Could the key to treating depression lay in your belly? A new review of research suggests that using probiotics to alter the trillions of microorganisms living in the intestinal tract could help ease symptoms of depression. Although there are some caveats to the research, the meta-study provides further evidence that probiotics could offer a new avenue to treat some mood disorders in the future. 

Reported in the journal BMJ Nutrition, Prevention, & Health, researchers from the Brighton and Sussex Medical School in the UK reviewed seven different scientific studies that assessed the effect of anxiety and/or depression after a course of probiotics and/or prebiotics, foods or supplements that induce the growth or activity of beneficial microorganisms in the gut microbiome. 

All of these studies, which used a range of different methods, showed "significant improvements" in depression symptoms when measuring the effect of taking pre/probiotics compared with no treatment or a placebo. Despite this apparent success, the evidence was not solid enough to conclude they helped with easing anxiety. 

The research looked at 12 different probiotic strains, with some investigating just one probiotic strain and others looking at combinations of multiple strains, including Lactobacillus acidophilus, Lactobacillus casei, and Bifidobacterium bifidium. Of this bunch of 12 probiotics, 11 were found to be potentially useful.

The review did not investigate why probiotics appear to ease depression, but the researchers draw on previous studies to explain why this might be the case. The gut and brain are deeply intertwined through what’s known as the gut-brain axis. Some of this is known to be mediated via microscopic pals that live in our intestinal tract, namely bacteria and fungi. Once again, the exact mechanism isn’t clear, but it’s thought gut microbes could help to produce, degrade, or modify certain neuroactive compounds. 

It’s also worth remembering that your gut is effectively the body’s “second brain,” equipped with its own network of hundreds of millions of neurons that play a role in this fascinating interplay between microorganisms and mood.

While independent experts in the field have described the new study as “good quality research,” they warned the studies were relatively small and short-term, so they should only be considered preliminary data for now. They also warned that heading to your local health food shop and stocking up on probiotics is not a good idea if you’re suffering from depression or any other mental health problems.

“Probiotics often contain different strains of bacteria and we do not know if the supplements, sachets and fermented milks you find on supermarket shelves will work, or only those probiotics used in the research studies,” commented Kevin Whelan, professor of Dietetics and head of Department of Nutritional Sciences at King’s College London, who was not involved in the study.

However, he added: “Given probiotics were not shown to worsen depression or cause other side-effects, then there is unlikely to be any harm in someone with depression trying probiotics in addition to the treatment recommended by their doctor.”