You have probably heard, at some point, that traveling on an airplane raises your risk of having a clot form in your leg veins. This is a complex phenomenon which may relate to a variety of things that occur during air travel including immobility (blood sitting still tends to clot), lower air pressure, and lower oxygen concentrations (both have been implicated in activating the clotting cascade in the body).
Now, in the March 2011 issue of the Clinical Journal of Sports Medicine, Parker et al. took blood samples from two groups of Boston Marathon runners, those who flew (over 4 hours) to the race, and those who drove (less than 2 hours) to the race. Three sample were taken, before the race, at the finish, and one day later, prior to returning home. In these samples, they measured different compounds which are known to play important roles in the formation of clots. The researchers found that from pre to post race, these compounds increased in both groups, but more so in the air travel group. They conclude that marathon running induces a hyper-coagulable state, presumably more so in those who travel by air. The study is interesting, but it was only conducted in 41 people and it does not measure actual clots, just blood tests that are markers for clotting.
While people do get vein clots while traveling by plane, they also occur during car trips and even seemingly without provocation. Other factors that increase your risk include birth control pills, age, and poor circulation from other medical conditions. Many people also carry minor deficiencies of anticlotting blood proteins, and many do not know they have them (Protein S or C deficiency and Factor V Leiden mutation). Dehydration also does not help, and that may be an additional concern for runners (also airplane air is very dry, about 20% humidity). Compression hose may help and staying hydrated while avoiding alcohol seem like good suggestions even if evidence of effectiveness is limited.
Crossposted at nevernotrunning.com