The gut microbiome may affect mental health symptoms like anxiety and depression, and possibly even autism
- Created: September 16, 2015
- Written by Ginny Fleming
Some very interesting work is being conducted to elucidate connections between the gut microbiome and mental health disorders such as anxiety, depression, and even autism. Scientific American has published several reviews on the subject. John Cryan and colleagues call it "a paradigm shift in neuroscience".
The gut is home to the enteric nervous system, which controls digestion and excretion locally. This "second brain" consists of sheaths of neurons, 100 million in all, embedded in the gut walls all the way from the esophagus to the anus. Neural communication between the brain and the enteric nervous system (gut-brain axis) goes in both directions with, surprisingly, as many as 90% of the fibers of the primary visceral nerve (the vagus) carrying information to the brain. More than 30 neurotransmitters are involved in the enteric nervous system (similar to the brain), and 95% of the body's serotonin is found in the gut. The enteric nervous system is thought to influence our state of mind and emotions. For example, electrical stimulation of the vagus nerve, which mimics and/or amplifies gut-to-brain neural activity, has been shown to be a useful treatment for depression.
The gut microbiota comprises about 100 trillion cells (several pounds of body mass) and represents up to 10,000 species of bacteria, eukaryotes, and viruses. The number of bacterial cells in the body outnumbers our own cells by 10 to 1, and the number of bacterial genes outnumbers our genes by 100 to 1.
Scientists now think that the gut microbiota play a key role in the gut-brain axis, using not only neural but also endocrine and immune pathways. In the last decade, gut microbiome influences in various mental health disorders including anxiety, depression, and autism have been reported.
- In patients with irritable bowel syndrome (IBS), probiotics not only relieved gastrointestinal (GI) symptoms but also reduced anxiety and stress response and improved mood. Probiotics are bacterial strains thought to be beneficial to health; common species include lactic acid bacterial strains (eg, Lactobacillus acidophilus, L. rhamnosus, L. johnsonii, L. delbrueckii subsp. bulgaricus, and Streptococcus thermophilus). Live probiotic cultures are ingested as fermented dairy products, other fermented foods such as tempeh, miso, kimchi, or kefir, or manufactured freeze-dried in tablets or capsules.
- Another study demonstrated reduced anxiety in rats and beneficial psychological effects and lower serum cortisol in human subjects when treated with L. helveticus and Bifidobacterium longum strains.
- L. reuteri decreased anxiety in the mouse elevated plus maze model and reduced stress-induced corticosterone release; in this study, vagotomy prevented these effects, suggesting that parasympathetic innervation is involved.
- B. infantis acted as an antidepressant in the forced swim test and relieved depression symptoms in the maternal separation model, both in rats.
Autism spectrum disorder (ASD) is a heterogeneous set of neurobehavioral diseases that can affect a patient's social interactions, communication (both verbal and nonverbal), behavior, and interests. The incidence of ASD in the US has increased dramatically from 1 in 150 children in 2000 to 1 in 68 in 2010, as reported by CDC, but it's not clear if that increase is due to increased surveillance and reporting, environmental factor(s), or other reasons. Many patients with ASD have GI comorbidities such as diarrhea, abdominal pain, IBS, and constipation, and altered GI motility and increased intestinal permeability have also been reported. Causes of these GI symptoms in patients with ASD are not clear, but Hsiao and colleagues list 8 studies from the literature in which an altered gut microbiome was identified in patients with ASD; elevated levels of Clostridium species were found in 3 of these studies.
In a recent review of this nascent field, Cheryl Rosenfeld summarizes the animal studies, human epidemiologic studies, and work on other microbiomes (eg, oral cavity, placenta), as well as possible mechanisms and therapies. I've selected one interesting animal study to review in detail.