Abstract
Microgravity-induced bone loss is currently a significant and unresolved health risk for space travelers, as it raises the likelihood for irreversible changes that weaken skeletal integrity and the incremental onset of fracture injuries and renal stone formation. Another issue related to bone tissue homeostasis in microgravity is its capacity to regenerate following fractures due to weakening of the tissue and accidental events during the accomplishment of particularly dangerous tasks. Today, several pharmacological and non-pharmacological countermeasures to this problem have been proposed, including physical exercise, diet supplements and administration of antiresorptive or anabolic drugs. However, each class of pharmacological agents presents several limitations as their prolonged and repeated employment is not exempt from the onset of serious side effects, which limit their use within a well-defined range of time. In this review, we will focus on the various countermeasures currently in place or proposed to address bone loss in conditions of microgravity, analyzing in detail the advantages and disadvantages of each option from a pharmacological point of view. Finally, we take stock of the situation in the currently available literature concerning bone loss and fracture healing processes. We try to understand which are the critical points and challenges that need to be addressed to reach innovative and targeted therapies to be used both in space missions and on Earth.
Highlights
Today, it is well established that the space “exposome”—the set of environmental factors related to space to which an astronaut is constantly exposed during a mission— can significantly influence human physiology [1]
We find physical stimulation therapies, such as extracorporeal shockwave treatment (ESWT), low-intensity pulsed ultrasound (LIPUS) or low-level laser therapies (LLLT) [99–102]
Because of reduced loading stimuli, this equilibrium is lost, and bone resorption prevails on bone formation, leading to bone mass loss at a rate of about ten times that of Earth osteoporosis [1,24]
Summary
It is well established that the space “exposome”—the set of environmental factors related to space to which an astronaut is constantly exposed during a mission— can significantly influence human physiology [1]. Within the first 72 h upon exposure to weightlessness, certain physiological systems exhibit altered function These include baroreceptor reflex, neurovestibular system, and gastrointestinal tract, leading to problems like nausea and vomiting, space motion sickness (SMS), sleep disturbances and headaches [7]. Among long-term effects on human physiology, we find muscle atrophy, bone demineralization and, alteration in calcium balance, immune function dysregulation, endocrine disorders, such as insulin resistance [10,11], and cardiovascular deconditioning that leads to orthostatic intolerance [12]. These physiological changes present major obstacles to long-term space missions. This review discusses alteration of human physiological parameters related to bone metabolism due to long-term weightlessness exposure and examines the effectiveness of both pharmacological and non-pharmacological countermeasures
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