Extended duration of vertical position might impair bone metabolism

Abstract. Bone‐remodelling is markedly influenced by vectors of gravitational forces. Sleep‐deprivation, common during military training, involves a change in the normal balance between horizontal and vertical forces enacting on the skeleton. Stress fractures are likewise prevalent among army recruits. In order to investigate the impact of sleep‐deprivation on bone‐metabolism, three groups of young, healthy volunteers were selected to exercise the following: 63 h of sleeplessness (17 participants, group A); vertical sleep in a seated position for three consecutive nights (9 participants, group B); controls who slept 6 h a night horizontally (14 participants, group C). During periods of wakefulness, all participants were kept in an upright position. Twenty‐four hours' urine collection was strictly observed from two days prior to the experiment until two days after it (1 week). Changes in levels of the most characteristic bone‐metabolites, calcium and hydroxyproline indicate an increased bone‐resorption in the two experimental groups, but not in controls. The calcium excreted in the fasting urine peaked significantly at 72 h after the beginning of the experiment (+ 170% in group A; +68% in group B, relative to the basal level). Qualitatively, similar results were obtained with hydroxyproline. On an individual basis, approximately 40% of the participants in either group responded by exceeding urinary‐calcium elevation. A comparison of pre‐test bone‐density between responders and non‐responders, reveals a significantly lower bone‐density (‐ 5%) in calcium and hydroxyproline excretors. These results suggest a pre‐disposition to bone‐resorption associated with responsiveness to changes in the balance between gravitational forces.