A seasoned heart specialist knows that depression is one of the early signs of heart disease - but has never understood why.
Depression can also make the problem worse in patients who have a heart problem, and it can cause all sorts of problems after by-pass surgery.
In fact, healthy people with depression usually have worse general health than people with specific problems such as asthma, diabetes, arthritis or heart disease.
But nobody has been sure why this is. To find out, researchers from the VA Medical Center in San Francisco tracked 1,017 patients with heart disease for nearly five years.
Those who were depressed were also 50 per cent more likely to suffer further heart problems, such as a heart attack or stroke.
But when researchers looked at lifestyle issues, such as exercise, the association went away. In other words, depression didn't cause heart problems, and heart problems didn't cause depression - but both were symptoms of a poor lifestyle, and lack of exercise.
(Source: Journal for the American Medical Association, 2008; 300: 2379-88).