The long-standing connection between depression and heart problems might be traceable to the fact that depressed people are less physically active than others, a new study of heart patients shows. A greater tendency in depressed people to smoke and to fail to take medications regularly may also play a role, researchers report in the Nov. 26 Journal of the American Medical Association.

Previous studies have suggested that depression seems to increase the risk of heart problems in people with no history of them, and that depression often coincides with worsening health in people who have an existing heart condition. Yet the medical reason for this association is unknown, and it’s not even clear whether depression leads to heart problems or vice versa.

Scientists have investigated possible side effects from antidepressant drugs, chemical imbalances in the brain, stress, diet, chronic inflammation, smoking and a lack of exercise as reasons for the link between depression and heart problems.

To sort out these possibilities, researchers began a study in 2000, identifying people visiting clinics in the San Francisco Bay area who had chronic but stable coronary heart disease. Of the 1,017 patients enrolled, tests showed that one-fifth, average age 63, had symptoms of depression at the start of the study. The other four-fifths were age 68 on average and weren’t depressed. Researchers monitored the health of all the volunteers using lab tests, checkups, interviews, death records. Follow-up averaged five years, and researchers logged the final data entries in early 2008.

During the study, the scientists periodically asked volunteers whether they had had any episodes of “heart trouble” or stroke that had necessitated a visit to a hospital. In cases where a volunteer had died or couldn’t respond, relatives or other caregivers provided information.

By the end of the study, 341 incidents were reported. These included cases of heart failure, heart attacks, strokes or deaths. After accounting for past medical histories and other differences between the depressed and nondepressed groups, the researchers calculated that people with depression had a 31 percent increased risk of having at least one such incident during the study, says study coauthor Mary Whooley, an internist at the San Francisco Veterans Affairs Medical Center and the University of California, San Francisco.

The depressed people were also slightly more likely to have high levels of inflammatory proteins in the blood, which may have explained some of these participants’ added coronary risk. Inflammatory cells and proteins contribute to plaque formation and vessel damage.

But the clearest differences between groups were behavioral, Whooley says. When researchers accounted for differences between the groups in smoking habits, exercise habits and discipline in taking medications, the heart risk apparently imparted by depression evaporated.

Meanwhile, the depressed people were nearly twice as likely to smoke and were more likely than the nondepressed group to fail to take medications on schedule. The depressed group also exercised less.

“This particular finding is important,” says cardiovascular epidemiologist Viola Vaccarino of Emory University in Atlanta. “In this particular group, behavioral risk factors, especially low physical activity, seem to explain away the depression risk.”

But she cautions that this explanation might not hold for other groups. For example, it’s unclear whether these findings apply to people who are outwardly healthy with no signs or history of heart trouble, but may nonetheless be at risk of heart disease.

On the other end of the spectrum, these findings also might not apply to people with acute coronary ailments, such as recurring chest pain. “It doesn’t really make any sense to ask them to up their physical activity,” Vaccarino says.

Meanwhile, Whooley and her coauthors note that it’s also difficult to determine whether a relative lack of physical inactivity is the cause or the result of depression, since the effect probably goes both ways.

Whooley and Vaccarino agree that it can be very difficult to change the behavior of depressed patients, who often aren’t very motivated, even while on medication. “They’ll [exercise] for a few months, then stop,” Whooley says.

She hopes these new findings make doctors more aware of the risks that depressed patients with heart disease run in maintaining a sedentary lifestyle and other detrimental behaviors.

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