At least 14 American astronauts have developed kidney stones in the last 5 years, say researchers at the University of Minnesota. Kidney stones are mineral deposits that can be “passed” through the urinary tract in a most painful manner.

The situation could become more accute if we ever venture back to the Moon or beyond.

“This becomes a real health concern, as the time astronauts spend in space and living in the space station is extended,” said study leader Manoj Monga.

Monga’s team put pairs of identical twins on inclined beds for 30 days to simulate effects of microgravity, then had one group exercise while their twins did not. Those who didn’t exercise had higher levels of urinary calcium.

In the July issue of the Journal of Urology, Monga and colleagues write that astronauts need to exercise and drink more water, as infrequent peeing is a bad sign. More on kidney stones here.