Ultra athletes are a special breed, running or cycling hundreds and sometimes thousands of miles to compete in exclusive races. But what really happens to their bodies hour after hour and day after day of intense exercise? A new study reveals some dramatic changes may take place.

The study, presented Sunday at the annual meeting of the Radiological Society of North America in Chicago, focused on 44 runners who took part in the TransEurope-FootRace in 2009, a 64-day race that covered about 4,488 kilometers (about 2,789 miles), going from southern Italy to Norway.

Researchers had a mobile MRI unit and tagged along with the runners, gathering information about how their bodies were changing. Half the runners had full-body MRIs taken every three or four days. The athletes also had their blood and urine checked, and some were given other tests such as electrocardiograms.

Overall, during the race runners lost an average 5.4% of their body volume, most of that occurring during the first half of the race. During that time they also lost 40% of their body fat, but lost 50% by the end of the race. Average muscle volume loss in the athletes’ legs was 7%.


Fat tissue was affected first by the excessive exercise. Loss of visceral fat--the fat around the abdomen that is thought to up the risk for cardiovascular disease--happened early on in the race. Most fat overall was lost in the beginning of the race as well.

Researchers discovered that some leg injuries don’t require being sidelined. Runners with intermuscular inflammation of the upper or lower legs, for example, can frequently continue to run without additionally damaging tissue. However, running with joint inflammation carries more possible risk.

Although ultra races are a niche event, researchers noted that the findings may spill over into marathon or recreational running. “The rule that ‘if there is pain, you should stop running’ is not always correct,” said study co-author Dr. Uwe Schutz, in a news release.