Gastric emptying during long-term bed rest head-down tilt: Effects of dietary sodium

Long term human space flights as operated since the early 70s of the last century have resulted in significant weight loss of many astronauts (“space anorexia“) but little is known about the underlying mechanisms. Bedrest studies with a 6° head down tilt (BR-HDT) are common ground-based models to simulate the effects of Zero-G on the human body.

Methods: Eight healthy male subjects (age: 26.12±3.94 years) participated in the study at the German Aerospace Research Facilities (DLR, Cologne, Germany) on two different occasions (M1: October 2005; M2: March 2006). On each occasion, 4 days of adaptation and baseline assessments were followed 14-days of strict BR-HDT and 3 days of recovery. During BR-HDT subjects received two different diets varying in sodium content (low: 50 mmol/day; high: 550 mmol/day) (LSD, HSD), such that half of the subjects received the LSD while the other half received the HSD during M1, conditions were switched for M2. Gastric emptying (assessed by 13C octanoid acid breath test) of a solid standardized meal was determined as T1/2 (half emptying time, in min) using an infrared spectroscopy technique (IRIS, Wagner Analysentechnik, Bremen, Germany) during the run-in phase (day 2) on a normal diet, and after 7 days of HDT-BR during both M1 and M2. Baseline T1/2 were compared to BR-HDT values for both diet conditions by t-tests (adjusted to α/3=0.05/3=0.16). In addition, percent 13C exhalation per hour data across the 17 breath sample were used to compute average 13C exhalation curves for baseline and BR-HDT conditions, and were analysed by repeated measure ANOVA.

Results: T1/2 during adaptation was not different for M1 and M2 (M1: 125.57±20.86min; M2: 129.68±23.87min). The comparison between LSD and HSD showed a significant difference (LSD: 122.68±13.36min; HSD: 137.02±11.76min; p=0.003). ANOVA revealed a significantly lower relative exhalation per hour during BR-HDT versus baseline independent of the diet (p=0.001).

Conclusion: We found that BR-HDT significantly prolonged gastric emptying and reduced overall 13C exhalation, and this effect was stronger with a high sodium content of food. Thus, “space anorexia“ may in part be due to a delay in gastric emptying during Zero-G, and dietary factors may further contribute to it.