Effects of long-duration space flight on calcium metabolism: Review of human studies from Skylab to the present

One of the major effects of prolonged weightlessness seen in long-duration space flights has been an extended loss of bone from the skeleton. The principal characteristics of this loss were shown in the metabolic studies carried out on the Skylab flights of 1, 2 and 3 months in 1973 and 1974. These studies now provide the background for a comprehensive review of the considerable number of subsequent calcium studies in humans during space flights from that time until the present. Because of the close similarities in pattern and degree between space flight and bed rest in effects on calcium metabolism, relevant long-term human bed rest studies have been included. An analysis is presented of the bone calcium loss data with respect to degree, duration and significance, as well as relative failure of reversibility or recovery following flights. Possible mechanisms of bone loss are discussed: the physiological condition of disuse atrophy, increase in bone resorption, decrease (later and lesser) in bone formation, decrease in intestinal calcium absorption, increase in glucocorticoids, along with the threat of urinary tract stone formation and proposed countermeasures. Considerable future research is needed, particularly on mechanisms of bone loss and on countermeasures, to be carried out on the International Space Station and via bed rest studies, before a mission to and return from Mars is undertaken.