It's not every day I get an email from someone in Taiwan about exercise, white blood cells, and menstruation. But in response to my post How Much Exercise Harms Your Immune System?, Guan-Da Syu from National Cheng Kung University Medical College dropped me a friendly note (if you can call an email with its own bibliography a "note") a few days ago. Syu is the lead author of the paper I'd discussed in that post, and he wanted to respond to some questions I raised.
The paper reported that after out-of-shape individuals engaged in sudden and intense exercise, their white blood cells died at an accelerated rate. An increase in reactive, oxygen-containing molecules seemed to be the culprit. But when those same people got consistent and moderate exercise--five days a week for 30 minutes--their white blood cells lived for longer. Furthermore, consistent exercise buffered the harmful effects of more strenuous exercise sessions on white blood cells.
I had asked whether we could be sure that shortening or increasing the life span of white blood cells (specifically, neutrophils) had a net negative or positive effect on individuals' immune systems. Might the body compensate somehow? Syu says that it's hard to quantify a person's immunity, but his findings fit with other research that linked extreme exercise with higher infection risk. Additionally, he says that after severe exercise, it takes about a half a day for the proportion of healthy white blood cells in the body to return to normal.
The consistent exercisers, Syu says, adapt to the oxidizing molecules, and begin to produce neutrophils that live for longer. I had asked whether prolonging the lifespan of short-lived cells might be a burden on the body somehow, but Syu points out that the number of neutrophils living in the body at one time remains the same throughout subjects' exercising or sedentary weeks. We don't have more white blood cells when we're in shape; we are able to produce fewer because they live for longer.
Finally, I'd pointed out that since the study only used male subjects, it's hard to generalize the results for woman, whose bodies don't necessarily react to exercise in the same way. Syu acknowledges that it's unknown how exercise affects women's white blood cells. But it's possible that the effect might depend on the time of the month. In a previous study, the same research group looked at women's platelets (the blood cells responsible for clotting). In the first half of the menstrual cycle, extreme exercise had a notable effect on platelet function. But in the second half of the cycle (from ovulation to menstruation), women's platelets didn't respond to severe exercise in any way.
So the effect of exercise on your immune system might depend on many factors: how hard you work out; how consistently you work out; whether you're a woman and what time of the month it is. It seems that consistent exercise protects your immune system, but going from zero to 60 when you start a workout routine is harmful. And women, for now, will remain a mystery.
Syu, G., Chen, H., & Jen, C. (2011). Severe Exercise and Exercise Training Exert Opposite Effects on Human Neutrophil Apoptosis via Altering the Redox Status PLoS ONE, 6 (9) DOI: 10.1371/journal.pone.0024385