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Especially on long flights, very fit people must remember to keep moving and stay hydrated

Kelly Wiebe's hopes of qualifying for this summer's Commonwealth Games in Glasgow were wiped out before he even reached the start line.

The 24-year-old from Swift Current, Sask., one of Canada's fleetest long-distance runners, was preparing for a race in California in April when he felt the first warning signs – a little more soreness than usual after his final prerace workout.

But this was no muscle pull or other typical sports injury. After jolting awake that night with a fever, he took a harrowing flight back to Canada and ended up in hospital for more than two weeks.

The unexpected cause: a blood clot in his leg, which had become infected.

If you're young and fit, blood clots probably aren't on your radar – with good reason, since common risk factors include obesity, heart disease and a sedentary lifestyle.

But researchers are now realizing that if you're really fit – a dedicated runner or cyclist, for example – your risk of blood clots and their potentially serious complications might be higher than expected.

"I always associated blood clots with immobility or lack of exercise," admits Wiebe, who is healthy again and back to easy training.

"Before this happened, I never thought in a million years that I would be susceptible."

Blood clots, or thromboses, are plugs that form in your blood vessels.

They're essential to stop bleeding when you have a cut, but they can cause trouble when they form in the absence of injury.

The most serious clots are those that form deep in your body rather than near the surface, because pieces from these deep-vein thromboses (DVTs) can break off and travel to the lung to cause a pulmonary embolism, a potentially fatal blockage of the lung's main artery.

The risk factors for blood clots and pulmonary embolism can be grouped into three categories: how prone your blood is to clotting; how smooth or damaged the walls of your blood vessels are; and how vigorously your blood flows.

As it happens, endurance athletes are prone to all three risk factors, explains Dr. Claire Hull, a medical researcher at Swansea University in Wales.

She is the lead author of a recently published case report on a 29-year-old marathon runner who suffered a blood clot that went undiagnosed for eight months.

Their findings:

  • Dehydration and inflammation make your blood more prone to clotting;
  • Physical trauma like muscle strains can damage blood vessel walls;
  • Low blood pressure and a low resting heart rate allow blood to pool during periods of immobility
  • And flow is further compromised when athletes take long flights to and from competitions.

In Wiebe's case, doctors suspect the second factor: A slight muscle tear during exercise might have precipitated the clot, and bacteria that were already in his system then congregated at the site.

So does this mean that athletes are ticking time bombs? It's difficult to know, because no one has done a study to compare prevalence in athletes compared to non-athletes, Hull says.

Still, "my personal experience of several other athletes leads me to believe that we are underestimating the problem quite simply because we do not have data," she adds.

To raise awareness of the issue, Hull and her colleague Dr. Julia Harris published a set of guidelines in the journal Circulation last year.

To reduce the risk of clots, they recommend staying hydrated after training, taking movement breaks and wearing compression socks during long plane and car trips, and avoiding sitting in cramped positions or crossing your legs.

The patient in Hull's case study initially reported calf pain that felt like sustained cramping.

Hull suggests watching for unexplained pain and swelling in the legs (though it can also occur in upper limbs), reddish or blue skin discoloration (more obvious in the bath) and a leg that is warm to the touch.

If you suspect a clot, head straight to the hospital, where doctors will confirm or rule out its presence and administer blood thinners to treat it.

While some of the advice about how avoid DVT is fairly familiar, it's surprising how few people obey it.

Wiebe recalls a flight to China for the World University Games a few years ago where he broke all the rules.

"I never moved around for the entire duration of the flight, I was dehydrated and I never wore compression socks," he admits.

"The end result was extreme swelling in my legs, which made my legs look like one long tube."

He's back on track now, slowly and painstakingly rebuilding his fitness – and he won't make those mistakes again.

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