Emotional issues such as depression used to be considered problems that affect the brain alone, but more recently it has become clear that a person’s mental state is closely linked to the function of his or her body, including the heart.

It’s estimated that some 21 million Americans have suffered an episode of major depression, which is defined as a depressed mood or loss of interest or pleasure lasting at least two weeks. This can include difficulties with sleep or eating, loss of energy or concentration, or persistent questions about self-worth.

The question of whether depression is an independent risk factor for heart disease or a secondary effect of lifestyle habits common to those with emotional difficulties contributes to the uncertainty about the relationship between depression and heart disease. After all, surveys show that people who are depressed are more likely to:

• Eat unhealthy foods

• Drink alcohol to excess

• Smoke

• Neglect to exercise

But mounting evidence indicates that depression itself is an independent risk factor. The American Heart Association has listed depression as a risk factor for poor prognosis after a heart attack or incident of unstable angina — which is chest pain at rest due to reduced blood flow to the heart.

One study found that the risk of death after a heart attack was three times greater for people who suffered from depression. Another study showed that patients are twice as likely to die if they develop depression after being diagnosed with heart disease.

In fact, depression is the strongest predictor of death in the first decade after a heart disease diagnosis.

People who suffer from depression have a 64 percent greater risk of developing coronary artery disease; and depressed patients with heart disease are 59 percent more likely to have a future cardiovascular event such as a heart attack, or suffer cardiac death.

When a person is first diagnosed with heart disease, suffers a heart attack, or undergoes heart valve surgery, bypass, or another major cardiac procedure, it’s normal to feel depressed or anxious, or have trouble sleeping. But if such problems continue, they could be signs of major depression.