During the space flight bone resorption occurs faster than new bone formation. The astronaut shows an increase in calciuria, averaging 250 mg calcium per day.
The exact cause of the osteoporosis is uncertain. It shares some features with disuse osteoporosis; however, it is more severe and continues throughout the flight (as opposed to the plateau seen with immobility).
After 1 week in space most bones lose 6-7% of their density (range 1% for vertebral spine to 11% for greater trochanter).
An astronaut may be at risk for fracture after returning from a long-duration flight if there is:
(1) significant osteoporosis
(2) balance problems and incoordination
(3) weakness secondary to significant muscle atrophy
Calcium redeposition following a mission occurs at 100 mg per day. The estimated time to recover is 2.5 times the duration of the spaceflight in days (250/100). Complete recovery in bone mass may take 2 years.
Prevention:
(1) Intense exercise is only partially effective in preventing the bone loss.
(2) Bisphosphanates may be helpful.
