When you think of bacteria inside your body, you probably think of nasty bacteria that can cause infections of all kinds. However, not all bacteria are bad; you have between 10 trillion and 100 trillion bacteria living in your gut that are necessary for your health. In fact, studies continue to link gut bacteria and mood in a few ways that may surprise you.
Back in the early 2000s, a psychiatrist named Ted Dinan took a position at the University College Cork, which was populated by some of the best microbiologists in the world. While he was there, he decided to give some lab rats he was studying for mental health some probiotics. What happened was profound. The rats who had previously displayed symptoms of anxiety and depression started to improve. Over time, they became normal, mentally healthy rats. Dinan decided to consider this further and attempt to explain the link between gut bacteria and food.
More recent research on the link between gut bacteria and mood has led to the coining of the term “gut-brain axis”, which describes how the bacteria in your intestines and your central nervous system communicate, almost as if on a two-way radio. In fact, researchers have manipulated the gut bacteria in growing mice, and they found that by adjusting the microbial environment in the intestines, they could influence the development of the mice’s brains – especially in the prefrontal cortex, which has a tremendous impact on mood.
Researchers have found that certain types of bacteria living in your intestines can secrete neurotransmitters, which are compounds that move between neurons and synapses and act as chemical messengers. A study published in the Journal of Psychiatric Research in April 2015 showed that by adding these types of bacteria to mice’s intestines, production of several neurotransmitters – including GABA, tryptophan, and acetylcholine – improved significantly. This is especially interesting when you consider that your digestive tract’s lining is part of your nervous system, called the enteric nervous system, which can react to the neurotransmitters produced by the bacteria and send signals to your brain.
In another study focused on the link between gut bacteria and mood, Dinan’s team took things a step further. First, they gave the mice probiotic microbes found in yogurt and observed. They found that the mice who ingested the probiotics seemed more relaxed than mice who didn’t get the probiotics, and they were more likely to step out into the open when on a maze course. Then, they transplanted feces from the mice fed microbes into the mice that didn’t receive the treatment, and they found that those mice began to behave similarly.
The vagus nerve connects your central nervous system to your intestines. Dinan believes that the neurotransmitters produced by the bacteria travel along the vagus nerve to signal the brain. In an interesting turn of events, Dinan found that when his team severed the mice’s vagus nerves, the effects produced by the probiotics or fecal transplants no longer occurred. The mice remained just as anxious as before, even though their bodies were producing more neurotransmitters in their guts.
Although the studies in mice are certainly promising, Dinan warns that this is just not enough evidence to accurately link gut bacteria and mood in humans. It’s not that probiotics won’t have the same effects in people, but merely a testament to the fact that it’s hard to study the complicated microbiome in the human gut and how it interacts with the brain. Per Johnathan Eisen, who is a professor of ecology and evolution at the University of California at Davis, there’s no tangible proof that human beings can guide influence the gut in their bacteria in such a way that they can change the way their brains develop, or even alleviate the symptoms of anxiety and depression.
The link between gut bacteria and mood is real – at least in mice. However, there’s just not enough human research to show that we can improve our mood by taking probiotics or eating foods designed to encourage the growth of healthy bacteria in our intestines. While probiotics and prebiotics are undeniably good for you, whether they can alleviate anxiety, stress, and even depression in human beings remains a mystery.