Participating in long-duration spaceflight can be exciting, but it also has a dark side: it accelerates the process of osteoporosis. Staying in zero gravity leads to undesirable consequences for the skeleton and loss of bone mass.
This makes sense if we think that the human skeleton is designed to withstand the constant gravitational loads of the planet we live on, Earth. Locomotion under the influence of gravity is what stimulates bone formation (produced by osteoblasts) and maintains bone homeostasis. This is why physical activity is a very important factor—perhaps the most important—to keeping your bones in tip-top condition.
Thus, among healthy people who play tennis or football, it was observed that bone mineral density (BMD) is higher in the dominant limb (DOI: 10.1007/s002239900467). For patients with osteoporosis, several studies and systematic reviews have confirmed that exercise. results in increased bone density regardless of whether they have had fragility fractures or not.
Finally, patients who are unable to ambulate adequately, as occurs in people with paraplegia or tetraplegia, suffer significant and rapid bone loss due to a sharp decrease in bone formation (called osteoblastic activity) together with a marked increase in bone breakdown (produced by osteoclasts) . Because less formation coexists with more bone destruction, these patients experience significant and rapid loss of bone mass, both in quantity and quality, causing a disease known as osteoporosis. Fragile fractures are one of the many clinical problems faced by these patients.
Osteoporosis in space
Likewise, in zero gravity, the gravitational loads that normally act on bones are dramatically reduced. This means that there is the same decrease in osteoblast activity and the same increase in osteoclast activity as with wheelchair immobilization, with subsequent loss of bone mass and changes in bone quality, which increases the risk of fractures.
Studies conducted on astronauts spending several months on the International Space Station (ISS) show a loss of 1-2% per month in BMD in the hips and lumbar spine. Excessively fast, considering that this is equivalent to the losses that an elderly person suffers in an entire year, especially in women after menopause.
To make matters worse, weightlessness affects bone microarchitecture, which is directly related to bone quality. Recent studies have shown that long-term exposure to the microgravity experienced in space reduces the quality of trabecular and cortical bone, contributing to an increased risk of fractures (DOI: 10.3357/AMHP.5007.2018).
Therefore, even if astronauts manage to restore bone mass after returning to Earth from space travel, bone microarchitecture may not fully recover (DOI: 10.1038/s41598-022-13461-1).
The dangers of a round trip to Mars
The main risk factor for bone changes has been found to be travel duration. The longer the weightless conditions persist, the greater the deterioration of the condition.
According to NASA, a one-way trip to the Red Planet will take at least nine months. Making the round trip would mean being in zero gravity for about 21 months, since we would have to wait on Mars for about three months to ensure that Earth and the neighboring planet are in the perfect place to return home. The time spent in zero gravity could be longer if, as some NASA reports indicate, astronauts make a stopover on the Moon.
If bone loss continues at a rate of 1-2% per month, astronauts could lose almost half of that mass en route to the Red Planet, not taking into account structural changes that are currently difficult to accurately quantify.
In addition, it must be taken into account that the risk of falls is higher in astronauts due to weak muscle function, which is lost in a similar way to bone mass as a result of the lack of gravity.
Finally, both when landing on Mars and when returning to Earth, the space probe will descend at tremendous speed. These impacts can cause fractures in a skeleton that was previously weakened, has lost more than half its bone mass, and has severely altered microstructure.
How can we avoid this problem?
Typically, astronauts are young and healthy people, well prepared physically, with proper nutrition, so they leave Earth in optimal conditions to avoid fragility fractures.
However, there are some recommendations that should be taken into account in order to take as much care of the bone health of space travelers as possible:
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Diet and nutrition. Inadequate intake of calcium and vitamin D during spaceflight can worsen bone loss. It is therefore important that adequate intake of both substances is maintained throughout the trip, which can be achieved by using medications containing these elements.
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Performing physical exercises during space flight. The use of exercise machines and resistance bands is now virtually standardized on all space missions precisely to minimize the loss of bone and muscle. These exercises are aimed at simulating the gravitational loads experienced by the skeleton on Earth. However, they are not enough to prevent bone loss.
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Antiresorptive drugs. Drugs such as bisphosphonates, which were used to treat osteoporosis on Earth, have been studied to preserve bone mass in astronauts (DOI: 10.1007/s00198-012-2243-z). Their use is not widespread, but during long journeys such as flight and return to Mars, they should be seriously considered.
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Simulation of artificial gravity. Some recent studies suggest using artificial gravity through rotating centrifugal force to reduce bone loss during long-duration spaceflight (DOI: 10.3357/AMHP.5855.2021).
Even taking all these measures, they are not enough to avoid the effects of long-term space travel on the skeleton: the lack of gravity will continue to cause decreased bone formation and increased bone breakdown. There is no other alternative but to continue researching how to prevent this.