Behind this paper was a simple question with scarce data behind it: when the human body is exposed to a short period of simulated microgravity, do women and men respond in the same way? We know that spaceflight causes rapid deconditioning across multiple systems, but most physiological datasets remain relatively small, fragmented, and historically male-dominated. At the same time, human spaceflight itself is changing. Recent short-duration commercial missions such as Inspiration4 (3 days), Polaris Dawn (5 days), and Fram2 (3 days) have shown that brief exposures to space are an increasingly relevant part of the modern spaceflight context. Looking ahead, the Artemis II mission sent on April 1st 2026 the first mixed crew around the Moon on a mission of about 10 days, further underscoring the need to understand short-duration physiological adaptation in diverse crews. In that context, a 5-day dry immersion model becomes especially valuable as it captures a duration that is relevant to the kinds of missions now being flown and those that are coming next.
Dry immersion is one of the most valuable ground-based analogs of microgravity because it combines several key features of spaceflight: extreme physical inactivity, removal of normal support loading, and a rapid fluid redistribution. The European Space Agency (ESA) proposed the VIVALDI I and II campaigns to compare 20 women and men using closely matched 5-day protocols and a broad physiological assessment. Over just a few days, the effects were significant. Plasma volume fell rapidly, orthostatic tolerance worsened, aerobic capacity declined, muscle function deteriorated, and glucose handling became less efficient. What stood out most was how coordinated these changes were: cardiovascular regulation, metabolism, vascular function, bone-related markers, and physical performance all shifted together. This reinforces the important message that adaptation to microgravity is not confined to one organ or one pathway, but reflects a rapid whole-body response to unloading and inactivity.
At the same time, the study also showed that most responses followed the same overall pattern in women and men, which is reassuring for the generalizability of short-term analog studies. But some differences did emerge. Women showed lower orthostatic tolerance after immersion, along with stronger signs of metabolic vulnerability and a larger rise in atherogenic index and bone resorption biomarkers. These were not opposite physiological responses, but potentially meaningful differences in magnitude, which is exactly the kind of nuance that matters when designing countermeasures for future crews rather than assuming an homogenous response.
For us, one of the most important messages of this work is that short-duration microgravity analogs can reveal important physiological changes very quickly. That matters for space agencies and commercial operators preparing increasingly diverse crews for missions lasting only a few days, but it also matters well beyond space medicine. Dry immersion compresses into a short, controlled time frame several processes that are highly relevant in clinical medicine on Earth: acute reductions in physical activity, cardiovascular deconditioning, impaired orthostatic tolerance, reduced insulin sensitivity, and early shifts in bone and vascular biology. In that sense, this model is not only useful for astronaut research, as it can also help us understand what happens to patients who become suddenly inactive, immobilized, or confined to bed. The translational value lies precisely there: by studying a clean, reversible model of unloading, we may better understand and prevent some of the same early deconditioning processes that affect hospitalized, bedridden, or acutely inactive individuals.
Finally, in 2025, we conducted the ESA-supported VIVALDI III campaign. Its goal was to directly compare 10 days of dry immersion with 10 days of head-down bed rest. The results will provide new insight into the time course of physiological adaptations in these two microgravity analogs, and help determine which model more closely reproduces specific aspects of the spaceflight environment across different physiological systems.