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Doctors have long known of an association between psychological and physical health, but mental illness wasn’t considered to be a major risk factor for ailments like heart disease, until now.

Depression has been linked to physical health risks including digestive disorders, chronic pain, stroke and even early death. Depression is also closely tied to heart health: New research suggests that it may be one of the top risk factors for cardiovascular disease.

The relationship seems to run both ways. Patients with heart conditions are more likely to become depressed as a result of their illness, and otherwise healthy people with depression are significantly more likely to develop heart disease than the general population.

The German study, published last month in the journal Atherosclerosis, found depressed mood and exhaustion to be on par with high cholesterol levels and obesity as a risk factor for cardiovascular disease ― the only greater risk factors being smoking and high blood pressure.

“An association between psychological health and disease has been appreciated for centuries,” Ahmed Tawakol, a professor of medicine at Harvard Medical School who has conducted research on the connection between stress and heart health, told The Huffington Post. “However, only over the past decades has mounting evidence suggested that stress and depression may be more than simple markers of heart disease; they might be important causes.”

How Depression Hurts The Heart

For the study, the researchers observed 3,500 German men between the ages of 45 and 74 years, over a period of 10 years, collecting information about their physical and mental health. The researchers analyzed the data, comparing clinical depression with other known risks for heart disease.

The analysis showed that the risk of fatal heart disease was as high in men with depression as it was in men with high cholesterol levels or obesity. In total, 15 percent of cardiovascular-related deaths were attributed to depression.

The findings point towards a greater need for heart health screenings among patients with depression, and mental health screenings for cardiovascular patients.

“In high risk patients, the diagnostic investigation of co-morbid depression should be standard,” Dr. Karl-Heinz Ladwig, a professor of psychosomatic medicine at the Technical University of Munich and the study’s lead author, said in a statement.

Just how depression impacts heart health isn’t entirely clear yet, but it’s likely that stress hormones play a role. It’s also still unclear whether treatments for depression could also improve heart health.

“Large randomized trials are needed to evaluate whether treating stress and depression results in reductions in heart disease,” Takawol said. “In the meantime, when treating individuals with cardiovascular disease, physicians should consider screening for these psychological risk factors, and addressing them if found.”

The Mind-Heart Connection

The link between psychological health and heart health is well established. One 2014 study, for example, showed that people with psychiatric conditions including depression, bipolar disorder, schizophrenia and neurotic disorders were twice as likely to have had a stroke or experienced heart disease than the general population. In another recent study, cardiologists showed how psychological stress can trigger neurological and physiological changes that create the conditions for heart attacks.

The new findings add heft to the growing body of work showing depression to be a strong risk factor for mortality from cardiac-related causes, leading some researchers to even claim that a depression questionnaire could be more informative than an intracardiac electrocardiogram ― a standard tool used by physicians to test heart activity ― for predicted heart problems.

The mechanisms underlying these associations, however, remain poorly understood. Most explanations have pointed to behavioral factors, suggesting that people with mental illness often adopt poor health behaviors like smoking, eating unhealthily, drug and alcohol consumption and an overly sedentary lifestyle, which in turn elevate the risk of heart disease. In addition to these behavioral considerations, other complex emotions and biological factors are also likely at play.

In the meantime, leading a healthy lifestyle been shown to be effective in preventing and treating both depression and heart conditions.