According to the World Health Organization (WHO), depression affects an estimated 5% of adults globally, which is around 1 in 20 people.
Although treatments are available — including medicines and talking therapies — there is no cure.
While incredibly successful for some people, talking therapies do not help everyone. Unfortunately, the same applies to medicines — not everyone benefits, and even if they do work, side effects can be significant.
Scientists are keen to find alternative, safe ways to help treat depression.
In this search for better interventions, some researchers are turning to probiotics — supplements containing live bacteria.
ZOE is a world leader in the study of the gut microbiome — the trillions of bacteria and other microbes that call your intestines home. We know they are vital for good health, and our research has identified bacteria associated with both positive and negative health markers.
It is now well established that gut bugs can influence physical health, but scientists are still unraveling their role in mental health.
If you’d like to find out which bacteria live in your gut, you can start by taking our free quiz.
How could gut bacteria influence depression?
Current antidepressant drugs boost the levels of certain neurotransmitters — the chemicals that our brain and nerves use to communicate.
In particular, they target three neurotransmitters that affect mood: serotonin, dopamine, and norepinephrine. And here’s where gut bacteria come in.
More than 90% of your serotonin and around 50% of your dopamine is produced in your gut. And your gut bugs help control how much is produced. Your gut microbiome also influences levels of norepinephrine.
In fact, some of your gut bacteria can directly produce dopamine and norepinephrine.
This begs the question: Do people with depression have a different gut microbiome than people without depression?
It seems so. One review looked at 17 studies, including data from 738 people with major depressive disorder and 782 controls without depression.
The authors found significant differences between the gut microbiomes of people with depression and those without.
So, all we have to do is boost the right bacteria to produce the right neurotransmitters and we’re done, right?
Sadly, it’s a little more complex. However, scientists are investigating, and some small studies have produced encouraging results.
The evidence so far
Most of the human studies are small, making it harder to extrapolate the findings to the population at large.
So, although we must keep our skeptical hat on, many of these studies have measured some benefits of probiotics.
For instance, one 8-week pilot study involving 10 participants found significant improvements in mood and sleep quality following a probiotic intervention with specific Lactobacillus helveticus and Bifidobacterium longum stains.
However, we should note that this study had no control group, so we can’t know whether simply being involved in the study helped boost mood (more on this effect later).
A slightly larger study also produced positive findings.
This time, the authors recruited 40 participants with depression — half of them received probiotic capsules that contained Lactobacillus acidophilus, Lactobacillus casei, and Bifidobacterium bifidum, while the other half acted as a control.
After 8 weeks, those who took the probiotic capsules each day reported fewer depressive symptoms at the end of the 8-week trial.
An even larger study that included 71 people also measured positive results after 8 weeks. There were improvements across the board, even for those taking a placebo.
This underscores the importance of a control group. Sometimes, simply being enrolled in a study and experiencing regular checkups can elicit mental health improvements for some people.
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That said, participants taking probiotics improved significantly more than those taking the placebo. Interestingly, the scientists didn’t identify any changes to the gut microbiome after 8 weeks of probiotics.
Not all studies have observed positive effects of probiotics on mood, though.
For instance, a study involving 69 people for 8 weeks saw no benefits when participants took Lactobacillus helveticus and Bifidobacterium longum.
However, in this study, the participants did not have a depression diagnosis. Instead, they were people who generally experienced low mood.
Could probiotic foods work?
Probiotic foods include natural yogurts, kimchi, kefir, sauerkraut, and some other fermented products.
Although most of the research into probiotics and depression focuses on supplements, some scientists have looked at probiotic foods. For instance, a study in South Korea used data from more than 26,000 people to look for links.
The authors found that compared with those who ate the fewest probiotic foods, those who ate the most had lower odds of having clinical depression.
And among individuals who did have depressive symptoms, they tended to be less severe in those who ate the most probiotic foods.
Although the study has limitations, the authors conclude that “probiotic food consumption is significantly linked with decreases in the prevalence and severity of depression in Korean adults.”
A future for probiotic treatments?
We still have a long way to go before doctors prescribe probiotics for depression, but it’s certainly not out of the question. A review on the subject, which includes data from 10 studies, concludes that probiotics “have great potential in the treatment” of depression.
Importantly, the authors also note that, at this stage, we don’t know which strains of bacteria will work best, how much people need to take, or how long treatment should last.
Probiotics may never cure depression, but it’s not too far-fetched to think they might help boost current treatments.
ZOE is at the leading edge of gut microbiome research. Our scientists are working hard to understand the many effects our gut bugs have on our health.
For instance, ZOE’s research has identified 15 “good” bugs associated with positive health markers and 15 “bad” bugs linked to negative health markers.
As gut microbiome research continues, it’s becoming clearer that a healthy gut microbiome is essential for good health.
A double-blind, randomized, placebo-controlled trial of Lactobacillus helveticus and Bifidobacterium longum for the symptoms of depression. The Australian and New Zealand Journal of Psychiatry. (2017). https://pubmed.ncbi.nlm.nih.gov/28068788/
Clinical and metabolic response to probiotic administration in patients with major depressive disorder: A randomized, double-blind, placebo-controlled trial. Nutrition. (2016). https://pubmed.ncbi.nlm.nih.gov/26706022/
Depression. (2021). https://www.who.int/news-room/fact-sheets/detail/depression
Effect of psychobiotics on psychometric tests and inflammatory markers in major depressive disorder: Meta-analysis of randomized controlled trials with meta-regression. Pharmaceuticals. (2021). https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8541446/
Gut feelings: A randomised, triple-blind, placebo-controlled trial of probiotics for depressive symptoms. Journal of Affective Disorders. (2019). https://pubmed.ncbi.nlm.nih.gov/31078831/
Gut microbiota variations in patients diagnosed with major depressive disorder—A systematic review. Brain and Behavior. (2021) https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8323045/
How antidepressant drugs act: A primer on neuroplasticity as the eventual mediator of antidepressant efficacy. Indian Journal of Psychiatry. (2010). https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3025168/
Long-term antidepressant use: patient perspectives of benefits and adverse effects. Patient Preference and Adherence. (2016). https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4970636/
Indigenous bacteria from the gut microbiota regulate host serotonin biosynthesis. Cell. (2015). https://www.cell.com/fulltext/S0092-8674(15)00248-2
Neurotransmitter modulation by the gut microbiota. Brain Research. (2019). https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6005194/
Peripheral dopamine controlled by gut microbes inhibits invariant natural killer T cell-mediated hepatitis. Frontiers in Immunology. (2018). https://www.frontiersin.org/articles/10.3389/fimmu.2018.02398/full
Probiotic food consumption is associated with lower severity and prevalence of depression: A nationwide cross-sectional study. Nutrition. (2019). https://www.sciencedirect.com/science/article/pii/S0899900718307329
The efficacy, safety, and tolerability of probiotics on depression: Clinical results from an open-label pilot study. Frontiers in Psychiatry. (2021). https://pubmed.ncbi.nlm.nih.gov/33658952/