'Straightforward' approaches to treatment are obviously are far more complex in a space station

Professor John Feehally, National Kidney Research Fund

Kidney stones can form anywhere within the kidney or bladder and range from tiny microscopic crystals to stones as large as walnuts.

They can move from the kidney towards the bladder causing a number of problems including excruciating pain.

If the stone completely blocks the tube draining the kidney, the kidney could stop functioning.

Calcium levels

The New Scientist magazine reports scientists at Nasa Johnson Space Center in Houston, are looking for a solution.

Peggy Whitson, a Nasa astronaut and biochemist, said even those who went on short 18 day space shuttle flights had been found to be affected.

"Once renal stones start to move they can be excruciatingly painful.

"You'd have an incapacitated crew member and potentially have to abort the mission."

Once renal stones start to move they can be excruciatingly painful

Peggy Whitson, Nasa

To test what happened to those who went on longer missions, urine samples were taken before, during and after astronauts went on 100 day Mir missions between 1995 and 1999.

They found fluid intake and urine volume were significantly lower than normal.

The astronauts also had significantly higher levels of calcium phosphate in urine.

This means calcium salts are more likely to crystallise and grow into stones.

Although levels returned to normal after a month, by that time stones could already have formed.

Bone density

Peggy Whitson said the loss of bone mass which occurs in microgravity could be one cause.

"During space flight, crew members lose bone density.

"And the calcium that's released probably ends up in the urine.

"The higher calcium levels are probably contributing to the increased calcium-stone forming potential."

Stones can usually be passed, painfully, without surgery.

Drinking plenty of water both helps pass the stones, and prevents them forming.

But experts say urinating in space can be difficult.

Peggy Whitson added: "Urinating in toilets in orbit is time consuming, crew members are very busy, and if they do extravehicular activity they don't have the option."

Vomiting because of motion sickness can also lead to loss of fluids.

Supplement solution

Joseph Zerwekh of Southwestern Medical Center at the University of Texas is involved in research aimed at finding a solution to the kidney stone conundrum.

He says although losing calcium will increase astronaut's risk of a fracture, kidney stones could develop long before that.

Dr Zerwekh said one solution could be for them to take potassium magnesium citrate supplements.

A trial of 60 patients given the supplements, and extended bed-rest to mimic microgravity, saw the risk of kidney stones reduced by about 85%.

Citrate inhibits stone formation, and previous research has found astronauts have low citrate levels in their urine.

He said: "One of our principal sources of citrate is fresh fruit. And that is something you don't find in space."

A study of how the supplements actually work in space is being carried out by the NASA team at the International Space Station.

Professor John Feehally, senior medical advisor for the National Kidney Research Fund said: "The fact that kidney stones are developing in astronauts is not completely unexpected.

"One of the responses to weightlessness is loss of bone density - the bones don't 'need' to be strong with no weight to carry around."

He backed the US scientists' conclusions and suggested solutions said although treatment of kidney stones, either by waiting for them to pass, or through surgery: "these 'straightforward' approaches obviously are far more complex in a space station!"

The research also features in the journal Nephron.