The Role of Probiotics in Depression Therapy and Prevention

Due to recent awareness campaigns, depression is now a well-known mental health condition. Discussion about depression and general mental health has risen in the media nationwide. Social media in particular has been a key platform for communicating information about psychological wellness and decreasing mental health-related stigma. Progress has been made in awareness, treatment and management of depression, but a great deal of information about its mechanisms is yet to be understood. 

Major Depressive Disorder (MDD), more commonly known as depression, is a complex psychiatric condition that has largely unexplained origins. Depression is a common mental health issue, and it will affect about 20% of people at some point in their lifetime. MDD has a debilitating impact on daily productivity and relationships by causing low mood, feelings of hopelessness, and loss of interest in typically enjoyable activities. It can also negatively affect physical health by disturbing appetite and sleep patterns. In extreme cases, depression can lead to suicidal thoughts or actions. Because of these devastating consequences, finding ways to manage and treat depression is crucial. 

You might be asking: What does this have to do with gut health? Interestingly, new research has emerged indicating a potential impact of the gut microbiome on the central nervous system. The connection between these two systems may suggest that improving gut health and improving mental health can go hand in hand.

The Gut-Brain Axis

The name for this proposed connection between the central nervous system and the gut microbiome is the gut-brain axis (GBA). The central nervous system includes the brain and the spinal cord, while the gut is innervated by a separate system called the enteric nervous system. Recent research has suggested a bidirectional communication between these two systems. More specifically, there is a link between the centers of the brain that control emotions and higher-order thinking and intestinal tract function. This link goes both ways, with each system communicating with the other. 

The key to this link seems to be the gut microbiome, according to recent evidence. Signaling from the gut microbiome to the brain and from the brain to the gut microbiome has been described in the literature. These connections are thought to be multifactorial, with involvement from the nervous system, immune system, vascular system, and hormones. In this way, the gut and the brain are intricately connected. 

Depression and the Gut Microbiome

The link between emotional processing in the brain and the composition of the gut microbiome implies involvement of gut microbiota in mood disorders, including depression. According to a 2015 study, individuals with depression have significant differences in gut microbiome composition when compared with healthy controls. Specifically, higher levels of Enterobacteriaceae and Alistipes were found in fecal samples of subjects with MDD. However, individuals with depression had reduced levels of Faecalibacterium, which is negatively correlated with severity of depression symptoms. This sheds more light on how the gut microbiome is involved in the physiology of depression. Increased levels of harmful gut microbiota and decreases in beneficial microorganisms may be the mechanism by which depressive symptoms are influenced by intestinal bacteria. 

Role of Probiotics in Decreasing Depressive Symptoms

Because symptoms of depression are correlated with gut microbiome composition, it follows that probiotics may be an effective treatment regimen to reduce severity of MDD. A 2022 study explored this possibility. In this randomized controlled trial, high doses of probiotics were implemented as an add-on therapy to traditional depression treatment. There was a significant difference in remission rates between a placebo group, who received standard treatment, and an experimental group, who took probiotics as an additional therapy. 

The probiotic group had higher overall rates of depression recovery than the placebo group. Patients who had the highest compliance to the probiotic regimen had the most significant reduction in depressive symptoms. An interesting additional finding was that, although probiotics significantly improved depressive symptoms, they did not have an effect on anxiety levels in subjects with anxiety disorders. This may be due to specificity of the connection of the gut to certain areas of the brain, although further research on this topic is needed. 

The gut microbiota of the subjects was then analyzed to determine whether the effects were mediated by changes in gut microbiome composition. It was found that taking the probiotic increased levels of beneficial bacterial species, including Prevotella and Lactobacillus. Lactobacillus in particular has been shown by other studies to be effective in reducing symptoms of depression, so these results further validate this conclusion. 

Probiotics and Depression Risk in Healthy Individuals

Not only can probiotics decrease the severity of depression by improving gut microbiome composition, but they can also reduce the risk of depression in those without mental health disorders. According to a recent review, several studies have found a reduction of depression risk in healthy individuals who took probiotic supplements. A subgroup analysis based on depressive state was conducted, and results showed a reduction in depression scale scores in both depressed and non-depressed subjects. This demonstrates that probiotics can be used not only as an addition to depression treatment in those diagnosed with MDD, but they can also be implemented as a preventive therapy against depression. 

Summary

Recent research has found a link between the gut microbiome and the central nervous system. Mediated by neural pathways, hormones, and vascular connections, this relationship is known as the gut-brain axis and is involved in the physiology of depressive symptoms. As a result, probiotics can be an effective add-on therapy for those diagnosed with clinical depression. They are also a potential mechanism by which to prevent depression in healthy individuals. Although more research is needed to validate these initial findings, there is promise for probiotic supplements as effective supplemental treatments and prevention mechanisms for the debilitating disease of depression.