During space travel, astronauts are exposed to a lack of gravity. This affects their physiology in different ways, including cardiovascular and musculoskeletal deconditioning, eye changes and immune dysfunction. Studies have also shown that astronauts lose bone density in space. This is a significant and rapid change (called an “adaptation”) for an organ that is generally not prone to change.

Bones are involved with functions important to our health, including bodily structure and protection. They produce white and red blood cells, store minerals such as calcium, produce and secrete hormones, and allow us to move.

The first observation of bone loss related to space travel was recorded in the mid-1970s when Skylab crewmembers demonstrated losses of 1 to 2% bone mass per month compared to before flight and people on the ground. Since then, there have been a number of studies investigating bone adaptations from space travel.

The general findings report that weightlessness reduces the loading on bones, which changes how biological processes in the bone occur. Bone adaptations rapidly occur in space, with increases in bone resorption (a breakdown of bone tissue and loss of calcium) seen within the first two weeks of spaceflight. The condition increases over time with flights of longer duration.

There are also long-term implications astronauts need to consider, too. Studies have shown that changes in bone health do not fully return, even after a year after returning from long-duration space travel. Researchers are exploring how bone cells are changing and adapting as a result of exposure to microgravity. We have much still to discover.

The space environment provides a unique perspective in studying health because it is the only environment where we can learn about the biological adaptations to a lack of gravity. Bones are gravity-sensitive and provide a mechanism to understand gravity’s role in controlling and shaping biology.

Studying bone health in this context also provides scientists an opportunity to discover new pathways involved in regulating biological adaptations on Earth. For example, as NASA plans to send people with disabilities, including those with paraplegia, into space, these avenues may help us discover new therapies and how different bodies adapt to space. In turn, the discoveries made from our journey into space will also improve life on Earth for all.