Can probiotics take the edge off anxiety?

Anxiety is a feeling that can often be helpful^[1], but too much of it can be detrimental to one’s quality of life^[2]. Some individuals who experience excessive anxiety are often diagnosed with an anxiety disorder and prescribed medication or therapy, which work well for many^[3]. However, these therapies aren’t always accessible, especially to people who have not been diagnosed with an anxiety disorder, but who still experience episodes of excessive anxiety.

Some researchers have proposed a connection between anxiety and the gut^[4], given that the microbiota seems to communicate with the nervous system^[5] and immune system. Furthermore, anxiety and mood disorders are comorbidities^[6] of functional gut disorders, or conditions where there is no clear biomarker for diagnosis, such as irritable bowel syndrome.

Gastrointestinal infections^[7] and frequent antibiotic use^[8] may precede such gut disorders, and both have also been associated^[9][10] with an increased risk of having an anxiety disorder. In addition, probiotic supplements have shown some success in clinical trials for reducing the symptoms of functional gut disorders^[11]. Thus, several researchers have explored whether they may have utility for reducing anxiety and mood disorders, too.

While these researchers have some justification to suspect that probiotics could influence anxiety, it’s also worth remembering that many things are correlated with gut function^[12]. It’s well known that correlation doesn’t equal causation, but sometimes, correlation doesn’t even imply correlation, in that some correlations may be spurious and vanish when looking at the population.

So, it would be irrational to assume that probiotics helped with everything correlated with gut function. The question is, when the hypothesis is directly tested, what impact do probiotics have on mood and anxiety disorders? The evidence to date seems to be mixed.

A 2016 systematic review^[13] of ten randomized trials found that certain probiotic formulations had some support for decreasing symptoms of anxiety and depression. However, it didn’t quantify the treatment effect with a meta-analysis. In 2017, a meta-analysis^[14] found that probiotics improved psychological symptoms, but did not specifically look at anxiety or depression on their own. This year, a meta-analysis^[15] of 10 randomized trials caught up on some of these research gaps but failed to find a statistically significant effect of probiotics on depression.

However, no meta-analysis has yet to quantify the effect of probiotic therapy on anxiety specifically. The study under review sought to fill this knowledge gap by quantifying the treatment effect of probiotics on anxiety in both animals and humans.

Anxiety can be helpful in the right amounts, but too much of it can be detrimental to quality of life. The gut microbiota and disordered gut function may be implicated in anxiety disorders, which has inspired researchers to explore whether probiotics can decrease anxiety symptoms. The study under review is the first meta-analysis to quantify the effects of probiotics on anxiety in both animals and humans.