Untangling the link between depression and gut disease

Depression and gastrointestinal disorders frequently occur together. UC Irvine experts wondered if knowing which tends to be diagnosed first could shed light on the well-established but not yet fully understood gut-brain connection.

They discovered the answer depends on the type of gastrointestinal disease.

Researchers analyzing large-scale electronic health records found a striking difference among three common conditions. Depression was typically diagnosed before irritable bowel syndrome. With Crohn's disease, the pattern was reversed, with the gastrointestinal disorder more often diagnosed first. Ulcerative colitis fell somewhere in between.

The findings add nuance to scientists' knowledge of the gut-brain axis and suggest that the relationship between mental and gastrointestinal health may differ with the ailment.

"IBS and Crohn's disease are fundamentally different diseases," says the study's corresponding author, Gha-Hyun Jeffrey Kim, Pharm.D., Ph.D., assistant professor of clinical pharmacy practice in UC Irvine's School of Pharmacy & Pharmaceutical Sciences and a member of the Robert A. Mah Molecular Innovation Center. "IBS involves changes in how the brain and gut communicate, including how gut signals are processed. That means problems on the brain side, like depression, can show up first and then shape what happens in the gut."

Crohn's disease, an inflammatory bowel disease, presents a different picture.

"In Crohn's disease, intestinal inflammation is the primary driver, and it reaches the brain through inflammatory signals and through the toll the disease takes on daily life," Kim says. "That fits the pattern we found. Crohn's disease was more often diagnosed before depression."

Different diseases, different patterns

The study first examined records from 633,547 participants in the National Institutes of Health's All of Us Research Program, a national research cohort designed to include populations historically underrepresented in biomedical research.

Experts then confirmed the findings using the University of California Health Data Warehouse, which pools de-identified records from more than 11 million patients across six UC academic health centers, including UCI Health. The analysis of diagnosis order focused on patients with both depression and a gastrointestinal condition whose first recorded diagnoses were more than six months apart.

Across both datasets, researchers found consistent differences among gastrointestinal diseases. In patients with IBS and depression, depression appeared first in the medical record in 73.2 percent of cases in the All of Us dataset and in 65.2 percent of UC dataset cases.

Crohn's disease showed nearly the opposite pattern. Depression was recorded first in 46.8 percent of cases in All of Us and in 35.1 percent of UC dataset cases, meaning Crohn's disease was more often diagnosed first. Ulcerative colitis showed a more mixed pattern.

The distinction matters because IBS and inflammatory bowel diseases such as Crohn's and ulcerative colitis are different conditions with different underlying biology. Grouping gastrointestinal diseases together can therefore obscure potentially meaningful differences in how they intersect with mental health.

A two-way conversation between the brain and gut

Scientists have increasingly studied the gut-brain axis, the complex communication system connecting the gastrointestinal tract and the brain. Kim says the new findings support looking at that relationship more specifically rather than assuming the same pathway applies to every gastrointestinal condition.

For IBS, the depression-first pattern points to a possibly stronger brain-to-gut contribution. Stress reactivity, increased sensitivity to signals from the gut and changes in serotonin signaling may all play a role in shaping symptoms. Low-dose antidepressants are already used sometimes in IBS treatment to target communication between the brain and gut.

Crohn's disease may involve a different pathway. Chronic intestinal inflammation can release inflammatory signals that affect the brain, while pain, hospitalization, surgery, medication side effects and the broader burden of living with a chronic disease can also affect mental health.

The study does not establish that depression causes IBS or that Crohn's disease causes depression. Because researchers retrospectively analyzed diagnoses recorded in electronic health records, the findings identify patterns in the timing and association of diagnoses rather than proving biological cause and effect.

That distinction is important: A diagnosis appearing first in a medical record does not necessarily mean the underlying condition began first biologically.

Recognizing symptoms that might otherwise be missed

The results could have practical implications for how clinicians think about patients experiencing both mental health and gastrointestinal symptoms.

For people with inflammatory bowel disease, particularly Crohn's disease, the findings reinforce the importance of paying attention to depression following diagnosis. For patients already being treated for depression, persistent gastrointestinal symptoms may warrant further evaluation rather than automatically being attributed to their mental health condition.

"These patterns can help inform what clinicians watch for - from persistent gut symptoms in patients with depression to changes in mood in patients with Crohn's disease," Kim says. "The opportunity is to recognize symptoms that might otherwise be overlooked when care focuses on just one condition."

The study also uncovered another intriguing association: Antidepressant use was linked with a lower likelihood of a subsequent gastrointestinal diagnosis, even after researchers accounted for several other health factors.

The finding does not demonstrate that antidepressants prevent gastrointestinal disease. But, Kim says, it raises an important question for future research.

Antidepressants are already used to treat IBS, serotonin is a major signaling molecule in the gastrointestinal tract, and some antidepressant medications have anti-inflammatory effects. Together, Kim says, those factors raise the possibility that effectively treating depression could also influence gut health, a hypothesis that would need to be tested directly in future studies.

For him, the broader message is that the connection between the brain and gut may be less like a single pathway and more like a two-way conversation, one that changes depending on the disease.

Understanding those differences could eventually help clinicians identify problems earlier, tailor screening to individual conditions and ensure that neither mental nor gastrointestinal symptoms are overlooked simply because a patient is already being treated for the other.