The link between gut problems and mental illnesses is the only consistent comorbidity that has been explored from the time of the ancient Greeks to the present. Galen, an immensely popular 2nd century Greek physician, noted the coincidence of mental disturbances with gut problems like constipation and indigestion. Using his theory of balancing the body’s humours (blood, phlegm, yellow bile, and black bile), his patients were given a change of diet as well as purgatives to restore mental health.
These days, we know the body functions using so much more than three bodily fluids, but the association between gut health and mental state has only become stronger. Let’s take a look at modern medicine’s take on the link between your gut and your mental health.
The gut-brain axis
The gut-brain axis is a system of approximately 500 million neurons, as well as hormones and neurotransmitters, connecting your brain and digestive system. The neurons of the gut-brain axis are governed by called the vagus nerve (Latin for “wanderer”, so called because it traverses the distance from the brain to the gut), a nerve that sends sensory and motor commands both ways. To ease the burden on the brain, the vagus nerve takes care of bodily processes that do not need conscious thought, like heart rate, respiratory function, and digestive function. For this reason, the gut-brain axis is sometimes called the “second brain”.
What makes the “second brain” changeable is that unlike your actual brain, which is not a popular breeding ground for bacteria, your gut is host to 500 to 1000 species of normal flora (some good, some bad), called the microbiome. Trouble happens when there is an imbalance of the resident bacteria in your gut, which can be due to an unhealthy diet, antibiotic use, chemotherapy or radiation exposure.
In an effort to bring your normal flora back to healthy levels, your immune system kicks in. The immune response can cause disruptions in the function of the gut-brain axis, inflammation to the GI tract, and a change in the neurotransmitters and hormones produced by the gut. This last one is the culprit for changes in mental health, as we’ll discover below.
Gut bacteria and mood disorders
We normally associate hormones governing mood with the brain, but the truth is, your gut produces 80-90% of serotonin, the hormone responsible for keeping your mood up.
Certain species of gut flora like the Candida, Streptococcus, and Escherichia species produce serotonin. A decrease in the population of these species leads to a reduction in the production of serotonin, leading to a drop or fluctuation in mood. A study on mice in California found that mice with no gut bacteria produced 60% less serotonin than those with normal gut bacteria levels. When the mice’s guts were recolonised with gut flora, serotonin levels normalised.
Mental health professionals are looking more and more towards dietary changes as a treatment for depression and other mood disorders. It’s a friendly adjunct to medications like SSRIs, with considerably less side effects.
Gut bacteria and anxiety
Anxiety is a mental health disorder where the body has an increased stress response to an absent or disproportionate stimulus. A study on the absence of gut biota in rats showed an increase in anxiety symptoms. In humans, Escherichia, Bacillus and Saccharomyces produce norepinephrine, a stress hormone. When the balance of gut biota is disrupted and these bacteria increase, it could result in an increased stress response in other areas of the body.
This can be illustrated when we look at the link between irritable bowel syndrome (IBS) and anxiety. IBS is a chronic condition where the intestines are more sensitive to certain foods, causing spasms and diarrhea. IBS is exacerbated by stress - the very hallmark of anxiety. The Anxiety and Depression Association of America estimates that up to 90 percent of IBS sufferers also have anxiety.
What’s worse is that it’s a chicken-and-egg scenario - are your IBS symptoms exacerbated by stress, or are you stressed because your IBS symptoms are acting up? "When we are anxious, our bodies become 'hyper-aware' of our symptoms, as if the nerve signals are amplified,” says Daniela Jodorkovsky, M.D., a gastroenterologist at Columbia University Medical Centre. “This makes the symptoms even more distressing than they would otherwise be."
Gut bacteria and schizophrenia
Less common than mood disorders and anxiety, schizophrenia is a chronic and severe mental health disorder that affects only 1% of the population. It’s characterised by positive symptoms such as hallucinations and thought disorders, negative symptoms such as apathy and reduced speaking, and cognitive symptoms such as poor decision making and concentration.
Risk factors for schizophrenia include infection, exposure to the parasite Toxoplasma gondii and gluten intolerance - all of which are related to gut function.
Immune function is regulated by gut-brain axis via the vagus nerve. Further, infection leads to the intake of antibiotics, which disrupts the microbiome and interferes with hormones secreted there.
Toxoplasma gondii exposure leads to an increase in dopamine, one of the hormones elevated during psychotic episodes in schizophrenia. 50% of the body’s dopamine is produced by the gut.
Food intolerances such as celiac disease are characterised by sensitivity to gluten. Eating foods with gluten damage the gut, cause inflammation, and change the gut microbiota. Many studies have explored how switching to a gluten-free diet alleviated symptoms of schizophrenia.
Using the link
All this to say, there is a link between the balance of gut bacteria in the microbiome and mental health. It’s pretty potent, and it goes both ways. But what now?
The next logical step to improve mental health is to restore gut health. To tweak your gut bacteria levels, eat probiotic foods that contain live bacteria to replenish depleted normal flora. Probiotics that specifically affect the brain called psychobiotics - think Greek yogurt, sauerkraut, kombucha, kimchi, and miso. The bacteria from these foods affect the immune system and nervous system to produce anxiolytic and antidepressant effects.
Restoring the balance of a thousand species of gut bacteria can alleviate symptoms of mental health issues. That’s way more than four humours, but perhaps, in essence, Galen had it right. ■