To synthesize evidence on urinary calcium and oxalate in astronauts, comparing short- and long-duration missions and temporal patterns across mission phases.

Studies involving astronauts that assessed microgravity as the primary exposure and collected urine specimens were included. PubMed and EMBASE were searched using Medical Subject Headings and Emtree terms. Results were synthesized narratively, with quantitative data summarized using descriptive statistics and medians with interquartile ranges where appropriate.

Thirty studies met the inclusion criteria, spanning early U.S. missions (Gemini, Apollo, Skylab), Russian Salyut and Mir missions, and more recent Space Shuttle and International Space Station flights. Most studies reported increased urinary calcium excretion, often peaking early in flight. Short missions showed heterogeneous and typically transient increases with rapid post-flight normalization, whereas long-duration missions demonstrated more consistent, sustained hypercalciuria with delayed recovery. Data on urinary oxalate were limited, with only 4 studies reporting in-flight measurements.

Hypercalciuria during spaceflight varies by mission duration, with the highest and most consistent risk observed during long-duration mission. Oxalate metabolism remains poorly studied in astronauts. As human spaceflight advances toward exploratory-class missions, further development and testing of countermeasures targeting hypercalciuria and other urinary risk factors for nephrolithiasis are needed.