Astronauts lose calcium in their bones and strength in their muscles while in space because of the zero-gravity environment. This calcium can end up in their kidneys, putting them at risk for developing kidney stones.

At least 14 American crew members have developed kidney stones in the last five years, and as missions become longer, the number is likely to grow. While astronauts have exercised in space to attempt to combat bone loss, the lack of gravity makes it difficult to achieve enough resistance to maintain their pre-flight fitness levels.

"This becomes a real health concern, as the time astronauts spend in space and living in the space station is extended," said Manoj Monga, M.D., professor of urologic surgery and lead investigator.

Researchers studied the effects of exercise in pairs of identical twins, since a portion of a person's risk for developing kidney stones is genetic. The study participants had no history of kidney stones and were placed on standardised diets.

The twins were put on bed rest on a tilted bed that positioned their head lower than their feet to simulate low gravity for 30 days. One twin per pair was randomly selected to exercise (while still reclining) in a chamber that put negative pressure, or resistance on their lower body, and the other twin served as a non-exercising control. The pressure in the chamber was roughly equivalent to what a person would experience running on Earth.

Monga found that the non-exercising study participants had higher levels of urinary calcium than the exercising group, and thus had a greater risk of developing kidney stones. "In combination with hydration therapy, exercise in a machine that simulates gravity could reduce the astronaut's risk of developing kidney stones, a condition that could be particularly painful and lead to an aborted mission," Monga said.

MEDICA.de; Source: University of Minnesota