What happens to your kidneys in space

What changes and why

A kidney stone forms when salts in urine, usually calcium salts, become so concentrated that they crystallize and clump together [1]. Spaceflight pushes urine in that direction in three ways:

• Calcium leaves the bones. Without body weight to carry, the skeleton loses mineral. Hip and spine bone density historically fell about 1–1.5% a month, and about 0.3–0.8% a month with today’s resistance exercise, varying widely [2]. Much of that calcium is passed in the urine (doctors call high urine calcium hypercalciuria) [1].
• Crews make less urine. In 11 astronauts and cosmonauts on long missions aboard the Mir space station, urine volume was 47% lower in the first 30 days and 39% lower after day 60 [1]. On the International Space Station, urine volume was lower in flight in every exercise group studied [3].
• The urine becomes more “stone-ready”. Calculated supersaturation (how close the urine is to crystallizing) rose by about 150% for one calcium salt (brushite) and 115% for another (calcium oxalate) during flight [1]. Diet matters too: high salt and high animal protein both add to the risk [1].

On a station crewmembers can be treated or brought home. On a mission to Mars, a stone that blocks the urinary tract would have to be handled on board. The U.S. space agency, the National Aeronautics and Space Administration (NASA), states that the ability to treat a stone during exploration missions is not yet available [4].

Timeline: days to a Mars mission

1. First daysShort flights

   Urine changes start quickly. In Space Shuttle crews, urine calcium did not rise on flights shorter than 6 days but did on 6- to 10-day flights, with less urine and more acidic urine [1]. If severe space motion sickness causes dehydration early in flight, fluids through a drip (intravenous fluids) may be considered [1].

2. WeeksFirst month

   Urine volume is lowest early: 47% below preflight before flight day 30 in Mir crews [1]. The NASA report notes that the early phase may carry more risk [1].

3. About 6 monthsTypical station stay

   Stone-forming risk stays raised even with today’s exercise equipment [3]. No symptomatic kidney stones have occurred in flight among U.S. crew on the International Space Station so far; one cosmonaut had severe abdominal pain that resolved and was later attributed to a stone [1].

4. Mars-length missionsAbout 2 to 3.4 years (730–1,224 days)

   NASA rates kidney stones a risk that “requires mitigation” for Mars missions. Its research plan lists, as an open gap: “We do not have the capability to mitigate renal stones in spaceflight” [4].

5. Back on EarthLanding and after

   Right after long missions, the type of risk shifts toward calcium oxalate and uric acid stones, because urine is low in volume and more acidic [1].

What crews do about it

• Drink more water. Stone risk rises when urine falls below 1.5 liters a day, and an effective hydration plan should produce 2.5–3 liters of urine a day [1]. A NASA model suggests in-flight intake of about 3.2 liters a day would bring the most common stone risk back to preflight levels, or about 2.5–2.7 liters a day if urine calcium is also lowered. NASA’s standard recommends at least 2.5 liters of water a day on the station, which is not always achievable [1]. These are prevention targets for spaceflight, not instructions for anyone on Earth.
• Limit salt, and watch oxalate and animal protein. NASA advises limiting sodium in flight; diets low in oxalate and animal protein may be advised [1].
• Protect the bones. On the station, heavy resistance exercise increased bone formation, and a bone-protecting medicine tested in one group countered bone breakdown, with nutrition support [3]. Even so, stone risk still rose in flight in every group, mainly because urine volume was lower [3].
• A medicine under study: potassium citrate. In a trial in long-mission crews, those who took potassium citrate had lower urine calcium, and their calcium oxalate stone risk stayed at preflight levels, compared with crew who did not take it. The trial measured urine chemistry, not actual stones [5]. NASA reports it has been used on short missions in astronauts with a past stone who had a medical waiver, with no side effects reported [1]. It is a prescription medicine; whether it suits anyone on Earth is a decision for their own doctor.
• Screening. Every active NASA astronaut has a kidney ultrasound every year and before and after flight, looking for early mineral deposits [1].

After landing and recovery

• Urine chemistry moves back toward normal after landing. Some measures briefly overshoot [1].
• The risk may last. A NASA review estimated that in the year after flight, astronauts’ stone rates may approach 2 to 7 times their preflight rates, with wide uncertainty [1].
• The long-term picture is unclear. In NASA’s lifetime health records, 19 of 36 stone events happened after astronauts’ careers had ended, in people who had flown short missions [1]. Age and other factors may explain this, so it is not settled how much of later stone risk is due to flight itself [1].

What it means for clinicians on Earth

These points are general education for clinicians, not advice about any individual patient. Coordinate with the person’s flight surgeon (the physician responsible for crew health) where there is one.

• Ask about spaceflight and bone loss. In flank pain or blood in the urine in someone who has recently flown, think of a kidney stone: spaceflight raises urine calcium and lowers urine volume, and stone rates may be higher in the year after flight [1].
• Imaging as usual. Standard practice applies. U.S. guidance favors non-contrast computed tomography (CT) first (ultrasound first in pregnancy) [6]; European guidance starts with ultrasound and confirms with non-contrast CT [7].
• Do not push fluids during an attack. A Cochrane review found no reliable evidence that high-volume (above-maintenance) fluids or diuretics improve pain or stone passage in acute ureteric colic [8]. This concerns forcing fluids, not treating dehydration.
• Prevention after an episode. General stone-prevention guidance (for example, a fluid intake of 2.5–3.0 liters a day, with water preferred, aiming for 2.0–2.5 liters of urine) applies to people who have flown just as to anyone else [9].
• Short flights count too. Urine calcium rose on Shuttle flights as short as 6–10 days [1], so a recent short orbital trip, including by spaceflight participants, is relevant history.

For a worked example, see the case file flank pain in month five.