Ten teams of researchers are poring over the data collected for NASA’s Twins study — an experiment to see how a year in space affected the health of astronaut Scott Kelly, compared to the health of his identical twin Mark who stayed on Earth. One of those research groups, from Northwestern University, have been focusing on the changes to Scott’s gut microbiome, the thousands of microbe species that live inside the GI tract and help with digestion. And they’ve found an interesting divergence between the two brothers’ bacteria, though they aren’t sure what these changes mean just yet.

There was a sustained shift in the balance between two major groups of bacteria

Based on stool samples from the twins, the Northwestern team found that there was a sustained shift in the balance between two major groups of bacteria — Firmicutes and Bacteroidetes — in Scott’s gut while he was in space. Meanwhile, his brother Mark only experienced minor fluctuations in this balance during the same time period. Once he returned to Earth, though, Scott’s bacterial balance went back to the way it was before the mission.

Scott spent 340 days on board the International Space Station between 2015 and 2016, a time that was referred to as the One-Year Mission. He took numerous biological samples before, during, and after his trip, to help catalog how his body changed while in space. The microgravity environment of lower Earth orbit is known to affect human health, such as causing changes to the cardiovascular system and eye health. However, the effects of long-duration stays in space are not fully known, which prompted NASA to do the One-Year Mission. As an added bonus, Scott’s identical twin Mark, a former NASA astronaut himself, acted as a control subject for the mission, since he shares many of the same genetics as his brother. He also collected samples the same time Scott did, but never left Earth.

The research teams just released the first round of findings from those samples last week at a meeting in Galveston. But there are still a lot of unknowns for the experts. The Northwestern researchers don’t know which group of bacteria had higher concentrations in Scott’s gut, only that there was a major shift in the ratio between the two. And it’s going to take some time to sort through the thousands of bacterial species to determine which ones had higher concentrations than others during the time in space.

“We don’t know what it is about the spaceflight environment that’s driving this change.”

As for the source of this imbalance, that’s still an open question. “We don’t know what it is about the spaceflight environment that’s driving this change,” Martha Hotz Vitaterna, co-leader of the gut bacteria study and deputy director of the Center for Sleep & Circadian Biology at Northwestern, tells The Verge. “We don’t know if it’s the diet, the radiation, the lack of sleep. We don’t know what’s driving it.”

The researchers do have a few theories though, and diet is definitely one of them. The astronauts on the ISS eat very clean, specially prepared food to lower the chances of contracting food borne illnesses. Our gut microbiome is largely influenced by the food we eat, so it’s possible a substantial change in diet may be to blame for the shift in Scott’s bacteria. In fact, the researchers thought the space station food might cause his gut bacteria to be less diverse in space, but the opposite turned out to be true.

“Some of the gut bacteria is normally introduced in the food, so we thought there might be a loss in some species,” says Vitaterna. “We were actually quite surprised to see no evidence of a reduction in the number of species present.”

It’s possible that the microgravity environment itself is to blame for the bacterial changes, but it’s going to be a while before Vitaterna and her team know for sure. They’re also eager to see what some of the other research teams come up with in the long run. For instance, there is a team focusing on how Scott’s immune system changed while in space, and the types of bacteria in the GI tract are partially influenced by changes in an individual’s immune function. “As we get more and more data from the 10 different teams together, it’s going to get more and more fun to look at this,” she says.

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