Human space exploration that started almost 60 years ago marked the beginning of a new field in biomedical research: an investigation of the effect of reduced gravity, an environment never faced by humans before, on our health. Bone loss in microgravity was first observed in astronauts of a Gemini 4 mission in 1965 and since then has been recognized as a serious risk for long-term space missions. Microgravity-induced bone loss has been linked to reduced mechanical loading of bones, alterations in the metabolism and calcium homeostasis, stress, and elevated radiation levels in space. Unfortunately, despite nutritional, exercise and pharmacological countermeasures developed based on these findings, the microgravity-induced bone loss remains an unresolved health risk jeopardizing future long-term missions to Mars and the Moon.
To date, 566 people have participated in spaceflights, which represents several decades of collected biomedical data that has not been utilized to its full potential to study the dynamics of microgravity-induced bone density change and factors contributing to it. Through a systematic search, our team identified that published literature contains an impressive dataset that includes bone density measurements for 148 astronauts and biochemical bone markers measurements for 124 astronauts. By employing meta-analytic tools, we have found that after the space flight bone density increases in the upper body and is lost in the lower body; that fast stimulation of bone resorption in microgravity is likely responsible for the bone loss, and that recovery after return to Earth depends on the flight duration and is likely limited by the length of the recovery phase. These findings allowed us to confirm that bone response to microgravity is linked to the gravitational vector, to demonstrate that factors other than mechanical loading are likely involved in the microgravity-induced bone loss, and to quantify a degree of individual variability that suggests that some people are less susceptible to microgravity-induced bone loss than others.
However, the true power of such a systematic review was in identifying the limitations of current studies to guide the planning of future missions. Combining data from many space travellers allowed us to estimate the degree of variability in spaceflight-related bone loss, which in turn is critical for estimating sample size required to detect changes when countermeasures are investigated. We showed that 10-20 astronauts per mission would be required to reach reliable conclusions. However, missions with such a high number of astronauts are currently not feasible. Therefore, it is crucial to standardize methodology and reporting so that data from different missions can be combined. Our study also demonstrates that dynamics of human adaptation to return to Earth environment is not fully understood, and suggests that larger-scale and longer-duration studies are needed to understand the long-term effects of space travel. Understanding how humans adapt to extra-terrestrial environment is a key step in becoming a spacefaring civilization.
This post is a collaboration between Mariya Stavnichuk and Svetlana V. Komarova.