EXTENDED DURATION OF VERTICAL POSITION MIGHT IMPAIR BONE METABOLISM

Bone-remodelling is markedly influenced by vectors of gravitational fo rces. Sleep-deprivation, common during military training, involves a c hange in the normal balance between horizontal and vertical forces ena cting on the skeleton. Stress fractures are likewise prevalent among a rmy recruits. In order to investigate the impact of sleep-deprivation on bone-metabolism, three groups of young, healthy volunteers were sel ected to exercise the following: 63 h of sleeplessness (17 participant s, group A); vertical sleep in a seated position for three consecutive nights (9 participants, group B); controls who slept 6 h a night hori zontally (14 participants, group C). During periods of wakefulness, al l participants were kept in an upright position. Twenty-four hours' ur ine collection was strictly observed from two days prior to the experi ment until two days after it (1 week). Changes in levels of the most c haracteristic 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 significant ly at 72 h after the beginning of the experiment (+170% in group A; +6 8% in group B, relative to the basal level). Qualitatively, similar re sults were obtained with hydroxyproline. On an individual basis, appro ximately 40% of the participants in either group responded by exceedin g urinary-calcium elevation. A comparison of pre-test bone-density bet ween 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 responsiv eness to changes in the balance between gravitational forces.