Heart disease and depression often go hand in hand. Long-term studies have found that people with depression have a significantly higher risk of subsequent heart disease, and vice versa. Recent research has revealed that the link begins at an early age and is probably caused by chronic inflammation.

A new study in the November 2014 issue of Psychosomatic Medicine by researchers in the U.S., Australia and China examined data from an ongoing study of health among Australians. The researchers looked at the scores of 865 young adults on a questionnaire that assesses depression symptoms and other measures of mental health. They also examined measurements of the internal diameter of the blood vessels of the retina, a possible marker of early cardiovascular disease.

After controlling for sex, age, smoking status and body mass index, the investigators found that participants with more symptoms of depression and anxiety had wider retinal arterioles than others, which could reflect the quality of blood vessels in their heart and brain. “We don't know if the association is causal,” explains study co-author Madeline Meier, a psychology professor at Arizona State University. “But our findings suggest that symptoms of depression and anxiety may identify youth at risk for cardiovascular disease.”

Other research shows that people with depression have more inflammation throughout their body and nervous system. “One theory is that stress and inflammation could play a causal role in depression,” Meier says. Such chronic inflammation is also a risk factor for cardiovascular disease. The relationship is complex: in some people, inflammation seems to precede depression and heart disease; in others, the disorders seem to cause or exacerbate the inflammation.

A study published last year suggests that atypical depression, one particular type of the disorder, may be more strongly associated with inflammation—and thus with cardiovascular problems. Atypical depression accounts for 15 to 40 percent of depression cases. It is characterized by more flexibility in mood than is found in typical depression—for instance, mood might improve in response to positive events—along with symptoms such as increased appetite, feelings of heaviness in the limbs and sensitivity to interpersonal rejection.

In the study, which was reported in the Journal of Behavioral Medicine, researchers found that blood levels of a telltale immune protein in young adults with atypical depression were at least 55 percent higher than in those with other types of depression or no depression. The number of participants with atypical depression who had readings that indicated high cardiovascular risk was almost double that of the others.

The good news is that treating depression symptoms may indeed help prevent heart disease, according to a trial reported in 2013 in Psychosomatic Medicine. Patients with depression—some with and some without heart disease—either got 12 months of treatment with antidepressants and psychotherapy, or they were simply advised to follow up with their primary care provider. Over eight years the patients without heart disease whose depression was treated had a 48 percent lower risk of heart attack and stroke than those who were not treated. There was no change in risk for patients who already had heart disease at the start of the study, further underscoring the need for timely intervention.