What do we know about the effects of spaceflight on bone?

This review of the peer-reviewed literature focuses on the effects of spaceflight on bone. Studies performed in humans and laboratory animals have revealed abnormalities in bone and mineral metabolism that suggest that long-duration spaceflight will have detrimental effects on the skeleton. However, because of large gaps in our knowledge, it is not;presently possible to estimate the magnitude of the health risk, individual variations in risk, effective countermeasures, or mechanism(s) of action. Specific recommendations are made for future research to ascertain risk and develop appropriate countermeasures.