Astronauts lose 1% to 1.5% of bone density a month in microgravity, much faster than the 0.5% to 1% a year lost by aging men and post-menopausal women on Earth. Weight-bearing bones like the spine, hips and legs are most affected. Melanie Coathup, a UCF professor of medicine, attributes the loss to the reduced stimulus of weight, which bone needs to maintain healthy mass.