Spaceflight osteopenia

Spaceflight osteopenia refers to the characteristic bone loss that occurs during spaceflight. Astronauts lose an average of more than 1% bone mass per month spent in space. There is concern that during long-duration flights, excessive bone loss and the associated increase in serum calcium ion levels will interfere with execution of mission tasks and result in irreversible skeletal damage.
History.
Bone loss has been observed during spaceflight since at least as early as Gemini in the 1960s. Although most early measurements of the amount of bone loss were not reliable, they did show bone loss in Gemini, Soyuz 9, Apollo, Skylab, Salyut 7, Mir, and the International Space Station. William E. Thornton, an astronaut and physician, was one of the biggest proponents of exercise as a way of preventing bone loss.
Cause.
Bone remodels in response to stress in order to maintain constant strain energy per bone mass throughout. To do this, it grows more dense in areas experiencing high stress, while resorbing density in areas experiencing low stress. On Mars, where gravity is about one-third that of earth, the gravitational forces acting on astronauts' bodies would be much lower, causing bones to decrease in mass and density.
Average bone loss of 1–2% was recorded in astronauts on Mir each month. This is in comparison to 1–1.5% bone loss in the elderly "per year", and 2–3% in postmenopausal women.
Countermeasures.
Since Gemini, exercise has been tried as a way of preventing bone loss, but it has not been shown to be successful. This may be in part due to lack of adequately designed studies (no controlled study had been done as of 2005, either in space or using bedrest as an attempt to simulate conditions which lead to bone loss). It is not known whether a different exercise regimen (perhaps including larger loads than past ones) would be effective.
Increasing dietary calcium and vitamin D is a standard countermeasure for osteoporosis. Clay is reportedly used by NASA for retaining calcium.
A variety of drug remedies currently used or proposed for osteoporosis may work for spaceflight, including hormone therapy (estrogen or progestin), selective estrogen receptor modulators, bisphosphonates, teriparatide, and others. Whether they can provide the same benefits for spaceflight as they do for osteoporosis is not yet known.