Lack of exercise and other unhealthy behaviours could explain why people with heart disease and depression seem to be at higher risk for heart trouble, a new study suggests.

Depression has long been recognized as a risk factor for the development of cardiovascular disease in people who are healthy. It can also increases the risk of a second heart attack among heart patients. Little is known, however, about what may cause the link between the two diseases.

In Wednesday's issue of the Journal of the American Medical Association, researchers said symptoms of depression were associated with a 31 per cent higher rate of cardiovascular events such as heart attack, stroke, heart failure and death over the average 4.8 years of follow-up, after taking into account other existing conditions and the severity of cardiac disease.

After adjusting for lack of physical activity, smoking and the tendency to stop taking medications properly, there was no longer a significant association between depressive symptoms and cardiovascular events, the researchers said.

"It's really difficult to tease out where the independent effect of depression is happening and where the other effects of cardiac disease are ending," said researcher Mary Whooley of the Veterans Affairs Medical Center in San Francisco.

"But the message is that once you account for those health behaviours the patients would not longer have this excess risk of cardiac events. The risk would go from one in 10 down to one in 15."

Depression may also lead some patients to exercise less, deepening their depression, she said.

The researchers looked at 1,017 heart patients whose conditions were stable. The average age of the 199 participants with depressive symptoms was 63, and the 818 participants without depression symptoms had an average age of 68.

Participants answered questions about the frequency of their depressive symptoms and exercise levels, such as whether they had done 15 to 20 minutes of brisk walking, swimming or played recreational sports in the past month. They also had echocardiograph, heart rate variability and cholesterol tests.

In September, the American Heart Association recommended screening heart disease patients for depression. Since then, a review of previous studies concluded there is little evidence that heart patients with depression live longer or fare better in the long run if they are treated with antidepressants and talk therapy.

Whooley and her colleagues concluded it is becoming more urgent to find ways of not only reducing depressive symptoms but also target how the depression leads to cardiovascular events.

With files from Reuters