A Healthy Gut Might Help People With Depression

Our gut has millions of good, live bacteria that help keep us healthy. These bacteria are called probiotics. The probiotics are part of the microbiota which also includes microorganisms such as viruses and fungi. The gut microbiota helps with bodily processes such as digestion and immune defences. We can find probiotics in foods such as yoghurts, fermented cheeses, pickles, dark chocolate, and pills that we can quickly get over the counter.

It is exciting to learn that two-way communication exists between our brain and gut microbiota. Scientists suggest that these good bacteria might help relieve low mood and that healthy microbiota might help healthy people keep healthy brains. But what about people who suffer from ongoing, severe sadness or cannot feel pleasure most of the time? These people suffer from clinical depression. May probiotics help them as well?

To answer this question, a team of scientists from King’s College London, led by Viktoriya Nikolova, recollected for the first time the past scientific studies involving clinically depressed people taking probiotic pills. They wanted to put them together to investigate if the scientific community agreed on whether probiotics could be a successful treatment for clinical depression. This type of research is called a systematic review, where you find, compare, and link past studies that worked on a specific question.

The researchers made sure each paper they selected had been published and answered the question of whether probiotics could help reduce clinical depression symptoms. They also chose research that compared a group of depressed people taking the probiotics with another one who did not. They checked the best they could that any differences in the reduction of depression could be traced back to taking probiotics and weren’t due to any other reason. After a thorough search in well-respected scientific online databases, the researchers picked 18 papers. Three of them met the requirements mentioned.

The three studies included 229 clinically depressed people. Each study lasted eight weeks. Two studies were done in Iran, and the participants took probiotics or a placebo (a substance that does not affect the body) in addition to antidepressants. The third study was done in New Zealand, and the participants took only probiotics or a placebo. The three studies used scientifically reliable psychological questionnaires to measure whether there was a reduction in depression.

So, what were these studies’ results? When the researchers linked the three studies as one single group, they concluded that the differences in the reduction of depression might have happened by chance and not necessarily because of having taken probiotics. However, when they analysed each study individually, they saw that the two Iranian studies showed that the people taking probiotics had a larger reduction in their depression than the placebo group. However, the New Zealand study found no differences between the two groups. It is interesting to remember that in the New Zealand study, the participants did not take antidepressants, and they self-reported their depression. In contrast, a mental health professional diagnosed the Iranian participants with depression, and all participants were taking antidepressants. With these differences in mind, the King’s College researchers checked the results excluding the New Zealand study and they saw a large impact of probiotics in helping reduce their depression. Therefore, they concluded that combining the probiotics with antidepressant medication might help reduce depressive symptoms. Moreover, the combination helps more than taking only antidepressants.

A limitation of this review is that it included a tiny number of studies. Science builds on studies replicating studies over and over. The purpose of a systematic review is to gather similar studies and see if they arrive at the same findings. If they do, science might have discovered facts that could lead to more effective therapies for a given illness. In this case, the reviewers could have had more participants by including, for example, unpublished studies. As long as they analysed the studies’ strengths and weaknesses, involving more participants could have allowed them to reach stronger probable conclusions. Moreover, regarding probiotic dosages and strains, there is a poor understanding of which ones are associated with better outcomes. Therefore, by adding more studies, the researchers could have created comparable groups depending on different probiotic-dosage ranges and strains. 

Importantly, we must be aware of the diversity in the participants – men and women together, different levels of severity of symptoms, types of medication history and dosage of probiotics. With such variety, it is impossible to arrive at reliable conclusions for a specific group. For example, we know that most of the participants in the review were women, and we do not know the severity of symptoms for all participants. Therefore, the results might reflect the effects of probiotics on women or on people with a level of depression that the study ignores. Besides, having had a mix of participants taking and not taking antidepressants added confusion to the results. Nevertheless, the reviewers were aware of this problem and excluded the study without antidepressants. This way, they highlighted a reason for future studies to focus separately on participants who are either taking or not taking antidepressants and report results accordingly. Moreover, it is important to remember that antidepressants might benefit depressed people by rewiring their brain connections. An intense, in-depth change such as brain-rewiring might as well affect the action of probiotics on the brain. That is why it is so important to conduct future studies separating the people taking antidepressants from those who are not.

Nevertheless, this review is valuable as it shows, for the first time, that science suggests that probiotics might be a successful treatment for depression in clinically depressed people when given in addition to antidepressants. This could mean, for example, people needing a lower dose of antidepressants and gaining the same beneficial effects. Also, it might benefit people who do not respond to antidepressants. Combining them with probiotics might make antidepressants act differently. However, it is important to remember that science needs more studies showing evidence of the beneficial role of probiotics on clinical depression. This paper put on the table that there are very few published studies on the topic, and we cannot draw firm conclusions on whether probiotics are helpful. Still, the results of the existing studies are promising and hopefully encourage the scientific community to keep researching this relevant topic.

References

Nikolova, V., Yawar Zaidi, S., Young, A. H., Cleare, A. J., & Sone, J. M. (2019). Gut feeling: randomized controlled trials of probiotics for the treatment of clinical depression: Systematic review and meta-analysis. Therapeutic Advances in Psychopharmacology, 9, 1-7. https://doi.org/101177/2045125319859963