Gut Bacteria's Role in Anxiety and Depression: It’s Not Just In Your Head

In the mid- to late 2000s, John Cryan of Ireland’s University College Cork was among the first to explore gut microbes’ effects on the brain. A neurobiologist by training, Cryan had shown that rats stressed from birth later showed signs of both irritable bowel syndrome (IBS) and mood disturbance. “When they grew up,” Cryan says, “they had a whole-body syndrome.” This finding echoed doctors’ observations that many patients with digestive symptoms also had mental health issues, and vice versa.
When researchers at Cryan’s lab sampled gut bacteria from stressed-out rats in 2009 and sequenced them, they hit on something surprising: Stressed-out animals — those more prone to mental health issues — had a less diverse assortment of gut microbes, or microbiome, than their more relaxed counterparts. “It got us thinking — if you stress an animal, [maybe] there’s a signature in the microbiome that’s persisting,” Cryan says.