Does Bacteria In The Gut Affect Mood And Behavior?

If you’re involved or invested in your overall health, then you already know the importance of the gastrointestinal system. It is not only the place where nutrient absorption and digestion occur, but it is also a key area for our immune system. You likely also know that our gut is filled with trillions of bacterial cells, both good and bad, which can have a major impact on our metabolism, weight, and overall wellness.

Bacteria Meme

Photo Credit : Flickr

However, many people argue that the gut also has a serious influence on our behavior and mood, which is a bit more difficult to understand. After all, behavior and mood are psychological aspects of our life, while the bacteria in our gut are more often associated with physical aspects like hunger and immune system strength.

Although it may seem incredible that the bacteria in the gut can have an influence on how humans behave, think and interact, it’s absolutely true!

The Secret Side of Bacteria

Human beings are extremely complex multicellular organisms, but there has been one thing that has remained the same over hundreds of millions of years of gradual evolution – the presence of bacteria in our bodies. In fact, the average human body contains roughly 100 trillion bacteria, representing a total biomass of 1-3 pounds in most people. With more than a thousand different species of bacteria in our bodies at any given time, it should not come as a surprise that many of those species have effects that go beyond the basics of metabolism, digestion, and immune response.

In fact, research has recently connected bacteria to a number of different aspects of our brain chemistry – including neural disorders and psychological conditions such as autism, depression, and anxiety.

The Gut-Brain Connection (Photo Credit: T. L. Furrer / Fotolia)

The Gut-Brain Connection (Photo Credit: T. L. Furrer / Fotolia)

Bacteria and Depression

Research has recently revealed that certain bacteria can also help our bodies modulate the levels of stress hormones in the body. A study showed that having higher levels of two particular strains of bacteria, bifidobacteria and lactobacillus, is linked to a reduced release of stress hormones into the bloodstream. This test has its basis in lab rats, but the early human experiments with these two strains of bacteria have produced similarly low levels of anxiety in response to typically anxiety-inducing activities.

A small fraction of the gut bacteria varieties (Photo Credit: rob3000 / Fotolia)

A small fraction of the gut bacteria varieties (Photo Credit: rob3000 / Fotolia)

Research has revealed that roughly 50% of neurochemicals are actually engaged in gut function, thus regulating appetite, digestive rate, and metabolism. These neurochemicals include a significant amount of the dopamine and serotonin in our bodies, which most people assume is produced and used in the brain itself. In truth, most of those neurochemicals are directly managed and produced in the gut!

“Good” bacteria in the stomach needs to be maximized, and one of the best ways to ensure a balanced microbiome is to “feed” those bacteria things it needs, such as prebiotics, concentrated carbohydrates that help those bacteria grow. By improving the bacterial health of the gut, a measurable decrease in anxiety was measured in human patients, because the bacteria were operating properly and releasing chemicals to soothe stress.


Bacteria and Autism

The recent surge in interest related to gastroenterology and psychiatric conditions has resulted in a great deal of research done on the topic. One interesting revelation has been the elevated levels of a particular chemical, 4-ethylphenylsulphate, also known as 4EPS, which seems to be produced by gut bacteria. Children with autism demonstrate statistically higher levels of 4EPS than those without, and lab studies have shown that mice injected with concentrated doses of this chemical also began exhibiting austistic symptoms.

The reason that this gut-brain connection matters so much is that for many neurological conditions, no cure is known, but that may be because researchers have been looking in the wrong place. Neurotransmitters, as the name implies, are largely thought of as being in the brain, but as it turns out, the gut also produces important neurotransmitters that can affect social behavior and brain chemistry. By narrowing down the candidates for a link between gut bacteria and autism, researchers could theoretically eliminate those bacteria, and the chemical that they produce, thus minimizing or even reversing the symptoms of autism.

Other research has even linked our psychological response to emotional stimuli to the presence or absence of gut bacteria. In a controlled study, two groups were given a probiotic and a placebo, respectively. When later shown images of people with emotional facial expressions, those who had been given the probiotic were more aware and responsive to emotions, something that people with an autism spectrum disorder often struggle with.

What Does This All Mean?

If we can understand which types of gut bacteria control what moods, emotions and chemical signals within the body that affect brain function, we could theoretically tailor a healthy bacterial balance in the stomach. Essentially, it could be possible to reverse or even inhibit certain chronic psychological or mental disorders. All we need are some mad scientists to make it a reality….

That being said, the exact mechanisms behind much of this gut-brain interaction is not understood. Theories include a key connection between bacteria and the vagus nerve (controlling digestive contraction and sensory input to the brain), but this has yet to be proven. The most recent research has focused on the wide range of metabolites, the drug-like chemicals that are produced by microflora that could potentially mimic, counter or complement the traditional neurotransmitters.

It is important to remember that evolution is typically extremely efficient – over a long enough timescale. Therefore, to improve its ability to reproduce, spread and survive, bacteria has likely altered our brain chemistry in myriad ways over time, perhaps even driving our social behavior to form communities and settle in close proximity to other potential hosts!


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The more we learn about bacteria, the more we realize that it does far more than digest our food – it dictates the way we experience, think about and approach the world around us!

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About the Author

John Staughton is a traveling writer, editor, publisher and photographer who earned his English and Integrative Biology degrees from the University of Illinois. He is the co-founder of a literary journal, Sheriff Nottingham, and the Content Director for Stain’d Arts, an arts nonprofit based in Denver. On a perpetual journey towards the idea of home, he uses words to educate, inspire, uplift and evolve.

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