Bone serves several important functions in the body, acting as a rigid structure necessary for movements and as a storage area for calcium. Thus, the deleterious effects of the space environment upon bone structures is a major field of research in space medicine. In a weightless environment, bone formation decreases rapidly. This phenomenon is due to the response of a series of cytokines and receptors on osteocytes, osteoclasts and osteoblasts (all involved in bone formation and turnover), which allow bone structures to “sense” changes in loading conditions. Thus, in the weightless conditions of space, bone resorption increases. In weight-bearing areas such as the hips and legs, bone loss occurs on a much greater scale than in non-weight-bearing areas such as the skull, where bone formation has been known to even increase.
The increase of serum calcium, and subsequent decrease of parathyroid hormone, in space also aids the loss of bone. Low levels of parathyroid hormone accompany a reduction in calcium absorption.
High levels of carbon dioxide (regularly found on a space station) can also trigger bone resorption, as high carbon dioxide levels can lead to a compensated respiratory acidosis. Because bone takes part in the neutralizing of excess acid, an increased acid load to neutralize can increase bone resorption.
Finally, low light levels in the spacecraft mean that little Vitamin D will be formed in the body. Combined with a suppression of parathyroid levels, the synergistic effect will be extremely low levels of Vitamin D, which can impair the absorption of calcium.
The overall picture of bone loss in space includes an increase in bone resorption, a decrease in bone formation, and lower levels of parathyroid hormone. For missions where astronauts return back to Earth, such bone loss can be accommodated through training programs and the allowance of time, but for missions landing on other planets (e.g., Mars), which will require immediate action upon landing, the most efficient countermeasures will be necessary.
Space is not the only environment in which bone loss can occur. In the U.S., at least 10 million people suffer bone loss called osteoporosis. Thus, this is not just an issue arising in space, but one we have faced for thousands of years here on Earth. As NASA has stated: “Researchers hope that solving the riddle of bone loss in space will reveal important clues about what causes osteoporosis (and other bone disorders) right here on Earth.”
Commonly used countermeasures include supplements such as calcium and Vitamin D. Calcium aids the stability of bone, while Vitamin D supplements prevent Vitamin D deficiency that can impair calcium absorption. Also, exercise programs targeting the lower, weight-bearing extremities can prevent some bone loss. Hip, femoral, tibial, lumbar spine and calcaneal loads can also help maintain bone mass, when used while the individual is harnessed onto a treadmill. Also, drugs such a bispgosphates, thiazide diuretics, potassium citrate, etc., can decrease bone resorption, increase the reabsorption of calcium, and deal with the side effects of bone loss (e.g., kidney stones resulting from excess free calcium). Perhaps the most effective countermeasure would be artificial gravity, as it would allow astronauts to avoid all the negative side effects of weightlessness, but such technology can be expensive and bulky.
In the field of Astro-Omics, a different array of countermeasures can be found. For instance, individual monitoring can be helpful in tailoring personalized countermeasures. Technology exists to analyze serum calcium, electrolytes, and other blood parameters, as well as biochemical markers of bone formation and bone resorption. Such equipment can be used to understand an individual astronaut’s health needs and to meet them. For instance, it has been discovered that by keeping urinary parameters in acceptable ranges, the recurrence of a kidney stone can be highly unlikely. This shows the power of individualized monitoring and treatment.
Also, by simply recognizing the differences in an astronaut’s profile, an individualized treatment plan can be created. For instance, the estrogen deficiency that accompanies menopause can increase bone turnover and bone loss. This would dictate a much different treatment for a postmenopausal astronaut. Similar deficiencies would also have to be diagnosed, treated, and monitored in space.
Genomic, proteomic, and metabolic markers can also play a key role in how an individual may react to the space environment. For instance, an astronaut with a one carbon metabolism can have increased osteoclast activity (increased bone turnover) and decreased osteoblast activity (decreased bone formation). Similarly, HFE, a genetic variant for hemochromatosis (which affects about one in every 200-250 Caucasians) causes high levels of iron, which has been linked with a decrease in bone mineral density. In both cases, a thorough assessment would have to be conducted, to better understand the issue and possible countermeasures. These countermeasures would be specifically designed to deal with the symptoms and the problem itself. For instance, an individual with HFE would not continue to receive the usual iron supplements, and would instead follow an iron-free diet.
Overall, due to a combination of genetic, physical, and environmental factors, the treatment plan would have to be flexible and individualized. An assessment of the major factors relating to bone would be the most effective to provide an Astro-Omics-based treatment plan. Also, setting clear-cut clinical requirements, as to who would not be a good candidate based on these factors, would help prevent possible medical emergencies in space.
For more information, visit the NASA website to learn about “Space Bones”: http://science.nasa.gov/science-news/science-at-nasa/2001/ast01oct_1/