In 1950, at Staten Island’s Sea View Hospital, a group of patients with terminal tuberculosis were given a new antibiotic called isoniazid, which caused some unexpected side effects. The patients reported euphoria, mental stimulation, and improved sleep, and even began socializing with more vigor. The press was all over the case, writing about the sick “dancing in the halls tho’ they had holes in their lungs.” Soon doctors started prescribing isoniazid as the first-ever antidepressant.
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The Sea View Hospital experiment was an early hint that changing the composition of the gut microbiome – in this case, via antibiotics – might affect our mental health. Yet only in the last 2 decades has research into connections between what we ingest and psychiatric disorders really taken off. In 2004, a landmark study showed that germ-free mice (born in such sterile conditions that they lacked a microbiome) had an exaggerated stress response. The effects were reversed, however, if the mice were fed a bacterial strain, Bifidobacterium infantis, a probiotic. This sparked academic interest, and thousands of research papers followed.
According to Stephen Ilardi, PhD, a clinical psychologist at the University of Kansas, Lawrence, focusing on the etiology and treatment of depression, now is the “time of exciting discovery” in the field of probiotics and psychiatric disorders, although, admittedly, a lot still remains unknown.
Gut microbiome profiles in mental health disorders
We humans have about 100 trillion microbes residing in our guts. Some of these are archaea, some fungi, some protozoans and even viruses, but most are bacteria. Things like diet, sleep, and stress can all impact the composition of our gut microbiome. When the microbiome differs considerably from the typical, doctors and researchers describe it as dysbiosis, or imbalance. Studies have uncovered dysbiosis in patients with depression, anxiety, schizophrenia, and bipolar disorder.
“I think there is now pretty good evidence that the gut microbiome is actually an important factor in a number of psychiatric disorders,” says Allan Young, MBChB, clinical psychiatrist at King’s College London. The gut microbiome composition does seem to differ between psychiatric patients and the healthy. In depression, for example, a recent review of nine studies found an increase on the genus level in Streptococcus and Oscillibacter and low abundance of Lactobacillus and Coprococcus, among others. In generalized anxiety disorder, meanwhile, there appears to be an increase in Fusobacteria and Escherichia/Shigella .
For Dr. Ilardi, the next important question is whether there are plausible mechanisms that could explain how gut microbiota may influence brain function. And, it appears there are.
“The microbes in the gut can release neurotransmitters into blood that cross into the brain and influence brain function. They can release hormones into the blood that again cross into the brain. They’ve got a lot of tricks up their sleeve,” he says.
One particularly important pathway runs through the vagus nerve – the longest nerve that emerges directly from the brain, connecting it to the gut. Another is the immune pathway. Gut bacteria can interact with immune cells and reduce cytokine production, which in turn can reduce systemic inflammation. Inflammatory processes have been implicated in both depression and bipolar disorder. What’s more, gut microbes can upregulate the expression of a protein called BDNF – brain-derived neurotrophic factor – which helps the development and survival of nerve cells in the brain.