Why Mental Illness Can Fuel Physical Disease

For people who suffer from depression or anxiety, being diagnosed with a physical health condition on top of a mental health issue can feel like the worst kind of bad luck. But recent research suggests that this type of dual diagnosis is more than just an unfortunate coincidence. Scientists are learning that seemingly unrelated psychological and physical issues may actually be closely connected.

Doctors once thought that the link between mental and physical health problems was purely behavioral. Depressed people are less likely to take their medications or practice healthy habits, for instance, so they get sicker. Sick people experience pain and impaired function, which affects their emotional state.

“But we have really come to learn that that’s not the case,” says Dr. David Gitlin, chair of the American Psychiatric Association’s Council on Psychosomatic Medicine and clinical vice chair at Brigham and Women’s Hospital. “These factors are certainly important, but there is also something physiological that’s happening.”

This mysterious mind-body connection seem to be at play in a new study published this week in the Journal of Investigative Dermatology, which focused on the physical and mental health of people with psoriasis, an autoimmune disease that causes red patches and flaky scales to form on the surface of the skin. Depression is common among people with psoriasis, who often deal with discomfort and social stigma related to their condition.

Researchers found that psoriasis patients diagnosed with depression were 37% more likely to also develop psoriatic arthritis—a complication that involves inflammation of and around the joints—than those without depression.

Depression can lead to behaviors that could trigger psoriatic arthritis or exacerbate an existing case, the authors say. For someone predisposed to the disease, factors such as lack of exercise, excess weight gain and poor diet can all affect the severity of symptoms. Yet the study authors controlled for many of these behaviors, and the association still held. This suggests that the depression itself, or the root cause behind the depression, has a direct influence on the development of psoriatic arthritis.

That makes sense, since psoriatic arthritis is triggered by inflammation in the body, says Gitlin (who was not involved in the study). Research over the last 20 to 30 years has shown that inflammatory processes can drive the development of depression, he says. “We now know that those processes are likely similar to those that drive some physical illnesses, as well.”

Elevated levels of the stress hormone cortisol might also be linking psychological and physical conditions, beyond just psoriatic arthritis. “We know that high levels of cortisol are associated with depression, and we know that they can contribute to an inflammatory state and to conditions like diabetes and heart disease,” says Gitlin.

Previous research has linked depression with an increased risk of conditions like stroke, diabetes and rheumatoid arthritis, and a 2016 study found that depressed mood—which included symptoms such as anxiety and fatigue—was as strong a predictor of heart disease as well-known risk factors like high cholesterol and obesity. Other mental-health diagnoses, including bipolar disorder and schizophrenia, have also been associated with increased risks of physical health problems.

But there’s a bright side. Treating depression and other mental health conditions (either with medication, talk therapy or a combination of both) may help improve physical symptoms or reduce the risk of future problems, says Gitlin, especially if doing so can lower cortisol levels and other markers of inflammation. Likewise, treating physical illnesses and getting symptoms under control can help improve mental health.

For scientists, further studies and a better understanding of the body’s inflammatory response may help fuel the development of treatments and preventive methods for a wide variety of diseases—both physical and psychological.

“Researchers are starting to look at anti-inflammatory agents to see if they can be helpful in treating both types of conditions,” Gitlin says, citing studies that have linked cholesterol-lowering statins with a reduced risk of depression, or antidepressants with a reduced risk of cardiovascular events. “We know inflammation is the singular common thread, and we’re getting better at figuring out new ways to target it.”