The human body is not adapted for life in space, and this is evident in our blood. Since humans began spending prolonged periods beyond Earth, researchers have consistently observed a peculiar and persistent decline in red blood cells among astronauts. This phenomenon, known as ‘space anemia,’ had its cause shrouded in mystery until recently. Some experts believed it was a short-term effect—a temporary adjustment to fluid changes in our bodies under microgravity.

However, a 2022 study suggests a more damaging and enduring mechanism.

During a six-month space mission, researchers discovered that the human body destroys approximately 54 percent more red blood cells than it normally would.

These readings, obtained from the breath and blood of 14 astronauts on the International Space Station (ISS), surpassed expectations and revealed a previously unknown aspect of red blood cell regulation in space and upon return to Earth, according to epidemiologist Guy Trudel from the University of Ottawa, Canada.

The measurements involved blood tests for iron and breath tests based on carbon monoxide. For every molecule of carbon monoxide exhaled, a molecule of the pigment found in red blood cells is also destroyed, serving as a useful approximation of red blood cell loss.

On Earth, astronauts were producing and destroying about 2 million red blood cells per second. In orbit, however, their bodies were destroying roughly 3 million blood cells per second.

Contrary to the earlier explanation for space anemia, which attributed it to compensating for a loss in blood volume, the recent study found that the loss of red blood cells continued throughout space flight.

Even after 120 days, when all the red blood cells in an astronaut’s body had been created in space, the loss persisted at a similar pace.

“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,” said Trudel.

The loss of red blood cells in space led to a higher-than-normal circulation of iron serum in their blood. This resulted in astronauts gradually approaching anemia, with five out of 13 astronauts reaching clinically diagnosable levels upon return to Earth.

Although red blood cell levels returned to normal about three or four months after landing, the astronauts’ bodies were still destroying 30 percent more red blood cells even a year after their space flight.

While the study did not measure red blood cell production, the absence of severe anemia suggests that astronauts may have been producing more red blood cells than normal in space.

If this is true, adjustments to astronaut diets may be necessary, as increased red blood cell production can strain bone marrow function and require higher energy consumption.

Without proper protection, astronauts could risk damage to their heart, lungs, bones, brain, and muscle systems upon their return to Earth.

“Thankfully, having fewer red blood cells in space isn’t a problem when your body is weightless,” explained Trudel. “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.”

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