How does gut microbiota affect mental health?
Bacteria, viruses, bacteria, fungi and archaea within the gastrointestinal microbiome interact with each other to support digestion and nutrient absorption, as well as immune function, metabolism and neurotransmission. In addition to genetics, environmental factors like age, diet, and geographical location can alter the composition of the gut microbiome. For example, the consumption of fiber-rich and fermented foods can support the proliferation of beneficial bacteria, whereas highly processed diets simultaneously poor in natural fibers can lead to gut dysbiosis, reducing beneficial bacteria and increasing the ratio advantaging the harmful ones.
Dysbiosis refers to an imbalance of microorganisms in the gut microbiota, which can lead to a wide range of health problems. Several studies have reported that gut dysbiosis can induce dysfunction in the gut-brain axis, which increase the risk of mental health disorders like anxiety and depression. Brain function can also be impacted by gut dysbiosis through its effects on the hypothalamic-pituitary-adrenal (HPA) axis, which subsequently alters the synthesis and secretion of neurotransmitters. The levels of neurotransmitters like GABA, dopamine, serotonin and acetylcholine that have been implicated in mental health disorders, can be affected by gut dysbiosis.
The gut microbiota and anxiety
Previous studies have reported that the microbiome is less diverse and numerically abundant in patients with generalized anxiety disorder (GAD) as compared to controls. These abnormalities persisted even during remission in one study, thus suggesting the role of the gut dysbiosis the in GAD development. The presence of certain bacteria species like Eubacterium and Coprococcus may increase, while Blautia and Butyricicoccus may reduce the risk of anxiety. Likewise, probiotic supplementation with Lactobacillus and Bifidobacterium may protect against mental illnesses by promoting the growth of beneficial microorganisms. Simultaneously, probiotics may also suppress the proliferation of pathogenic species such as Eschirichia-Shigella, Fusobacterium and Ruminococcus gnavus. These pathogenic bacteria can increase gut permeability and systemic inflammation, both of which are implicated in anxiety symptoms.
What lies beneath?
Gut dysbiosis is associated with reduced epithelial integrity, which allows inflammatory molecules to enter the bloodstream. Chronic inflammation has been attributed to increased anxiety and altered brain functioning. Beneficial bacteria produce GABA and other metabolites that affect vagal nerve signaling from the gut to the brain. GABA is an inhibitory neurotransmitter that reduces the intensity of anxiety and stress responses. Therefore, low levels of these beneficial bacteria can amplify the intensity and frequency of these responses. Short-chain fatty acids (SCFAs) like butyrate, acetate and propionate exert anti-inflammatory properties within the brain and participate in neurotransmitter activity. Faecalibacterium prausnitzii and Blautia are SCFA-producing bacteria in the gut microbiome. Thus, gut dysbiosis can interfere with SCFA metabolism to increase the risk of mental health disorders.
The effect of probiotics
Numerous probiotic formulations have been developed and investigated for their potential to alleviate symptoms of anxiety and depression with varying levels of success. Supplementation with Lactobacillus plantarum P8, for example, was found to reduce stress and anxiety symptoms while also reducing inflammatory cytokine levels. Similarly, a probiotic intervention containing Bifidobacterium longum led to reduced perceived stress and improved sleep quality as compared to placebo recipients. Bifidobacteria, in addition, seem to be more effective against depression rather than anxiety, adding the information that they better controlo stipsis, a commont symptom associated to depression. Multiple probiotic formulations have also been shown to reduce anxiety, depression, and stress levels.
For example, one study found that patients who took both 25 mg of sertraline and a probiotic consisting of Bifidobacteirum bifidum, Bifidobacterium lactis, Bifidobacterium longumn and Lactobacillus acidophilus led to a significant reduction in anxiety levels than the placebo group. It is relatively difficult to ascertain how this effect was most related to the drug or the probiotici preparation themselves. However, previous data states that probiotic treatment of depressive syndromes with probiotics only takes much longer to exert sharp effects on mood, compared to conventional antidepressants. There is more to that: antidepressants usaually start to take effect after least 3 weeks of administration and are knowingly associated to adverse side effects, including an unwanted higher risk for suicide.
This might be related to the improvise boost of neurotransmitters lie dopamine and serotonin induced by the drug therapy. Probiotics (best if associated with prebiotics or soluble fibers) do not seem to induce these side effects and have the advantage to modulate better the neurochemical production: dopamine and serotonin may boost the mood, but there’s need for inhibitory control over them with GABA and acetylcholine. This is why a combined approach of psychiatric conditions like anxiety and depression with a conventional drug and a probiotic plus prebiotic may represent the best choice for management. Beside, although probiotics may improve anxiety symptoms, additional studies are needed to validate these findings and determine the appropriate strains, dosage, and duration of therapy for this indication.
- Edited by Dr. Gianfrancesco Cormaci, PhD, specialist in Clinical Biochemistry.
Scientific references
Nikel K, Stojko M et al. Nutrients 2025; 17(6):993.
Javaid SF et al. Middle East Curr. Psych 2023, 30:44.
Shoubridge AP et al. Mol Psych. 2022; 27:1908–1919.
