Patients diagnosed with a mood disorder such as depression or anxiety are frequently dealing with another chronic health condition—a recent study found that 50 percent of patients with depression also had chronic pain.
This wasn't a surprise: Researchers and doctors have known about the association for years. "We have decades' worth of data on the relationship between depression—though not as strongly with anxiety disorders—and comorbidity with almost all medical disorders," says David Gitlin, the chair of the American Psychiatric Association's Council on Psychosomatic Medicine and an assistant professor of psychiatry at Harvard Medical School, who was not involved with the study.
The new study, published in May in the the Journal of Affective Disorders, strengthens that association by adding data from South America, where little had been previously documented. Researchers from Columbia Mailman School of Public Health, in collaboration with psychiatrists and epidemiologists from the University of São Paulo, analyzed data from more than 5,000 residents of the greater São Paulo area who had been diagnosed with a mood disorder or an anxiety disorder.
The researchers found that 50 percent of people with a mood disorder also reported suffering from chronic pain. Other chronic health issues were also prevalent—33 percent of participants with mood disorders had respiratory issues, 10 percent had heart disease, 9 percent had arthritis, and 7 percent had diabetes. There were similar numbers for people with anxiety disorders; people who had two or more chronic diseases were much more likely to be diagnosed with mental illness.
Watch More From VICE: Stopping HIV: The Truvada Revolution
"This study may serve as a model of mental and physical comorbidity patterns that might be seen in other megacities within Latin America or other rapidly developing nations," the researchers wrote.
Though well established, the association is still just that; scientists are just starting to understand the mechanisms that might explain the relationship between chronic pain and mood disorders. It's become pretty clear that the conditions are bidirectional—depression can make people more likely to develop physical disorders, in part because they are less motivated to do things like exercise and eat well, and people with physical disorders are more likely to develop depression, because, well, being sick really sucks.
But the two conditions have something in common on a more fundamental level: The same chemicals in the brain that modulate mood are the same ones that affect a number of physical systems in the body, and especially pain, Gitlin says. Studies have shown that people suffering from depression are more sensitive to pain.
Those receptors can be altered by inflammation, the body's chemical response to trying to fix something that's wrong, which seems to plays a role in both. Chronic illnesses create an increased inflammatory state, which could in turn affect the brain. The increased presence of cytokines, compounds from the immune system that drive inflammation, has been known to increase the symptoms of depression. Conversely, some types of psychiatric conditions can throw off how much the body produces cortisol, a stress hormone, which can boost inflammation even more.
"It's likely that there is this end-pathway process of worsened inflammation in the body, particularly the brain, that underlies both [chronic health and psychiatric conditions], and that's why there is more comorbidity," Gitlin says.
Thinking of chronic physical conditions and psychiatric disorders as "two manifestations of a singular process" can help doctors effectively treat both, Gitlin says. That's not to say that taking the right antidepressant can make your heart disease simply go away, he adds. But treating a chronic health condition can ease depression, and treating depression can make medical symptoms less severe, or at least improve the experience of being ill.
Now researchers are focusing on better understanding the causal relationship between mood disorders and chronic health conditions. They're still in the early stages, Gitlin says. But they're starting to parse through the effects of different inflammatory factors and how that can help doctors choose the most effective treatment for patients. "For patients with depression, are there specific causal factors in the setting of a medical illness that would make them more or less likely to respond to particular treatments?" Gitlin asks, noting that the current process for finding the right antidepressant for a specific patient is like "throwing the kitchen sink at people."
The questions that remain are not easy ones, but their answers can help millions of people worldwide better treat their physical and psychiatric conditions.
Read this Next: I Thought I Was Depressed But It Turns Out I'm Bipolar