Being depressed may raise your risk of heart problems, researchers have found.
Depression may have a physical effect
A study by Emory University in Atlanta found depressed people are more likely to develop an irregular heart beat - which can put them at risk of sudden death.
The study on twins found that the worse the depression, the higher the risk.
Details were presented at a meeting of American College of Cardiology.
The research found that people who were depressed were more likely to show symptoms of a condition called decreased heart rate variability (HRV).
HRV is a measure of the heart's ability to adapt to current circumstances. For instance, during exercise the heart needs to beat faster to pump more oxygen to the muscles, while during sleep a fast heart rate is unnecessary.
A decreased HRV therefore means that the heart is less able to adapt. It also increase the risk of developing an irregular heart beat - known as an arrhythmia - which in extreme circumstances can trigger sudden death.
The heart beat is controlled by the autonomic nervous system, which is responsible for regulating basic body functions that are not under our conscious control.
Therefore, a decreased HRV is a sign that the autonomic nervous system is not regulating heart beat in the way that it should.
The researchers studied 50 pairs of male twins, who were all free from any signs of heart disease.
Their hearts were monitored over a period of 24 hours and they were questioned
about symptoms and histories of depression.
The researchers found that people who showed signs of being depressed were more likely to have a decreased HRV.
However, there was no link between decreased HRV and a previous history of depression - suggesting that the state of being depressed has a direct effect on the autonomic nervous system.
Researcher Dr Viola Vaccarino said the results suggested that even minor episodes of depression could affect the heart.
He said: "The data suggest that treatment of clinical depression, and perhaps also
minor forms of depression, should reduce the risk of coronary heart disease via normalisation of HRV."
Depression Alliance, a charity which raises awareness of the problem, called for more research into the link.
"The brain is a complex organ and it is no wonder that 'mental' illness often comes with many physical effects or that physical illness may be accompanied by 'mental' ill health.
"Indeed, recent research reveals that having depression at the time of undergoing heart bypass surgery doubles the chances of dying.
"Whilst it remains unclear as to the precise link between depression and other serious illness we would also call for the implementation of depression screening during routine medical examinations."
Belinda Linden, of the British Heart Foundation said: "It is important that we learn more about the influence of depression on heart rate variability and its effect on cardiac disease.
"It must be remembered that heart rate variability is also dependent on age and inactivity.
"Although depression and isolation are clear factors that need to be addressed, it is also vital to consider lifestyle issues such as levels of physical activity, diet and smoking to reduce the risk of coronary heart disease."