Whenever a marathon runner suffers a heart attack during a marathon, the media immediately revisits the debate over the effect of marathon running on the heart. On this topic, data overwhelmingly support that marathon running is safe for the the vast majority of runners. However, less attention is paid to other risks associated with marathon running. Today, then, I focus on some of my own research investigating blood clots, marathon running, and air travel.
My interest in this somewhat obscure topic began when my sister suffered a deep vein thrombosis (DVT, a blood clot in one of the deep veins of the leg) following the combination of doing a long run with me in Connecticut and then later flying home to Seattle. Blood clots are extremely rare in healthy younger adults, and I immediately set out researching possible causes (out of self-interest, predominantly. Could it happen to me?) Endurance exercise alone does not increase the risk of blood clot formation, as both clot formation and clot breakdown are augmented with exercise such that hemostatic balance is maintained. However, since air travel approximately doubles the risk of DVT, I wondered whether the combination of sustained endurance exercise AND air travel could predispose an otherwise healthy athlete to blood clots. Indeed, there are several case studies depicting exactly such a phenomenon in triathletes and runners, but no rigorous study data.
Consequently, we conducted a study at the 2010 Boston Marathon where we assessed markers of blood clot formation and breakdown in athletes who flew cross-country to and from the marathon versus local runners who drove a short distance to compete. Interestingly, the athletes who flew to the marathon showed dramatically elevated levels of clot formation relative to local, non-traveling athletes. In a subsequent analysis, we found that d-dimer, a clinical biomarker of DVT, was also significantly greater immediately after the marathon in the travel group of athletes. In fact, 6 of the travel subjects (versus no local controls) had d-dimer values that exceeded the clinical threshold for preliminary diagnosis of DVT.
What does this mean? First, a note of caution. Similar to the risk of running on the heart, the risk of experiencing a DVT following sustained endurance exercise and air travel is extremely rare. In fact, in our studies, no travel group athlete experienced a documented DVT, suggesting that even in subjects with elevated biomarkers of clot formation, the DVT risk was still low. However, athletes who travel long distances to and from races should be aware of the effect of travel and exercise on the balance between clot formation and breakdown. In particular, athletes who have other risk factors for blood clot formation– use of estrogen-based oral contraceptives or a clotting disorder, for example– may benefit from using common precautions taken to minimize blood clot risk during air travel, such as wearing compression socks, getting up to walk around regularly and staying hydrated.