The impact of simulated microgravity on human kidney function in men and women: lessons from bedrest studies

Astronauts have been reported to have an unusually high rate of kidney stone formation during spaceflight, which represents a risk for the health and for the space mission success. Our studies in humans adapted to simulated microgravity (bedrest) demonstrate that exposure to microgravity results in alterations of renal function, fluid redistribution, and bone loss, which is coupled to a rise of urinary calcium excretion thus increasing the risk of renal stone formation. Vasopressin, the main hormone involved in water balance regulation, promotes urine concentration and reduces bone mass. Interestingly, measurements of vasopressin in astronauts (from 24 h to 8 days after launch), revealed concentrations considerably elevated compared to ground, suggesting a critical role for this hormone in renal stone risk in microgravity. In line, in a previous 10-days bedrest study, we provided evidence that bedrest is associated with an early increased risk of stone formation that was monitored by novel identified biomarkers. The impact of simulated microgravity in risk of stone formation is currently under investigation in a more recent 21-day bedrest campaign as part of Science for Bed Rest program of Italian Space Agency (ASI). This study involves, for the first time, men and women, thus allowing a more general understanding of sex difference in hormonal response to simulated microgravity. Preliminary results indicate a tendency toward increased vasopressin levels, evaluated as copeptin, a validated and reliable surrogate marker of vasopressin, over 21-day bedrest both in men (n=6) and women (n=6) with a peak at day 15. These preliminary data suggest that both men and women may respond to changes in blood volume or osmolality during bedrest and release higher amount of vasopressin to restore decreased plasma and extracellular volume known to occur consequently to adaptation to microgravity. This abstract was presented at the American Physiology Summit 2026 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.