As Medical Daily’ s own Samantha Olson knows, one has to undergo months of grueling fitness and nutrition regimens to prepare for running a marathon. They can be so rough that many compare the experience both physically and emotionally to childbirth. But what about people who run ultramarathons, which go even further than the 26.2-mile marathon? According to a new study, running for this long can actually shrink the size of the brain.

Ultramarathons tend to run between 31 and 100 miles, and often send runners off the pavement to run on dirt road trails. Running for so long, however, can cause hallucinations, temporarily blurred vision, and hypothermia, among other adverse effects, according to The Washington Post. The Trans Europe Foot Race is among the most grueling of these ultramarathons, covering 2,796.17 miles from Italy to Norway in 64 days. The race, which equals about 100 normal marathons, has no days off, which is exactly why researchers were interested in seeing the effects it had on the human body.

After spending the last six years studying ultramarathon runners, Uwe Schütz and his team at the University Hospital of Ulm in Germany presented their findings at the Radiological Society of North America annual meeting in Chicago. The team spent its time using portable MRI scanners to assess the legs, feet, heart, brains, and cardiovascular systems of 13 volunteer runners. They scanned before the race, twice during the race, and then eight months later for a final follow-up scan. They also took blood tests and urine samples.

Results showed runners’ brains shrank by about 6 percent over the course of the two-month race, New Scientist reported. Though the drop in volume could be attributed to undernourishment or fatigue, Schütz believes it might actually be due to lack of stimulation. The researchers found that one of the brain’s temporal regions, which deals with visual processing, was affected over the course of the race. They hypothesized this was caused by 64 straight days of running while focusing on nothing but the road in front of them.

As worrisome as losing brain volume would seem to runners who want to continue running ultramarathons, the researchers found that after eight months, runners’ brains returned to normal. “The reversibility of the changes at follow-up supports the finding that [gray matter] changes in chronic diseases like chronic pain are... functional and reversible. Accordingly, relevant and persisting long-term effects on the brain’s integrity in ultramarathon runners are less likely to occur. Still, this conclusion awaits further research and replications,” the researchers wrote.

Aside from testing brain size, the researchers also used MRI to scan the runners’ feet and leg joints every 900 kilometers. They were trying to determine how much damage the shock-absorbing cartilage of the runners’ knees were taking by looking at the amount of water the cartilage released. They found the cartilage started to break down entirely after 1,553 miles. Surprisingly, after that distance, they also noticed the cartilage began to regrow itself. “It was thought that cartilage could only regenerate during rest,” Schütz said. “We have shown for the first time that it can regenerate during running.”

It's important to note the study was extremely small, with only seven runners making it to the eight-month follow-up scan. For this reason, only further research will prove whether these brain changes actually occur.

Source: Schütz, U, et al. Regionally accentuated reversible brain grey matter reduction in ultra marathon runners detected by voxel-based morphometry. Radiological Society of North America annual meeting. 2015.