Bone Metabolism and Nutritional Status during 30-day Head-Down Tilt Bed Rest

Bed rest studies provide an important tool for modeling physiologic changes of spaceflight. Markers of bone metabolism and nutritional status were evaluated in 12 subjects (8 male, 4 female; ages 25-49 yr) who participated in a 30-day head-down tilt diet-controlled bed rest study. Blood and urine samples were collected twice before, once a week during, and twice after bed rest. Data were analyzed using a mixed-effects linear regression with a priori contrasts comparing all days to the second week of pre-bed rest acclimation. During bed rest, all urinary markers of bone resorption increased about 20 % (P \u3c 0.001) and serum parathyroid hormone decreased about 25 % (P \u3c 0.001). Unlike longer (\u3e60 d) bed rest studies, neither markers of oxidative damage nor iron status indices changed over the 30 days of bed rest. Urinary oxalate excretion decreased about 20 % during bed rest (P \u3c 0.001) and correlated inversely with urinary calcium (R = -0.18, P \u3c 0.02). These data provide a broad overview of the biochemistry associated with short-duration bed rest studies, and provide an impetus for using shorter studies to save time and costs wherever possible. For some effects related to bone biochemistry, short-duration bed rest will fulfill the scientific requirements to simulate spaceflight, but other effects (antioxidants/oxidative damage, iron status) do not manifest until subjects are in bed longer, in which case longer studies or other analogs may be needed. Regardless, maximizing research funding and opportunities will be critical to enable the next steps in space exploration