Putting your body through a long-distance race can cause your brain to shrink by up to 6 percent in size, according to a new study by researchers in Germany. Exactly why this happens isn't clear, but the team also found that the brains of all the runners in their study returned to their original size within the space of six months.

What makes the findings surprising is the fact that previous research has found strong links between exercise and good brain health: getting up off the couch can stave off depression and reduce the risk of degenerative brain diseases. One early theory is that excessive amounts of running cause the brain to shrink because of fatigue and poor nourishment.

There's no need to cancel your weekend run just yet, though. The runners monitored for the purposes of the study were participants in the Italy-to-Norway foot race, which covers a distance of 4,500 km (or 2,796 miles) over 64 days. There is no evidence that running a single marathon or jogging around the park causes the same results.

The team from the University Hospital of Ulm found that the area of the brain affected the most was responsible for visual processing, which lead them to suspect it might be related to the impact of staring at a road for 64 successive days. They hypothesise that the brain could be reorganising itself to reroute valuable energy resources to parts of the body that need it more. 

More positively, the study found that cartilage between the bones, which protects them from shocks and damage, degraded for the first 2,500 km (1,553 miles), but then began to regenerate after that point. It was previously believed that cartilage could only repair itself when the body was at rest, so perhaps there's something to be said for putting your body through such a punishing test.

The researchers have presented their findings at the annual meeting of the Radiological Society of North America. Portable MRI scanners and blood and urine tests were used to record data on the runners every 900 km (559 miles).

"It is hard to explain what's going on," lead researcher Uwe Schütz told New Scientist. "But we do see total recovery after six months."

While exercise is almost always good for us, it would seem there are limits beyond which the body starts to suffer, even if it is only temporarily.