There are many things that affect our mental health, from work-related stress to past traumas to whether or not we meditate each day. The one factor that often gets overlooked is our belly, or more specifically, the bacteria that inhabit our intestines.
The bacteria and other microorganisms that inhabit our insides are collectively known as the gut microbiome. In recent years, these microbes have been implicated in many aspects of our health, including obesity, irritable bowel syndrome and even how we respond to cancer immunotherapy.
Now scientists are exploring a new frontier for our gut microbiome — our brains. Research shows that bacteria in the gut may play a role in the development of mental illnesses like depression, anxiety and even schizophrenia and bipolar disorder. In addition, altering the community of gut bacteria may alleviate symptoms of those illnesses, in much the same way that antidepressants and other medications are supposed to do.
Although research into the influence of the gut microbiome on mental health has taken off over the past decade, it’s not completely new. In 1910, Dr. George Porter Phillips gave patients with melancholia a fermented milk drink known as kefir. Like yogurt, this is rich in the “friendly” bacterium lactobacillus. His approach worked. Eleven out of 18 of his patients were cured, with two others showing improvement.
The thought of being able to improve our health, including our mental health, simply by altering our gut microbiome is so enticing that it has led to an onslaught of overhyped claims and questionably marketed probiotics and other natural supplements.
Behind this noise, the science of the gut microbiome is very real, pointing toward new treatments for mental illness and ways to generally improve people’s mental health.
A radically different approach to mental illness is sorely needed. Some research shows that about four to six out of every 10 people with severe depression improved while taking antidepressants. This is only slightly better than the results for people taking a non-active pill — two to four out of every 10 people.
Much of the research on effects of the gut microbiome on mental health has been done in mice. In one study at Kyushu University in Japan in 2004, researchers noticed that mice that were bred to have no microbes in their gut or on their body — “germfree” mice — had higher levels of stress hormones. This suggested that gut bacteria have an important role in regulating stress responses.
When the researchers gave germfree mice the bacterium lactobacillus — the same group that is found in kefir and yogurt — their stress hormones decreased, as did their stress response. They were still more “stressed” than mice raised under normal conditions, but they were better off than completely germfree mice.
Another animal study found that when bacteria from people with major depressive disorder were transplanted into germfree mice, the mice showed mouse signs of depression — giving up earlier on a swimming task and seeking safer locations in their habitat.
The results of animal studies don’t always hold true for people. But studies in people are starting to provide interesting results. One study found that people with depression have lower amounts of certain types of bacteria in their gut. So far, though, no one bacterium has been identified as the “depression germ” or the “anti-depression germ.”
Some researchers think that the mental health effects of the microbiome is more a question of the overall gut community. One reason is that bacteria in the gut interact with each other, not just with the body. So “good” bacteria may have a direct effect on mental health, or they may work by inhibiting “bad” bacteria. Overall diversity of the gut microbiome is also important. In one study, patients with schizophrenia had a decreased diversity of their microbiome.
Scientists have a lot of ideas for how the microbiome affects our mental health. An unbalanced microbiome may lead to a “leaky gut,” which not allows molecules to cross from the gut into the blood when they shouldn’t, but also triggers an immune response. This low-level immune response may lead to symptoms of depression, such as tiredness and low mood, similar to what we feel when we have a cold or flu.
Microbes in the intestines could also affect how we digest our food, which can alter the levels of neurotransmitters in our blood that affect our mood. Or gut microbes may release chemicals that change the signals flowing through the vagus nerve, which runs from gut to brain.
It’s likely a two-way street, with microbes influencing the brain and the brain influencing the gut microbiome.
Although research into the link between the gut microbiome and mental health is promising, a gut-based approach may not work for everyone. An unbalanced gut is only possible cause of mental illness. The trick for scientists is figuring out which patients would benefit most from probiotics, prebiotics or fecal transplants.
Probiotics are pills or powders that contain beneficial bacteria. Prebiotics are foods that encourage certain gut bacteria to grow. Fecal transplants involve seeding a person’s intestines with a stool sample taken from a healthy person in order to change their gut microbiome. So far, a lot of studies looking at altering the gut microbiome have been small, which limits their usefulness. The results of this research has also been mixed.
In the future, doctors may first test a person’s microbiome to see which bacteria are present. This kind of microbiome “fingerprint” could provide insight into whether a person might benefit from a traditional antidepressant medication or should switch to a Mediterranean diet, which has also been shown to have beneficial effects on the gut microbiome.
For more on the effects of the gut microbiome on mental health, check out the BBC’s Microbes and Me.