Marathon runners who travel by air to the race may end up with higher blood levels of molecules that have been linked to clots, a new study shows.

That doesn't mean flying is actually likely to trigger a blood clot in endurance athletes, researchers say, or that air travel is a no-go. But it does suggest a possible explanation for the rare but mysterious reports of clots in otherwise healthy marathoners who flew to a race.

"It seems that the two activities could have a compounding effect when they are carried out back-to-back," said Beth Parker at Hartford Hospital in Connecticut, who led the research.

Blood clots usually form in veins, and can be dangerous if they break off and block blood supply to the lungs or heart.

The Centers for Disease Control and Prevention estimates that between one and two per 1,000 Americans experience a life-threatening blood clot each year. The elderly and those with genetic risk factors are at increased risk.

In the new study, published in the American Journal of Cardiology, Parker and her colleagues took blood samples from 41 healthy non-smokers who participated in the 2010 Boston Marathon.

Twenty-three of them lived more than a four-hour plane flight away, while 18 participants -- the comparison group -- lived within a two-hour drive of Boston. The researchers collected blood after the runners landed in Boston, immediately after the marathon, and again when the participants were back home the next day.

After returning home, six people in the air travel group had elevated levels of a substance called D-dimer, which has been used as a sign of possible blood clots. By contrast, none of those who lived close to Boston did.

Air travel is known to double or triple the risk blood clots in the veins of the legs, called deep vein thrombosis, according to Parker.

And endurance exercise, such as marathon running, may also raise the number of blood molecules that aid clotting.

What the elevated D-dimer levels meant is unclear. None of the athletes showed actual symptoms of deep vein thrombosis, such as leg pain or swelling, shortness of breath or rapid heart rate.

The researchers also found that older athletes had significantly higher levels of an inflammatory blood compound that could be associated with heart attack risk, regardless of whether they flew to the marathon or not.

One expert who wasn't part of the study cautioned that none of the molecules the researchers looked at has been shown to clearly identify individuals at increased risk of clots or heart disease.

"I wouldn't let these findings alarm runners traveling to marathons," Dr. Kenneth Bauer of Beth Israel Deaconess Medical Center in Boston told Reuters Health by email.

Parker agrees that while the research suggests marathon running and air travel to get to marathons could contribute to a slightly increased risk of blood clots or other cardiovascular events, the benefits of endurance running far outweigh the small risks for most people.

Simple precautions will reduce the risk of blood clots while flying, said Dr. Mary Ann Bauman, medical director for women's health at INTEGRIS Health in Oklahoma City.

She recommends staying hydrated, wearing loose clothes, and not cross your legs for long periods of time. Walking around the cabin every one to two hours and wearing compression stockings can help too, she Told Reuters Health.

One of the researchers on Park's team consults for AstraZeneca, which makes the cholesterol-lowering drug Crestor, and a number of other major pharmaceutical companies that make heart drugs.