Evaluation of Fluid Loading Protocols for Use Before Returning from Spaceflight
Authors: Lytle Jason , Lee Stuart , Poczatek Matthew , Pickering Sarah , O’Grady Cambria , Melvin Stephanie , Wu Xulei 
Published: May 2024

Reductions in plasma volume (PV) during spaceflight contribute to post-spaceflight orthostatic intolerance (OI) during re-exposure to a gravitational environment. Both the National Aeronautics and Space Administration (NASA) and the Russian Space Agency (RSA) utilize fluid loading prior to re-entry to expand PV and mitigate the risk of post flight OI, but the timing of ingestion differs. Before landing NASA crewmembers consume 1-1.5 L of water with salt tablets to produce an isotonic solution in ~1 hour, whereas RSA cosmonauts consume the same, over ~4 hours. Anecdotal reports suggest low palatability and gastrointestinal (GI) distress with salt tablet and water fluid loading. To address this concern, the NASA Space Food Systems Laboratory developed an electrolyte solution. The purpose of this study is to test the effcacy and palatability of the two fluid loading compositions ingested in 1 hour versus 4 hours. Seven participants (2 Female, 39±9 years, 178.2±9.9 cm, 74.9±8.6 kg) completed four randomized visits separated ≥ 48 hours. The four visits were (1) no fluid loading (control), (2) 4 hour ingestion of the electrolyte solution (4hr-Electro), (3) 1 hour ingestion of the electrolyte solution (1hr-Electro), and (4) 4 hour ingestion of salt tablets and water (4hr-Salt). Participants reported to the lab at ~7:00 a.m. fasted, and PV was measured using a carbon monoxide re-breathing technique. During each visit subjects consumed 500 mL of water representing normal daily water intake through the first five hours of the session along with breakfast and lunch meals identical to those provided to astronauts. During the control visit, no other fluid consumption was allowed. During the three fluid loading visits, participants also consumed up to 1.2 L of additional fluid, either water with salt tablets or the electrolyte solution. After completion of the fluid loading protocol no additional fluid was consumed during the final 3.5 hours of the test. Participants remained seated for all portions of the study. Hemoglobin and hematocrit were measured at 0, 1, 2, 3, and 3.5 hours after cessation of food and fluid intake to assess changes in PV. In these preliminary data, PV was not significantly different from baseline at any timepoint during the control and 4hr-Electro sessions ( P>0.20). PV increased from baseline at all timepoints for the 1hr-Electro session ( P<0.003) and at all timepoints, other than 0 ( P=0.17 ), for the 4hr-Salt condition ( P<0.01 ). GI distress was reported by participant during the 4hr-Electro (n=2), 1hr-Electro (n=1), and 4hr-Salt protocols (n=4). No GI distress was reported during the control visit. The most severe GI symptom (18/20) was nausea reported with the 4hr-Salt protocol. These data suggest that both the 4hr-Salt and 1hr-Electro fluid loading protocols successfully expands PV, however the 1hr-Electro results in fewer participants reporting GI symptoms. Supported by Mars Campaign Development - Exploration Capabilities. This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.