Space travel poses significant challenges to human health. The lack of gravity can lead to motion sickness, known as space adaptation syndrome, and over time, it can result in muscle loss, bone density issues, and various health complications.

NASA and its global partners have acknowledged these risks and devised effective strategies to mitigate them. However, as they have addressed the hurdles associated with living in zero-gravity, new health concerns have surfaced.

A 2016 study led by NASA and published in the International Journal of General Medicine indicated that time spent in space disrupts the human immune system. The research found that many markers associated with immune function—including T cell signaling and cytokine production—become imbalanced in space, which may lead to several documented health issues.

For instance, astronauts frequently experience skin rashes, allergies, and infections in the respiratory and urinary systems. Additionally, dormant viral infections, such as those from various herpes viruses, can reactivate, leading to cold sores, shingles, and mononucleosis, which can also spread among astronauts.

Concerns persist that immune-related alterations from prolonged space travel could increase astronauts' long-term risks for cancers, cardiovascular diseases, and gastrointestinal disorders such as inflammatory bowel disease (IBD).

Numerous factors in space travel may disrupt astronauts' immune systems, but NASA researchers are increasingly investigating the human microbiome—the vast array of bacteria that inhabit our bodies and play crucial roles in immune function.

Findings from this research could provide valuable insights applicable to people on Earth.

The human body harbors various microbial ecosystems, including distinct communities in the skin, gut, and nasal passages. "In each of these environments, microbes fulfill different roles in maintaining health," states Hernan Lorenzi, PhD, an assistant professor in Infectious Diseases at the J. Craig Venter Institute in Maryland.

Lorenzi has led research projects examining the microbiome's response to extended space missions. In a 2019 study published in Scientific Reports, he and his team analyzed samples of skin, nasal, tongue, and gut bacteria from nine astronauts who spent six months to a year on the International Space Station (ISS). "We observed that many of these microbiomes changed during their mission, and these changes were linked to shifts in immune responses," he notes.

"We identified alterations in the gut microbiome related to immune dysregulation that, if persistent, can lead to various health issues, including cancer."

Lorenzi's research revealed a decrease in Proteobacteria on astronauts' skin, which are known to provide protection against allergic reactions. "Extended space travel often results in skin rashes and infections, and these alterations in skin bacteria might be a contributing factor," he explains.

The team also noted significant changes in the astronauts' gut microbiomes.

"We found that modifications in the intestinal microbiome correlated with immune dysregulation, which could have long-term health consequences, including cancer," he comments. These changes encompassed a decline in three beneficial bacterial species—Fusicatenibacter, Pseudobutyvibrio, and Akkermansia—that help maintain lower gut inflammation levels. "We also observed a decrease in bacteria that support the health of the intestinal epithelium," he adds, highlighting that dysfunction in the epithelial barrier is closely associated with the rise of IBD in recent decades.

The myriad ways in which bacteria affect human health cannot be overstated. It is well-established that our microbiota are not merely passive inhabitants; they actively shape and regulate our immune and nervous systems, among other functions.

The composition of an individual's gut microbiome is particularly significant, with imbalances linked to a range of health issues, from gut disorders and autoimmune conditions to mental health problems. Emerging research also indicates connections between gut microbiome factors and susceptibility to diseases like COVID-19.

Most astronauts’ microbiomes returned to baseline levels within two months of returning to Earth. However, the question remains: what aspects of space travel led to these disruptions?

"That's an interesting question," Lorenzi acknowledges. "We suspect diet might play a role, although astronauts consume a diet specifically designed for their health." Stress, weightlessness, and disruptions to their circadian rhythms could also contribute to the microbiome changes.

However, the ultra-sterile conditions on the ISS are likely significant factors. "Many believe maintaining a clean environment is healthier, but our findings suggest that being in an excessively clean space may not be beneficial," he states.

The notion that environments can be too clean is not new.

The "hygiene hypothesis," which posits that our health might rely on interactions with bacteria and even certain germs, has gained traction since at least 1989. That year, epidemiologist David Strachan identified a correlation between "unhygienic contact" and a lower incidence of conditions like hay fever and eczema in children.

For a time, the hygiene hypothesis faced skepticism due to the lack of understanding regarding its mechanisms. While some germs are harmful, the perception of the body's bacteria as mere opportunistic organisms rather than essential allies has changed in recent years, especially with the growing acknowledgment of the microbiome's critical role in health.

"If one resides in an overly clean environment, it's reasonable to expect a less diverse microbiome due to the absence of regular contact," Lorenzi explains.

Current research has firmly established that early exposure to beneficial bacteria is crucial for populating the body's microbiomes, which then guide immune system activity. It's also becoming evident that the indiscriminate use of antibiotics, especially during childhood, can lead to conditions such as obesity, asthma, and autoimmune disorders.

While we know dietary choices impact gut microbiome composition, the idea that overly sterile environments might pose a risk has only recently come to the forefront. This perspective remains contentious, yet mounting evidence suggests that regular interactions with bacteria—through nature, pets, or social contact—are vital for maintaining microbiome health.

"If you're in an ultra-clean environment, your microbiome is likely to become less diverse because you miss out on constant reinfection through contact," Lorenzi notes. "There have been discussions at NASA about whether creating a greener, more natural environment inside spacecraft could allow astronauts to engage with the microorganisms that support their immune health."

However, this presents challenges in space exploration. "One primary objective is to search for life on other planets, so minimizing contamination risks is crucial, including those posed by astronauts and their spacecraft," he clarifies. "This is a key reason for maintaining sterile conditions."

Here on Earth, Lorenzi's findings, along with a plethora of recent microbiome studies, suggest that our efforts to disinfect and sterilize our surroundings may be depriving us of beneficial bacteria rather than protecting us from pathogens. This deficiency may be contributing to the rising prevalence of immune-related and chronic diseases, including IBD, metabolic syndrome, cancer, and autoimmune conditions.

Even as we strive to combat the coronavirus, the CDC has recognized that the likelihood of contracting the virus from contaminated surfaces is "less than 1 in 10,000," and that proper handwashing before meals virtually eliminates this already minimal risk.

"When I was younger, people believed all contact with microbes was harmful," Lorenzi reflects. "Now we understand that only a small number of microorganisms are pathogenic, while many are essential for our survival."