One consequence of space flight is a significant loss of bone mass (both matrix and mineral) in a weightless environment. During the early Gemini and Apollo flights, x-ray densitometry studies of the calcaneus of astronauts indicated that bone loss may be extensive and rapid even during a brief period of weightlessness. In later Apollo and Skylab space missions, a more precise technique, photon absorptiometry, was employed to assess the preflight and the postflight bone mass of the calcaneus. Findings showed a direct dose-response relationship between time spent in a weightless environment and loss of bone mass, although wide variations between astronauts were observed. These findings were corroborated by Mir space flight studies of changes in bone mass carried out on Soviet cosmonauts. Overall, for the proximal femur and spinal vertebra locations, there is an average loss of 1.0% to 1.5% bone mass and density for every month in space. In relative comparison, this level of bone loss would occur during menopausal osteoporosis over a 10- to 12-month time period.

The just-mentioned bone losses occur mostly in the weight-bearing bones of the lower limb and axial skeleton, with the greatest changes seen in the trabecular bone tissues having high-remodeling rates. After return from space, skeletal mass is gradually restored, although the recovery times on Earth are usually much longer than the actual space flight duration times. In some cases, bone mass recovery on Earth after a space mission is not completely restored, even after a prolonged recovery in Earth’s gravitational field. For these cases with greatly prolonged weightlessness, full recovery is not anticipated because of irreversible loss of the surface scaffolding that is necessary for bone cell activity. Studies of metabolic balance, performed on the Skylab crew during flights lasting 28 to 84 days, revealed a striking increase in urinary calcium levels, which reached a plateau after 28 days in space. Fecal calcium levels, however, continued to increase, with no sign of leveling off even after 84 days in space. After only 10 days of weightlessness, the preflight positive calcium balance was abolished and a net negative calcium balance prevailed for the remainder of the flight. Apparently, dietary calcium sources used by the Skylab crew were not effectively absorbed during space flight. Rather, systemic calcium homeostasis was seemingly maintained in these astronauts by release of calcium from the skeleton.

Space flight’s loss of total body calcium was much greater than that predicted from bed rest studies. Assuming that bone loss was continuous without a stabilizing end point during space flight, these findings led Rambaut and Johnston to calculate that a year of weightlessness might result in a loss of up to 20% of the body’s total calcium reserve removed principally from trabecular bone surfaces. As occurs with immobilization, hypercalciuria is accompanied by hydroxyprolinuria, indicating that bone’s organic matrix as well as mineral is lost. This finding was confirmed by the detection of elevated levels of collagen degradation products in the urine of Skylab astronauts. Results of studies of metabolic balance also showed a profound loss of total body nitrogen, reflecting a concomitant precipitous loss of muscle mass. Astronauts on later Skylab missions exercised vigorously, but these particular exercises did not restore or lessen their calcium loss.

Within 10 days after return to Earth, the astronauts’ urinary calcium levels returned to normal but the fecal calcium content remained elevated. Thus, total body calcium balance remained negative even 20 days after the space flight.

Osteoporosis caused by weightlessness is more severe and unrelenting than any form of disuse osteoporosis here on Earth. The roles played by muscle contraction, periosteal tension, circulatory physiology, and bioelectric and piezoelectric properties of bone in weightlessness provide intriguing topics for further investigations. The study of bone physiology in the weightless environment of space is important for several reasons. First, the prolonged time in space required for interplanetary travel could result in the severe and permanently disabling complications of profound osteopenia, which would elevate the risk for fracture on reattaining weight-bearing activities under normal gravity loads. Therefore, protective countermeasures must be developed to mitigate bone loss during long space missions. Second, the weightless environment provides a natural opportunity for studying the complexities of normal bone physiology as well as a multitude of osteopenic conditions.