Going to space is challenging both physically and mentally. The adaptations that the human body goes through are extensive and various health conditions can develop in the process. Among them is space anemia. And researchers have just discovered some crucial insight into the condition.

The leading cause was believed to be a physiological response to the fluids in the astronaut’s body shifting towards the head, as they no longer have to fight off gravity. Astronauts lose 10 percent of the mass in their blood vessels in the first 10 days to compensate for that. But now, work published in Nature Medicine argues that the picture is a lot more complex.

It appears that the bodies of astronauts in space destroy more red blood cells than they would on earth, a phenomenon called hemolysis. The team established that astronauts were losing 54 percent more red blood cells than average on Earth. Using the amount of carbon monoxide released by lungs as a proxy, the team established that astronauts lose 3 million red blood cells a second.

“Space anemia has consistently been reported when astronauts returned to Earth since the first space missions, but we didn’t know why,” lead author Dr. Guy Trudel, a rehabilitation physician and researcher at The Ottawa Hospital and professor at the University of Ottawa, said in a statement. “Our study shows that upon arriving in space, more red blood cells are destroyed, and this continues for the entire duration of the astronaut’s mission.”

The benchmark value on Earth is about 2 million red blood cells being destroyed and created every second. The increased destruction rate is partially compensated by increased production, otherwise, the astronauts would become severely anemic. This change lasted for the whole six months of permanence in orbit and persisted even once astronauts returned to Earth.

The study looked at the profile of 14 astronauts. Thirteen of them had their blood taken when they came back to Earth and five of them were clinically anemic. The space anemia was reversed between three and four months. But the level of red blood cell destruction remained high. After a year, astronauts were still destroying 30 percent more red blood cells than non-astronauts.

“If we can find out exactly what’s causing this anemia, then there is a potential to treat it or prevent it, both for astronauts and for patients here on Earth,” said Dr. Trudel.

“Thankfully, having fewer red blood cells in space isn’t a problem when your body is weightless. But when landing on Earth and potentially on other planets or moons, anemia affecting your energy, endurance, and strength can threaten mission objectives. The effects of anemia are only felt once you land, and must deal with gravity again.”

Both astronauts and space tourists should be screened for blood conditions that can be exacerbated by space anemia. More work is necessary to completely understand how the condition develops.