Biomedical Research 



resorption of bone raises the theoretical concern of susceptibility to fracture. Also, 

 hypercalciuria, or increased urinary excretion of calcium, may predispose 

 individuals to nephrolithiasis (kidney stones), especially when phosphate excretion 

 is increased. Although bone demineralization during weightlessness has not 

 caused acute or chronic adverse health effects during or following space flight, the 

 likelihood of such an eventual occurrence is not negligible when the heterogene- 

 ous renal transport kinetics that vary between individuals are considered. It is 

 important to recall that much of the architecture of bone (examples are the 

 cancellous bone struts in the femoral head and neck) results directly from 

 gravitational stress acting on body weight, and that the reshaping of bone after 

 fracture is closely related to the lines of weight-bearing force. The influence of 

 gravity on both the macrostructure of the skeleton, and the microstructure of 

 cortical and cancellous bone, is unquestioned. 



Urinary excretion of calcium and phosphorus observed among the Apollo and 

 Skylab crews paralleled the losses previously reported in healthy, immobilized 

 bed-rest subjects on Earth (13). Urinary excretion of hydroxyproline and total and 

 nonglycosylated hydroxylysine (indicators of bone matrix turnover) was also 

 elevated in Skylab subjects. It is particularly troublesome that the continuous 

 increase in calcium excretion during space flight showed no tendency to plateau. 

 A conservative extrapolation of the amount of calcium lost during relatively short 

 periods in space suggests that a 6-month mission would result in a loss of 2-3 

 percent of total body calcium (13). Measurements of bone density have 

 corroborated the evidence of negative calcium balance from bone calcium loss, and 

 there is added concern that this loss may not be recoverable after the flight and 

 that this may result in less dense (i.e., weaker) bone in crew members subsequent 

 to their missions. 



Current evidence has not demonstrated an increase in morbidity or mortality from 

 altered calcium metabolism after short space missions. However, the clinical effect 

 of longer missions (greater than 90 days) on calcium metabolism and skeletal 

 performance is unknown. Furthermore, although the obvious role of altered 

 calcium metabolism on bone structure and function has taken priority in current 

 studies, the data on hand suggest other concerns. For example, the negative 

 calcium balance is associated with an increased loss in magnesium. Since 

 hypomagnasemia is associated with altered coronary vascular reactivity and 

 ventricular ectopy (14), it is quite possible that hypomagnasemia may increase the 

 likelihood of cardiac dysrhythmias during acceleration or space flight. 



In microgravity the need to maintain skeletal muscle integrity is decreased since 

 there is less need for active opposition to gravity to maintain posture or move 

 limbs. Anthropometric measurements, stereometric analysis, and electromyographic 

 data have demonstrated that with space flight, there is loss of muscle strength, a 

 decrease in muscle mass, an increase in protein catabolism, and a persistently 

 negative nitrogen balance (13). 



Programs for prevention of muscle atrophy and skeletal demineralization are 

 hampered by insufficient understanding of the metabolism of bone and muscle in 



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