“I want to make a difference in the world. Not drag myself from A to B until I die,” wrote a sufferer of irritable bowel syndrome (IBS)—a condition characterized by symptoms of pain and cramping, bloating, and cycles of diarrhea and/or constipation. It’s a statement of extreme hopelessness. And it comes from a person whose diagnosis is a digestive one.

A digestive-related diagnosis, yes, but one that has known links with psychological conditions like anxiety and depression. In fact, of the 10-15% of the worldwide population that suffer from IBS, around 50% complain of significant psychological symptoms.

Doctors once believed that the brain was the cause of gut symptoms in IBS—after all, IBS is a disorder for which no biological cause can be identified (that is, a “functional” disorder). Yet there is now ample evidence that IBS does not have its origin in the brain. A 2016 study that tracked 1,900 Australians over time, for example, found that subjects who had IBS-like gut symptoms with no mood problems at the beginning of the study showed higher levels of anxiety and depression one year later. And of all those with a functional gastrointestinal (GI) disorder at the end of the study, two thirds had seen gut symptoms show up before the psychological symptoms, rendering it unlikely that the brain had initially caused the GI symptoms.

According to the current medical consensus, the psychological symptoms of IBS could at most play into the perceived severity of the GI symptoms and take away from a sufferer’s quality of life.

Stress or other psychological factors don’t cause the gut symptoms of IBS, but they do accompany them much of the time. Yet so far, researchers haven’t been able to pinpoint something else that could cause both the psychological and the gut symptoms.

A more complex understanding of the disorder is emerging by looking at the gut microbiota of affected individuals. Many studies have now shown alterations of the normal gut microbiota in those with IBS, even if the exact nature of the changes varies from study to study. Overall, however, those with the IBS diagnosis seem to have a community of gut microbes that is less diverse and more unstable than their healthy counterparts.

To be clear, the gut microbiota is not shown to cause IBS—neither the gut nor the brain symptoms. But scientists are indeed certain that the two-way communication network between the GI tract and the brain (called the gut-brain axis) is somehow involved in IBS. And the gut-brain axis is known to be influenced by the gut microbiota. For example, by studying mouse models, scientists have found that conditions of stress (either acute or chronic) can change the gut environment in a way that shifts the gut microbiota composition; this altered microbiota can itself be responsible for ongoing anxious and depressive-like behaviours in the mice.

Backing the concept of the human gut microbiota’s involvement in IBS-linked brain symptoms is a 2012 study that found specific microbiota patterns (called ‘signatures’) correlated not only with the speed at which materials moved through the colon, but also with reported levels of depression.

Currently scientists’ best guess is that the gut microbiota in IBS is a factor that either initiates psychological symptoms or contributes to their persistence. But much more human research is required to untangle the complex origin of this disorder.

Because symptoms and microbial communities in IBS seem to be a moving target, it could be years before doctors really understand what IBS is about. Yet links between IBS and the gut microbiota, as they emerge, could be good news. They raise the possibility of modulating the gut microbiota as a treatment for either gut or brain symptoms of the disorder. If it turns out that some of these troublesome symptoms depend on changes to the gut microbiota, targeting the gut with interventions like probiotics could make a big difference for sufferers.

Some probiotics research is already ahead of the game, as evidence exists for the idea that targeting the gut microbial community with live microorganisms can reduce pain and overall severity of symptoms in those with IBS. Research on probiotics to address anxiety and depression in IBS, however, is still scant. But an important goal in the years ahead will be to find out if probiotics are a useful tool for addressing a disorder that’s not just ‘all in your head’.

Kristina Campbell
Kristina Campbell
Science writer Kristina Campbell (M.Sc.), from British Columbia (Canada), specializes in communicating about the gut microbiota, digestive health, and nutrition. Author of the best selling Well-Fed Microbiome Cookbook, her freelance work has appeared in publications around the world. Kristina joined the Gut Microbiota for Health publishing team in 2014.  Find her on: GoogleTwitter