Changes in Total Body Water During Spaceflight

Total body water (TBW) changes occurring in humans as a consequence of prolonged exposure to microgravity were measured in five male crewmembers of Space Shuttle missions STS-61C and STS-26. It was found that the inflight mean TBW values were significantly different from the preflight and postflight values, while the preflight TBW values were not significantly different from the postflight values. It was also found that individuals may differ in the rate at which they respond to weightlessness. Of the three crewmen who reported experiencing no symptoms of space motion sickness (SMS), two had not exhibited a decrease of TBW at the time of measurements (24 hrs after launch), while the two crewmen who reported SMS of intermediate severity showed a decrease of several kg by 24 hrs, suggesting that dehydration might be an important factor affecting the rate of TBW decrease.