Bone Metabolism During Strict Head-Down Tilt Bed Rest with and without CO2 Exposure

Spaceflight and spacecraft have many negative physiological effects on the human body. One such factor is the exposure to elevated levels of carbon dioxide (CO2). In fact, the CO2 levels on board the International Space Station (ISS) have often reached levels 10x higher than outdoor terrestrial levels (1). Among other effects of CO2, it is possible that this exposure alters bone metabolism, leading to an increase of bone tissue resorption and mineral efflux, thus jeopardizing bone fidelity and mission success. Bed rest is a common analog to simulate the effects of microgravity on bone as subjects are placed at a -6° head-down tilt (HDT) position which reduces the mechanical load on bone (2).