A large study published in April of 2013 in the journal Psychoneuroendocrinology followed 667 people with heart disease for 5 years and found that depression appears to lead to inflammation rather than inflammation leading to depression.
Excessive inflammation is one of the main causes of heart disease, because it is associated with blockages in the blood vessels that deliver blood to the heart. And those blockages are what cause heart attacks.
Two thirds of the people had no depressive symptoms during the 5 year followup. 13 percent had depressive symptoms at one followup visit but didn’t ever have symptoms again. 21% had depressive symptoms at multiple followup visits.
All the subjects had about the same level of white blood cells (involved in inflammation or excessive immune system activation) at the beginning of the study, but those who had depressive symptoms showed an increase in white blood count during the five years of the study.
Another report from the same group found elevations in other chemicals in the blood that indicate increased inflammation.
However in that second report, the markers of inflammation were more strongly associated with smoking and other behaviors that are known to be harmful… in other words, depression was associated with more smoking and less exercise, and those appeared to be causing increased inflammation.
If these results are confirmed it could explain why it is that people who are depressed after a heart attack have about three to five times the risk of early death compared to those who aren’t.