut occurs when the intestines become excessively permeable, which can happen as a function of an unhealthy microbiome. Impoverished microbiota in the intestines has been shown to disrupt the intestinal barrier, which is important for regulating inflammation. Emerging evidence suggests that an unhealthy and imbalanced gut microbiome is associated with autism and schizophrenia and scientists are exploring the possible causal pathways in such mental health conditions.
These are early days in terms of understanding the gut – brain relationship and mental health, but Hippocrates was on to something 2500 years ago. Since forever, parents have told kids to eat a balanced diet. It now looks like we have another reason to do so. High fiber, complex carbohydrates, and naturally fermented foods like sauerkraut, pickles, miso, and yogurt contain probiotics that are good for our gut. And good for our gut may be good for our mental health, too.