Calcium loss from bones while in the weightless state has been found to be continuous in all space flights so far studied. There appears to be no reversal in this trend even when extra calcium is added to the diet. This situation could lead to bone fragility, to urinary calculi, etc., and thus limit the time workers can remain in space. There is a fairly rapid reversal upon return to earth, and it appears that the bone may return almost if not completely to normal. There is a possibility that with repeated trips to space there may be some irreversible damage to bone structure. Muscle tissue reduction has taken place in all space flights to date. There is some evidence from Skylab that a regimented exercise program may, at least partially, reverse this trend. Fluid electrolyte changes occur early in space flight and are sustained throughout flights up to 84 days in duration. These changes are accompanied by changes in hormones, some of which regulate kidney function. Thus, these changes may be a combination of fluid shifts due to weightlessness and hormonal changes with cause and effect interrelated. Nutrition needs in the weightless state, in general, appear to be the same as those on the ground except that as time in space is extended there appears to be need for more nutrients, i.e., fat loss continued throughout the 84-day Skylab flight. Calcium loss (as discussed above) remains puzzling and protein intake may need to be lowered because of its possible effect on calcium metabolism. Nitrogen balance remains a puzzle, especially as related to muscle loss. Vestibular middle ear balance center problems such as "space-motion sickness," dizziness, vertigo, spatial disorientation, sensations of rotation, etc., have affected a high percentage of space travelers. The physiological mechanisms underlying these sensations is unknown. The motion-sickness seems to lessen after about seven days; some of the other sensations persist longer. There is as yet no way to predict which persons will be most affected. Drugs used for other types of motion~sickness have some effectiveness but are far from a completely satisfactory treatment of the problem. No ways are currently known to avoid or completely ameliorate the problem. It appears that these problems increase with increased physical activity and with increased space to move about in; thus they may interfere greatly with performance efficiency in space and may be especially critical for construction workers.
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