We all know that stress is probably not too good for us, but the truth is stress can be a killer. A Harvard Medical School study has found that chronic stress — the kind that goes on for weeks and months — increases our supply of white blood cells called leukocytes, worsening inflammation and causing the buildup of plaque, or atherosclerosis, in the arteries.

The chronic stress studied was that faced by physicians working in an intensive care unit.

The results of the study help explain the link between stress and heart attacks and stroke, which are both caused by atherosclerosis.

Researchers studied the chronic stress faced by physicians working in an intensive care unit. When they compared leukocyte counts in samples taken during the doctors’ work hours to those taken while they were off duty, they found the number of leukocytes had increased after just one week.

In addition to their human subjects, the investigators also looked at the effect of chronic stress in healthy mice. They induced stress by isolating the mice or tilting their cages and found that stress activated nerve fibers in the bone marrow of the mice, compared to non-stressed controls, leading to stem cell proliferation and increased leukocyte production.

This increase in leukocytes appeared to encourage plaque formation in the arteries of the mice. The plaques in these stressed mice resembled those found in humans that are most likely to rupture, causing a heart attack or stroke.

Risk factors such as smoking, poor diet, and a sedentary lifestyle can also contribute to this inflammation, researcher Matthias Nahrendorf, an associate professor of radiology at Harvard, told TheDoctor.

Since we can't always eliminate the stress in our lives, it is important to do what you can — quit smoking, eat properly, and stay physically active. These, too, can reduce the proliferation of leukocytes stress triggers.

“We recommend doing things such as yoga to reduce stress,” said Nahrendorf.

Look closely at the situation causing the stress, and find ways to make changes that may help reduce it, whether it's taking an evening walk or eating a better diet. You may not be able to change the stressors in your life, but you can change aspects of your life that could help you cope with them better.

The study is published in Nature Medicine.