“Probiotic” has become a popular health-food buzzword in recent years, but many people don’t take the time to stop and think about what the term actually means. In contrast to antibiotics, which are intended to kill infectious bacteria but often have wider-reaching effects, probiotics are supposed to deliver beneficial bacteria that help maintain internal balance and promote health. The research that inspired this health fad showcases the intricacies of biological systems and the many ways in which they are interwoven, even inside our own bodies.
Because of the negative ways in which “germs” and bacteria are generally discussed, most people think of bacteria as harmful, threatening, and unwanted. In reality, humans couldn’t survive without bacteria. Researchers who study the microbiome, which is the collection of all microorganisms in the human body, recently estimated that the human body is actually composed of an approximate one-to-one ratio of human cells to bacterial cells. Trillions of bacteria – over one thousand different species – inhabit our bodies. The majority of these are found in our digestive system and are collectively referred to as gut microbiota. Much of their function is related to food breakdown and nutrient absorption, as might be expected given their location. It turns out, however, that gut bacteria are also linked to other bodily processes, including brain function.
Although gut microbiota and their influence on total health have only recently started to enter mainstream public awareness, scientists have actually known about the link between the gut and the brain for over 100 years. As early as the 1830s, doctors noted a correlation between changing moods and digestive secretions. By the mid-1900s, researchers who studied stress biology had confirmed a link between gut function and stress response, both mental and physical. This connection made sense, since the brain is known to control various body systems including the gastrointestinal (GI) tract. But exciting new research has revealed that influence can also go in the other direction – GI function and gut microbiota can also have a significant impact on the brain, including regulation of mood and mental state.
Mark Lyte, a researcher at Texas Tech University, has found that gut bacteria actually produce neurotransmitters. These chemicals, which are responsible for signaling in the human brain, include norepinephrine, gamma-aminobutyric acid (GABA), serotonin, dopamine, and acetylcholine. “[Humans and bacteria] make the same stuff. Maybe all this communication has an influence on our behavior,” said Lyte excitedly in an interview with the New York Times. This neurotransmitter connection may help explain why mental disorders like depression and anxiety often coincide with GI disorders.
Not only can gut microbiota affect day-to-day brain function, but they can also have a far-reaching impact on brain development at different stages of life. Neurological studies in mice showed that disruption of gut microbiota was associated with a decrease in levels of brain-derived neurotropic factor (BDNF), a chemical that helps keep neurons healthy and promotes the growth of neurons and the development and maturation of connections. Deficient levels of BDNF in early life can cause cognitive deficits, while these deficiencies later in life can be associated with symptoms of neurological diseases like dementia. Other studies have found possible links between gut-brain interaction and autism spectrum disorders.
In light of this newfound recognition of the two-way interaction between the brain and gut microbiota, researchers like Lyte have begun to explore the possibility of developing targeted therapies based on modification of the microbiome. “Psychobiotics” is an emerging term for microbes that can be administered to produce changes in mood or mental state. Mouse studies revealed that administration of Lactobacillus rhamnosus, a bacterium that produces the inhibitory neurotransmitter GABA, led to a marked decrease in symptoms of anxiety in mice. Researchers noted that the effects of the bacteria were similar to those of commonly prescribed anti-anxiety drugs like Xanax or Prozac. Preliminary trials with healthy volunteers have demonstrated that the psychological changes produced by artificially modulation of gut microbiota are evident in humans as well, though the effect appears more subtle than in mice. Recent studies have found that disruption in gut microbiota also plays a significant role in alcoholism and possibly other substance abuse disorders, and scientists are hopeful that modulation of gut bacteria may play a part in future alcoholism treatments. In the future, it is possible that microbes may play a key role in both diagnosis and treatment of neurodevelopmental disorders as well as digestive diseases.
As promising as these potential treatments seem, they are still far from clinical implementation. Lyte says the excitement around psychobiotics’ potential has outrun the extent of current scientific knowledge. He warns that far more research studies and clinical trials are necessary before people begin treating themselves regularly with advertised psychobiotics or other probiotic concoctions. Even when probiotics become a reliable treatment option, therapy that isn’t designed by an expert in probiotics and the human microbiome could prove dangerous. Lyte believes that microbiota control is the future, but he remains cautious and says that future is still a long way away.
Grace Niewijk is a junior in Calhoun College. Contact her at firstname.lastname@example.org.