Bone Loss

Abstract

While in the space environment, healthy astronauts can lose bone mass at a rate 10-fold faster than do post-menopausal women here on Earth. Not only does bone mineral density (BMD) decline roughly 1% per month in-flight, but equally important changes occur in bone geometry and architecture that diminish bone strength and its resistance to fracture; volumetric BMD of cancellous bone at the femoral neck and lumbar spine declines even more rapidly, up to 2.5% per month. Early increases in bone resorption markers in-flight are accompanied by smaller declines in bone formation markers. The primary risk of fracture occurs when crew members transition back to the full gravity of Earth or perhaps even to the 3/8 g environment of Mars in the future. There is tremendous individual variability in the rate of bone mass loss in-flight and during recovery after return to the 1 g of Earth, which we need to understand better. Evidence to date suggests that exposure to galactic cosmic radiation, ubiquitous in the space environment, may exacerbate bone loss due to the unweighting effect of microgravity; future work needs to investigate whether continuous, long-term exposure to high energy radiation has the same effect. Current countermeasures utilized aboard ISS (resistance exercise and adequate caloric and Vitamin D intake) effectively minimize changes in BMD, but we have less information on their ability to mitigate negative changes in bone geometry. Pharmacological interventions used to treat osteoporosis on Earth may prove useful adjuncts to exercise and nutritional intake on long-duration exploration missions to protect bone integrity.