Effects of Recumbency and Space Flight on Bone Density

Abstract

Evaluations of bone mass changes in terms of calcium hydroxyapatite equivalency have been made for the central section of the os calcis and for hand phalanx 5-2 of men participating in seven bed rest units consisting of an equilibration period, a 14-day bed rest period, and a reconditioning period, with the level of calcium intake during bed rest ranging from 300 to 2,000 mg daily. The same type of bone mass determinations was made on the crews of the Gemini IV, Gemini V, and Gemini VII missions. The method of radiographic bone densitometry was used in evaluating skeletal changes in both groups of subjects. A significant negative coefficient of correlation was found between bone mass losses in the central section of the os calcis and the mean levels of daily calcium intake after 14 days of horizontal bed rest. Significant positive correlations were found between intake of dietary calcium and outgo of urinary and of urinary and fecal calcium. In four bed rest units in which the same level of dietary calcium was fed in the prebed rest ambulatory equilibration period as during the bed rest phase, the urinary and the combined urinary and fecal calcium output during bed rest surpassed that during ambulation by significant differences throughout. With respect to the three groups of astronauts, the duration of the orbital flight evidently was not the sole factor in losses of bone mass, since the astronautts engaged in the longest space flight experienced the lowest negative changes in bone density. Although mean daily calcium consumption was found to be related negatively to bone density losses in subjects during bed rest, the same could not be stated unequivocally concerning astronauts during space flight with the evidence now in hand because of uncontrolled variables, such as stress and dietary factors in addition to calcium. Also the exercise program introduced into the Gemini VII flight imposed a new variable which contributed to the reduction of bone mass in the os calcis as supported by ground based trials with bed rest subjects. Because of the small number of subjects in the space flight study to date, further data will need to be acquired from future flights in order to understand the interrelationships more fully.