Effect of prolonged bed rest on urinary calcium output.

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Abstract
The effect of prolonged supine position on the urinary nitrogen and calcium output was studied on young healthy men. Bed rest increased the excretion of calcium. The nitrogen output did not show any consistent response. Exercise on a bicycle ergometer in sittting or in supine position failed to change the course of calcium excretion. Supine exercise up to 4 hrs./day did not decrease the urinary calcium output which was previously elevated by a complete bed rest. Quiet sitting for 8 hrs. combined with 16 hr lying did not prevent the rise of calcium output. On the other hand, 3 hrs./day quiet standing proved to be sufficient to induce a slow decline of the elevated calcium excretion in 4 out of 5 subjects. Following a complete bed rest in the recovery phase when the subjects resumed their normal up-and-about activities, both the nitrogen and calcium excretion rapidly decreased below the base-line value of the individual. It is concluded that the increase in urinary calcium output in prolonged horizontal position is due to the absence of longitudinal pressure (weight bearing) on the bones rather than the physical inactivity during bed rest.