just published in Medicine & Science in Sports & Exercise, which I explore in my Globe and Mail column this week.
The key difference about the new study is that, unlike hundreds of
previous studies, they measure immune function "in vivo" rather than in
test tubes or animals:
While hundreds of studies have explored the links between
exercise and immune function, nearly all use rodents or approximate
immune function by measuring the abundance of certain markers in the
blood or saliva. But the immune system is a co-ordinated network
throughout the body that includes neural and hormonal responses, so
testing the response of a few cells in a test tube doesn’t give a
reliable prediction of immune behaviour, Walsh says.
In a study published this month in Medicine & Science in
Sports & Exercise, Walsh and his team administered a chemical called
diphenylcyclopropenone (DPCP) through a patch on the lower back, 20
minutes after exercise bouts of various lengths and intensities. DPCP is
an antigen that triggers the development of a brand new immune
response; the strength of the response can be assessed by applying more DPCP to the skin four weeks later and measuring the resulting redness and skin thickening.
Using this technique, the researchers compared 30 minutes of
"moderate" treadmill running, 30 minutes of "intense" running (where
you'd struggle to maintain a conversation), 120 minutes of "moderate"
running, and a control group with no running. Here's one of the
resulting measures of immune function, skin thickening (MI is moderate
intensity, HI is high intensity):
In this context, the conclusion is clear: a long duration of exercise
(120 minutes) causes a temporary weakening of immune function, while
short (30 minutes) intense exercise doesn't. How generalizable is this
finding? That's very hard to say. For example, would long bike rides,
which are generally less "stressful" on the body compared to long runs,
produce the same effect? What role does overall fitness play – i.e. if
you're used to running two hours every weekend, does it cease to be a
stress? Do all aspects of immune function follow a similar pattern as
this particular one measured in the study? Is 120 moderate minutes
simply a harder workout than 30 intense minutes? All these are open
questions, and I certainly don't think we should oversimplify the
takeaway message. But it's an interesting new approach to studying
exercise immunology, and it offers some food for thought about how
duration and intensity affect immune function.