http://www.scientificamerican.com/article/how-does-spending-prolong/

How does spending prolonged time in microgravity affect the bodies of astronauts?
August 15, 2005
Jeffrey Sutton, director of the National Space Biomedical Research Institute and Nitza Cintr¿n, chief of NASA's Space Medicine and Health Care Systems Office, explain.
Space is a harsh environment that affects the body in many ways. In microgravity, bone loss occurs at a rate of 1 to 1.5 percent a month, leading to an acceleration of age-related changes similar to osteoporosis. Decreases in bone density and strength are more pronounced in some skeletal regions, such as the pelvis, although much of the loss is reversible upon return to Earth. Prolonged exposure to weightlessness also increases the risks of kidney stones and bone fractures, which are both associated with bone demineralization. In addition, studies suggest that microgravity alters the ability of bones to heal after fractures.