Having depression and heart disease is deadlier than having either condition alone, a new study suggests.

Results showed that adults with both coronary heart disease and depression were five times more likely to die during the five-and-a-half year study than a healthy person, the study said.

Those with only coronary heart disease were 67 percent more likely to die in that time period than a healthy person, and those with only depression were twice as likely to die in that time period as a healthy person.

"People with depression and heart disease more than double their risk of death [relative] to those who have heart disease alone or depression alone," said study researcher Hermann Nabi, of the Centre for Research in Epidemiology and Population Health at Paul-Brousse Hospital in Paris.

People should seek help for symptoms of depression, especially when they are diagnosed with heart disease, Nabi told MyHealthNewsdaily.

Previous studies found depression to be a risk factor for heart disease, and a 2009 British Journal of Psychiatry study found that people who are depressed and people who smoke have equal risks of dying over the sameCK time period.

In the new study, researchers tracked 6,000 middle-age British adults for five-and-a-half years. In that time, 170 people died, including 47 from heart attack or stroke.

Having the two conditions simultaneously also tripled the risk of death from any cause over that time period, and quadrupled the risk of dying from a heart attack or stroke, according to the study.

Further research is needed to find why the relationship between depression and heart disease is so deadly, Nabi said.

However, some medical reasons might include inflammation or high blood pressure, which occur in people with both conditions. Lifestyle-related reasons could include smoking, physical inactivity and high alcohol consumption, he said.

The study was published online on Sept. 15 in the journal Heart.