Skeletal effects of long-duration head-down bed rest.
BACKGROUND
Skeletal unloading during spaceflight causes regional loss of bone mineral density (BMD), primarily in the spine and lower body regions. This loss of skeletal mass could adversely affect crew health during and after spaceflight and jeopardize mission success. Bed rest has long been used as a spaceflight analog to study the effects of disuse on many body systems, including the skeleton. This study was undertaken by the NASA Flight Analogs Project (FAP) to collect control data for upcoming countermeasure studies.


METHODS
There were 13 subjects who participated in 42, 44, 49, 52, 60, or 90 d of continuous, head-down bed rest. DXA scans (dual-energy X-ray absorptiometry) were obtained before and after bed rest to measure changes in BMD of the whole body, lumbar spine, hip, heel, and wrist; the 90-d subjects were also scanned at the 60-d time point. Follow-up DXA scans were performed after 6 mo and 12 mo of reambulation to assess BMD recovery.


RESULTS
BMD changes were consistent with earlier bed rest and spaceflight studies, with statistically significant losses averaging 1% per month in the hip, pelvis, and heel. Recovery data were also consistent with data obtained after spaceflight. Bone biomarker data are described, and support the findings of previous studies. Specifically, the process of normal bone remodeling is uncoupled: increased bone resorption with no concomitant change in bone formation.


CONCLUSIONS
The FAP appears to be a valid test bed for skeletal disuse studies, and should provide a useful research platform for evaluating countermeasures to spaceflight-induced bone loss.
Spector
Elisabeth R
ER
Bone and Mineral Laboratory, Human Adaptation and Countermeasures Division, NASA Johnson Space Center, Houston, TX 77058, USA.
Smith
Scott M
SM
Sibonga
Jean D
JD
D015502:Absorptiometry, Photon; D000328:Adult; D001510:Bed Rest; D015519:Bone Density; D016723:Bone Remodeling; D001862:Bone Resorption; D001842:Bone and Bones / Q000000981:diagnostic imaging / Q000378:metabolism / Q000502:physiology*; D005260:Female; D018475:Head-Down Tilt; D006801:Humans; D008297:Male; D009141:Musculoskeletal System / Q000503:physiopathology; D011187:Posture / Q000502:physiology; D016730:Program Development; D015397:Program Evaluation; D011446:Prospective Studies; D013026:Space Flight; D013997:Time Factors; D014481:United States; D018557:United States National Aeronautics and Space Administration; D014893:Weightlessness / Q000009:adverse effects