Competitors in races like the Ironman triathlon and the TransEurope-FootRace (TEFR) know before they ever start training that these races will be tough on their bodies. But what about their brains? A recent study suggests that the brain does take a hit during ultramarathons. Fortunately, the researchers also concluded that the damage is temporary.

An ultramarathon is any footrace longer than a marathon’s 26.2 miles. Distances usually range from 31 to 100 miles, and the races are often conducted off road on trails or through parks. The longest races last for days. Runners frequently experience blisters, gastrointestinal distress, stress fractures in their feet, and even hallucinations. Still, ultrarunning enthusiasts say it’s all worth it. 

The researchers studied 44 runners in the 2009 TEFR, which involved a 64-day trek from Italy to Norway with no rest days. The race was the equivalent of about 100 marathons. Understandably, scientists took an interest. Just what does a challenge like that do to the human body?

The research team brought portable MRI machines, checking up on the runners’ legs, feet, hearts, and brains along the way. They used a technique called voxel-based morphometry (VBM) to scan the brains of 12 male runners before, during, and after the race. They also monitored a control group made up of physically fit men of the same ages as the race participants. 

Two findings stood out. First, the runners’ cartilage began breaking down in the first half of the race, but recovered as the race continued. “It was thought that cartilage could only regenerate during rest,” researcher Uwe Schütz told New Scientist. “We have shown for the first time that it can regenerate during running.”

They also learned that the ultrarunners’ brains were shrinking by as much as 6 percent as they ran. The culprit? Schütz suspects under-stimulation. Running through the same landscape day after day would offer little in the way of new information for the runners’ eyes, he told New Scientist. 

But the loss was only temporary. Eight months later, the runners’ brains were back to their pre-race baselines. “It is hard to explain what’s going on,” Schütz admitted to New Scientist. He noted that regular marathons won’t have the same effects.

Schütz shared his findings this week at the annual meeting of the Radiological Society of North America.