Space is not a welcoming place for the human body. The radiation-infused vacuum is not exactly an oasis where life (as we know it) can thrive. And yet, the stars keep calling our name like a beacon in the night.
Fortunately, we have (and continue) to develop spaceships that can shield us from the worst of the radiation and the vacuum. Spaceflight is an incredible feat of human ingenuity and exploration, but even with spaceships at our disposal, it comes with a unique set of challenges for the human body. Microgravity, radiation exposure, confinement, circadian derailment, and disruptive workloads are just some of the stressors that astronauts face while in space.
The effects of spaceflight, especially of long duration, on the human body are complex and multifaceted. Perhaps the most well-known is sarcopenia or muscle wasting. In our daily lives, simply moving around requires our muscles to work against gravity. Think of this as a continuous mini-workout. In spaceships or space stations that lack earth-level gravity, this means no more workout and bye-bye muscles. That’s why astronauts have to follow a rigorous exercise program before, during, and after their missions.
Muscle, though, is far from the only tissue affected. Bone density goes down, the circulatory system starts to sputter, and all the fluids in the body — blood, lymph, cerebrospinal fluid, etc. — are distributed differently in low-gravity conditions. For example, more cerebrospinal fluid can put pressure on the back of the eye, leading to vision problems.
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On a molecular level too, things begin to go awry during a long-duration trip to space.
Let’s take a closer look at the immune system. A 2019 study of 12 cosmonauts who participated in a long (>140 days) spaceflight mission found several undesirable changes in their immune systems. Another 2019 study of 2 ISS crew members study suggests that these changes might be partially driven by changes in circulating microRNAs — small molecules that can alter gene expression.
While the cosmonauts’ cortisol levels stayed relatively constant, their endocannabinoid system — also part of the immune system — kicked into high gear. More specifically, the cosmonauts’ immune system shifted to a pro-inflammatory state characterised by an aberrant peripheral blood leukocyte distribution, elevated neutrophil activity, and highly amplified TNF and IL-1β response (interestingly, especially against fungal antigen stimulation). What this means is that their immune systems were in panic mode.
Additionally, the researchers observed a reduction in anti-inflammatory capacities. Some of the immune shifts persisted for at least a month post-flight, suggesting a prolonged alteration in adaptive immunity.
The immune system panics, and does not cool down. Together with bacteria’s boosted gene exchange in microgravity, these immune system changes are why superbugs are a real threat during space expeditions. And this is where we can begin to draw a parallel with ageing.
Inflammaging, in space
Ready for a quiz? Here is a list of symptoms: muscle loss, bone density decreases, the cardiovascular system has problems keeping up, and the immune system goes haywire.
That sounds a lot like what we encountered above. It’s also what happens during ageing. The researchers of the cosmonaut study even explicitly write:
“…the results of this study indicate long-duration spaceflight could trigger inflammaging…”
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“Inflammaging” is a type of chronic, low-grade inflammation that, in most people, is linked to the many ageing processes in their bodies. If we look at some common molecular correlates of old age — oxidative stress, DNA damage repair problems, and telomere shortening, for example — we tend to observe a direct or indirect influence of (and, in a vicious cycle, on) inflammation. Inflammaging is the idea that this inflammation is an important driver of several age-related problems.
So, if going to space mimics inflammaging, do astronauts age faster? For now, we do not have an unambiguous answer. Even with epigenetic clocks, it’s not always straightforward to measure the rate of ageing. The NASA Twins Study found signs of DNA damage and inflammation during spaceflight. Oddly, the researchers also observed telomere lengthening in the astronaut twin, but that came with a caveat: after a return to earth, he showed an increased number of shortened telomeres.
Several changes reverted back to baseline after a few months on earth. However, some lingered: gene expression changes, increased DNA damage, and a decline in some cognitive domains. Sounds a bit like ageing…
Safe in space
What do we do against that, now that we are eager to venture into the starry beyond? It’s not — pun intended — rocket science. Despite some recent headlines, we can’t turn back the biological clock. Still, if inflammation is a common denominator in several of the detrimental physiological effects of spaceflight, we might have a few tricks up our sleeves.
A recent review collates the current evidence for space-related inflammaging and suggests a few countermeasures, some of which are already part and parcel of daily astronaut life.
- A good exercise regime is a big one; cardio supports the cardiovascular system and resistance training helps maintain muscle mass as well as mitigate chronic inflammation.
- A diet filled with micronutrients. Personalised biomarker monitoring could also point to potential deficiencies that can be remediated with supplements. (Recent work suggests that tailored probiotics can play a role here too.)
- A sleep schedule that mimics the diurnal cycle on earth. Not only does this prevent mental and physical fatigue, but it also ensures that the astronauts’ bodies can keep following the circadian rhythm that drives the proper functioning of several physiological processes.
- If we take a (very) speculative look decades ahead, we might also entertain the idea of specific gene edits. For example, the radiation-protective Dsup protein found in tardigrades has already been tentatively shown to work in human cells.
Space is a big, dangerous place and its effects on the human body are not trivial. Yet, the same can be said for crossing oceans in medieval sailboats. As we learn more about the effects of space travel on the human body and improve spaceship technology and physiological travel support, our yearning for the stars will sound ever louder.