Exercise may prevent stress on telomeres, the protective caps on the ends of chromosomes that are a measure of cell age and cellular health. Elizabeth Blackburn, from University of California/San Francisco (UCSF), and colleagues report that while psychological stress leads to shorter telomeres, exercise may prevent this damage.  The team focused on three groups: post-menopausal women who were the primary caregivers for a family member with dementia; young to middle-aged adults with post-traumatic stress disorder; and healthy, non-smoking women ages 50 to 65 years. They examined telomeres in leukocytes, or white blood cells, of the immune system, which defends the body against both infectious agents and cell damage.  "Our findings suggest that traumatic and chronic stressful life events are associated with shortening of telomeres in cells of the immune system, but that physical activity may moderate this impact," reports the team.