It’s estimated that 350 million people are affected by depression globally. The financial burden of depression in the U.S. is over $70 billion annually, and this number is expected to rise. In developing countries, depression is a leading cause of disability. 

While the complete etiology remains unknown, certain physiological factors have been associated with depression. Imbalances in the serotonergic system are the major hallmark of depression. Gut microbiota have been shown to modulate neuroactive metabolites including serotonin and gamma-aminobutyric acid (GABA), influencing the serotonergic system. The gut-brain axis may therefore play a bidirectional role in mood health. 

In addition, some inflammatory markers including interleukin (IL)-6 and tumor necrosis factor (TNF)-α, while certainly not unique to mood disorders, have been included in clinical studies related to depression and other psychological illnesses. Oxidative stress has also been linked to certain parameters related to mood health. Probiotics have been shown to support both antioxidative status and a normal inflammatory response.

A recently published systematic review and meta-analysis by Lin and colleagues analyzed data from randomized controlled clinical trials (RCTs) involving probiotic use and mood disorders such as depression. Thirteen RCTs involving 776 participants were included in this study. Upon subgroup analysis, probiotics significantly improved parameters related to depression. The effect of probiotics was greater in individuals over 40 years of age. 

Significant differences in IL-6 and nitric oxide levels were reported when comparing treatment and placebo groups. There were no significant differences in TNF-α levels. However, the authors note that only a few studies in this meta-analysis assessed this parameter, so significant results were not achievable. TNF-α is thought to influence the inflammatory response and may also increase the permeability of the blood-brain barrier; it therefore may be a compelling data point to capture in future RCTs.  

Drawbacks and challenges to this meta-analysis include relatively small sample sizes and the use of varying probiotic strains. In addition, changes in the gut microbial composition were not analyzed in this study. The authors also note that the included RCTs focused on depression without accounting for other psychiatric symptoms. More repeatable studies with standardized measurement tools and more study participants are needed before clinical conclusions can be made. 

While the biochemistry of mood health is multifactorial, recent research has explored the potential role of probiotics to support mood health. Probiotics can be found in fermented foods such as sauerkraut, kimchi, tempeh, and kefir. 

By Dr. Cory Ambrose, ND, MAT