Bone metabolism during strict head-down tilt bed rest and exposure to elevated levels of ambient CO2

Published in Healthcare & Nursing

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The :envihab facility in Germany (Source: DLR)
Astronaut Samantha Cristoforetti on the International Space Station (Source: NASA)

What is the backstory of this research?

            This paper would not have been possible without NASA’s commitment to mentorship and collaboration. As an undergraduate student at Cornell University, I applied to NASA’s 2021 Summer Space Life Science Summer Institute internship program and was selected to work as a virtual summer intern in NASA’s Nutritional Biochemistry Lab with Drs. Smith and Zwart. After conducting a literature review of the effects of CO2 on bone, I was given the project of analyzing data from two bed rest studies that had been conducted at the :envihab facility in Germany. One study occurred at terrestrial CO2 levels and the other at 0.5% CO2, which is comparable to the concentration inside the International Space Station. The German Aerospace Center (DLR) built the :envihab facility to allow for ground-based studies to evaluate the effects of spaceflight on the human body to better understand, and ideally counteract, spaceflight risks for astronauts. One unique aspect of the :envihab facility is the ability to control the environment, even to be able to alter the gas mixture of the air. This allows for studies of elevated ambient CO2 concentrations, as are often found on spacecraft.

How can spaceflight impact human bone, and why does it matter?

            Spaceflight conditions are so fundamentally different from terrestrial conditions that there is bound to be a level of dysregulation within the human body. Bone is especially sensitive to such changes since it serves both structural and homeostatic capacities. Therefore, preserving bone fidelity is of utmost importance for preserving the health of humans in space.

Our research focused on two ways spaceflight may impact bone. First, the microgravitational environment removes mechanical loading on bone and stresses its capability as a structural organ. Second, the higher ambient levels of carbon dioxide (CO2) that exist in a closed spacecraft may trigger blood acidosis and require the homeostatic capability of bone. We sought to assess whether the combination of these two factors may impact the structure and fidelity of the bone through loss of bone mineral and tissue- and how this may impact human health in spaceflight moving forward.

Is there a human analog for the study of the spaceflight effect on bone?

            Yes! Human analog studies of bone loss in spaceflight have taken place for nearly 75 years. In the early studies, horizontal bed rest was used to mimic microgravity through disuse. In the 2000s, standardized bed rest protocols were created which involved participants placed at -6°  head down tilt (HDT), in an attempt to simulate both disuse and the fluid shifts associated with microgravity exposure. In an attempt to increase fluid pressures in the head, which might be causing ocular issues, researchers modified the head down tilt protocol by not providing a pillow and not allowing subjects to lean on an elbow while eating. This “strict HDT” bed rest analog has not been evaluated with regard to bone effects of bed rest. Thus, one aim in the studies reported here was to determine whether the strict HDT analog was an effective analog for the study of spaceflight-associated changes in bone metabolism.

Why is this research important?

            Elevated CO2 exposure in spaceflight is a significant health concern for astronauts. High levels of CO2 can provoke astronaut concerns such as the onset of headaches and irritability. As NASA’s Artemis missions push exploration beyond low Earth orbit towards Mars, it is necessary to fully understand the effects of elevated CO2 exposure on bone health. Therefore, this research is especially important as a proof of concept of the strict HDT model for bone unloading.

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