Is the post-marathon “cold/cough” really an allergic reaction?

It’s pretty well established that heavy training — the type you might do to prepare for a marathon — can reduce immune function a bit, leaving you more susceptible to colds. Same thing with the race itself, which can trigger a temporary lowering of immune function: it’s well documented that runners have an elevated risk of catching an upper respiratory tract infection (URTI) in the week or so after a marathon.

Except… are those symptoms (e.g. congestion, cough, watery eyes, sneezing, “nasal discharge”) really the result of URTIs? In a new paper in Medicine & Science in Sports & Exercise, a group of mostly British researchers suggest an alternate explanation: allergies. And they provide some intriguing data to support this claim. They studied 208 runners who ran the 2010 London marathon. Before the race, the runners completed an allergy questionnaire and did a blood test to look for reactions to common inhaled allergens (like pollen). After the race, they filled out questionnaires daily for 15 days describing any possible URTI/allergy symptoms. The key results:

• 47% of the runners suffered from the symptoms of an apparent URTI after the marathon (i.e. they reported symptoms on at least two days in a three-day period during the 15-day follow-up).
• The researchers also surveyed non-runners who were living with the runners in the study; only 19% of the non-runners reported URTI symptoms during the period, which (the researchers say) argues against the symptoms being due to an infectious disease like cold or flu.
• 40% of the runners had some form of allergy, based on either the questionnaire or blood test — and the allergy questionnaire was a “significant predictor” of the whether the runner would suffer URTI symptoms after the race.

So what’s going on here? According to the researchers:

The shifting of breathing during exercise from nose to combined mouth and nasal breathing results in a greater deposition of airborne allergens, and unconditioned air, to the lower airways.

They go on to note that these symptoms may be associated with asthma, but that, “worryingly,” few of the runners were being treated pharmacologically for asthma — which they attribute to the fact that asthma meds are restricted by the World Anti-Doping Association. Given that the average finishing time of their subjects was slower than five hours, I find this an extremely strange argument to make! They go on to recommend that recreational runners should be screened for allergies and consider treatment.

I think this is a really interesting topic. It’s been known for a long time that endurance athletes (particularly winter athletes) are far more likely to suffer from conditions like exercise-induced brochoconstriction (basically exercise-induced asthma symptoms) than the general population. Why is this happening? What makes athletes’ airways more sensitive? Is it the airways being dried out by large volumes of air passing through? Is it particulate matter being inhaled and damaging airways? Or is it allergens that are causing inflammation due to all the deep breathing? It would be great to see more research looking into these questions.

That being said, I think this study has some pretty important limitations. I found the presentation of the data to be a bit lacking — it’s very hard to get a precise sense of just how different the allergy and non-allergy groups were, to see how strong the data really is. But the more fundamental issue is simple: selection bias. The subjects “were recruited randomly during the 4-day registration exhibition before competing in the 2010 London Marathon.” So which people would be more likely to agree to fill out a questionnaire about allergies and give a blood sample for allergy testing? Probably people who have, or believe they have, allergies. So when the results tell us that a very high proportion of runners have some indication of allergies, that doesn’t necessarily hold true for all runners!

Still, a thought-provoking paper: maybe the famous “post-marathon cold” isn’t quite what we thought.