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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.”
It seems to follow from the graph that high stress and vigorous execise is healthier than low stress and vigorous execise. Or is the unit on the vertical axis relative to some standard value for high and for low stress?
(Put differently: I feel a bad cough comming up. Should I increase my work stress?)
@RH
That’s a funny (and very confusing!) point. I just went back and re-read the paper, but I can’t find any indication that the data in the graph were re-normalized. Some of the results in the paper are expressed as “incidence rate ratios,” which are normalized to 1. But the graph appears to show the number of infections per person per week. So that leaves us with two options:
(a) You’re right: we should all make sure to be highly stressed (and get lots of exercise) to avoid infections;
(b) There’s something wonky with either the data set or the analysis.
My hunch is that (b) is the right answer. The authors do indirectly address this point:
“We used the 14-item validated version of the Perceived Stress Scale by Cohen et al. (7) to assess perceived stress during the previous month, but we did not find an overall effect of high perceived stress on URTI… However, it is possible that perceived stress assessed in this population-based study was not severe or long term enough to affect immune function.”
Of course, if the subjects weren’t stressed enough for it to affect immune function, then it’s hard to take their conclusions seriously about the interaction effects between stress and exercise. Thanks for pointing out this problem, RH!
If I had to take a guess, I’d say that the number of men reporting low stress in the previous month (or low stress and high METS) was somehow pretty low, since that curve is the outlier. (Statistic on one, but I can’t remember a month without job stress and if I had no job, I’d be pretty stressed too)
Another thing: From the abstract is isn’t entirely clear if stress and METs were assessed every three weeks, along with URTI, only once, at the start of the study. In the latter case, the classification of people in the high or low stress group may not be very meaningful.
“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.”
An hour a day does sound pretty high for a “regular person.”
I can say that in an experiment of one (i.e. me) when I stopped running for 6 weeks during my first year of law school, it was very damaging to my health. Stupid move. I was basically too stressed and made the rookie mistake of thinking running was adding to the stress, not helping. Since then I train like a poorly-dedicated recreational runner (4-5 sessions a week, random workouts at paces too fast for my fitness but that I can complete because of my background). Seems to be working.