The effects of spaceflight on the human body are well known, with astronauts seeing some changes to bone density, muscle strength and other physical alterations, such as puffy faces or changes to their eyes. But a new study suggests the changes may go even further – they may cause harm to the immune system and lead to cancer.

The study, which was published in November 2018 in the Journal of Applied Physiology, highlights the negative impact on natural killer (NK) cells for astronauts who have spent approximately six months on the International Space Station.

"What NASA and other space agencies are concerned about is whether or not the immune system is going to be compromised during very prolonged spaceflight missions," said Richard Simpson, senior author and associate professor of nutritional sciences at the University of Arizona, in a statement. "What clinical risks are there to the astronauts during these missions when they're exposed to things like microgravity, radiation and isolation stress? Could it be catastrophic to the level that the astronaut wouldn't be able to complete the mission?"

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Simpson, whose team looked at blood samples from eight astronauts, said there were stark differences when compared to control samples.

"When we look at the [NK-cell] function of the astronaut samples during flight compared to their own samples before they flew, it goes down," Simpson added. "When we compare them to controls who stayed on Earth, it still goes down.I don't think there's any doubt that NK-cell function is decreasing in the spaceflight environment when analyzed in a cell culture system."

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As such, cancer is a "big risk" to astronauts who are in space for prolonged periods of time, Simpson said, due to radiation exposure. "When you're in the space station, it's a very sterile environment - you're not likely to pick up the flu or a rhinovirus or some community-type infection - but the infections that are a problem are the viruses that are already in your body," Simpson added. "These are mostly viruses that cause things like shingles, mononucleosis or cold sores; they stay in your body for the rest of your life, and they do reactivate when you're stressed."

NK cells help "control several types of tumors and microbial infections by limiting their spread and subsequent tissue damage," according to a 2008 study. "Although NK cells might appear to be redundant in several conditions of immune challenge in humans, NK cell manipulation seems to hold promise in efforts to improve hematopoietic and solid organ transplantation, promote antitumor immunotherapy and control inflammatory and autoimmune disorders."

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