“Our data challenge the concept that short, high intensity exercise decreases immunity,” says Dr. Neil Walsh of Bangor University in Britain. Instead, “it appears that exercise lasting two hours or more decreases the immune response.”
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Contrary to earlier test-tube studies, 30 minutes of intense running had no effect on immune response. In contrast, 120 minutes of slower running did spark a significant reduction in immune response. One theory is that prolonged exercise triggers a gradual rise in blood stress hormones such as cortisol, which in turn temporarily suppresses immune function. That dip explains why runners are more likely to catch an infection in the week immediately following a marathon. The results are consistent with previous studies on exercise and immune function, according to Dr. Peter Darlington, a professor in Concordia University’s department of exercise science who studies immunology.