absorption (x-ray). The bone loss observed may not simply result from an increase in the bone resorption but also from a decrease in bone formation. The results reported here pertain to studies performed on manned Soviet and American flights of a duration from 2 weeks to more than 6 months, and to a post mortem investigation that was carried out on bones from the cosmonauts killed by accidental decompression of the Salyut-1 space station after a flight of 24 days [11, 12, 14, 15, 16]. The finding during American and Soviet space flights have shown a calcium loss beginning as soon as the astronauts became weightless. The mean calcium loss is small (50 mg/day) during the first month, but it increases during the second month up to a level of 200mg/day by the sixtieth day (the total mass of exchangeable calcium is 1 kg). The data suggest that after the 84-days Skylab flight the astronauts had lost a total of 25g of calcium [15]. Both fecal and urinary excretion are increased, but their relative importance varies between different flights. Studies of bone structure in the immediate post flight period have shown a greater demineralization of weight-bearing bones such as the calcaneum than on other bones such as the radius. After a flight, the calcium loss persists for a period of at least 18 days, but the calcium balance tends to become less negative and should eventually return to an equilibrium, [13, 14, 15]. It may be that some calcium loss is permanent: measurements on Skylab astronauts five years after their last flight showed that the mineral density of the calcaneum was lower than before the flight and lower than in control subjects [14]. Recent fragmentary reports of Soviet CAT Scan show little or no evidence of vertebral bone loss [6]. In addition to the calcium loss, an increase in the bone turnover or resorption is suggested by the elevated urinary hydroxyproline during the first 30-40 days of weightlessness. Then its level tends to stabilize [15]. Bone growth in weightlessness has been studied in young rats aboard the Cosmos flights for a total duration of 18-24 days: after 11-12 days, the bones' growth is stopped for the weight-bearing bones, but does not seem affected for the other bones. Flight rats allowed to readapt to earth gravity resumed bone formation at a rate similar to that of control animals [13, 14, 15]. The hypothesis is that there is, in a weightless environment, a persistence or an increase of the bone resorption, with decreased bone formation. The major cause of these changes seem to be weightlessness, as suggested by the predominance of effects on weight-bearing bones and the absence of major variations in the levels of the hormones involved in phosphocalcic regulation [13]. Without the feedback provided by the receptors sensitive to gravity, the cells responsible for bone resorption are activated, and they remove from the bones calcium and phosphorus which are then eliminated. The effects of long-term flights and the long-term effects of the bone loss already incurred by astronauts seem difficult to extrapolate. Most data suggest that the calcium balance returned towards equilibrium, i.e. excretion is equal to the intake, but it is possible that a permanent loss of calcium may have occurred. If bone formation does cease in astronauts exposed to weightlessness, it is possible that there may be an irreversible component to the bone loss [12, 14, 15]. With advancing age, this may result in increased risks of bone fracture, and particularly of femur and vertebral damage. Countermeasures of various types have already been tried on American and Soviet flights. They act on both the physical and biochemical aspects of bones' physiology. The physical measures have been experimented with and refined from the earliest
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