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The standard picture of how exercise affects your immune system is sometimes called the “J-curve hypothesis“: your immune system is strongest at some “moderate” level of physical activity, and gets weaker if you exercise too little or too much. Stress, on the other hand, has a simpler curve: the more stressed you are, the more susceptible to infection. A new study in this month’s Medicine & Science in Sports & Exercise takes a look at how these two factors interact, with some interesting results.
The study, from the Karolinska Institute in Sweden, followed 1,509 adults for four months, recording self-reported levels of stress, physical activity, and upper respiratory tract infections (e.g. cold and flu). They saw the following (the top graph is everyone, middle is men, and bottom is women):
The key point: for men in particular, the more stressed you are, the more exercise helps to keep you healthy. Why is this not the case for women?
The ‘‘fight-or-flight’’ response to stress, although present in both men and women, is proposed to be stronger in men and a ‘‘tend-and-befriend’’ response more common in women in response to stress (35). These response differences could explain why men might benefit more from physical activity while under stress than women.
The researchers also note that the J-curve response isn’t very pronounced in their data, likely because very few of the respondents were exercising at anywhere near the intensity and duration of a marathon runner: “A participant classified as having a high physical activity (>55 MET*h/d) in our study would for example be someone with a sedentary job that goes jogging or to the gym for an hour each day and is moderately active the rest of the day.”