Research update

Probiotics, taken by themselves or in combination with prebiotics, may help to ease depression by altering gut bacteria, according to a systematic review published in BMJ Nutrition Prevention & Health.

Interest in the possible influence of gut microbes (the range and number of bacteria in the gut) on mental health has grown; partly fuelled by a growing understanding of the two-way relationship between the brain and digestive tract, known as the gut-brain axis.

From 2016-2017, 1.4 million people in the UK were diagnosed with mental health issues; 53% had anxiety or stress related disorders, while 33% had depression. Currently, prescribed treatments for mental health issues involve pharmaceuticals, many of which have negative side effects. This study aimed to investigate whether probiotics and prebiotics were associated with an improvement in symptoms. Probiotics are foods that broaden the profile of helpful bacteria in the gut while prebiotics are compounds that help these bacteria to flourish.

After searching for studies published between 2003 and 2019 that had investigated the potential therapeutic contribution of pre-and probiotics in adults with depression and/or anxiety disorders, the team found seven out of 71 met the inclusion criteria.

All seven investigated at least one probiotic strain, while four looked at the effect of combinations of multiple strains.

The studies varied considerably in design, methods used, and clinical considerations, but all concluded that probiotic supplements, alone or with prebiotics, may be linked to measurable reductions in depression.

Each study also showed a significant fall or improvement in anxiety symptoms and/or clinically relevant changes in biochemical measures of anxiety and/or depression with probiotic or combined pre-probiotic use.

However, the authors cautioned that it was difficult to draw any firm conclusions about whether the effects were long-lasting, or whether there might be any unwanted side effects associated with prolonged use, as all of the studies were short-term and used small samples.

Nevertheless, the authors suggested that on the basis of the preliminary evidence to date, pre- and probiotic therapy may warrant further investigation.

They suggested that probiotics may help to reduce the production of inflammatory chemicals, such as cytokines, as is the case with inflammatory bowel disease. Or they may help direct the action of tryptophan, a chemical thought to be important in the gut-brain axis in psychiatric disorders.

However, as anxiety disorders and depression affect people very differently, the authors said that treatment approaches should take account of these complexities, stating: "In this way, with a better understanding of the mechanisms, probiotics may prove to be a useful tool across a wide range of conditions".