The human body did not evolve to handle life in space, and it shows in our very blood.

Since our species first started to spend extended periods of time beyond our planet, researchers have noticed a curious and consistent loss of red blood cells among astronauts.

The phenomenon is called 'space anemia', and until recently, its cause was a mystery. Some experts have argued space anemia is only a short-term phenomenon – a brief compensation for the fluid changes in our bodies under microgravity.

A 2022 study, however, points toward a more destructive and lasting mechanism.

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

The readings are far higher than expected, and they come directly from the breath and blood of 14 astronauts on the International Space Station (ISS).

"This is the best description we have of red blood cell control in space and after return to Earth," said epidemiologist Guy Trudel from the University of Ottawa, Canada, back in January 2022.

"These findings are spectacular, considering these measurements had never been made before and we had no idea if we were going to find anything. We were surprised and rewarded for our curiosity."

The measurements were made via blood tests of iron and breath tests based on carbon monoxide.

For every one molecule of carbon monoxide exhaled, a molecule of the pigment found in red blood cells is also destroyed, which makes it a useful approximation of red blood cell loss.

While still firmly grounded on Earth, astronauts in the study were creating and destroying about 2 million red blood cells a second. During their time in orbit, however, their bodies were destroying roughly 3 million blood cells a second.

In microgravity, the human body loses about 10 percent of the liquid flowing through our blood vessels, as blood accumulates in our head and chest. That's why astronauts sometimes look swollen in their videos from the ISS.

For years, this was the explanation for space anemia. Perhaps the loss of red blood cells was our body's way of compensating for a loss in blood volume.

But that's not what the recent study found. Instead of equalizing the makeup of our blood, the loss of red blood cells appears to continue unabated 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 of red blood cells continued 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.

When the astronauts were in space, the loss of red blood cells appears to have led to a higher-than-normal circulation of iron serum in their blood. Without as many red blood cells to transport iron around the body, the astronauts gradually approached anemia, which can be classified into mild, moderate, and severe.

When they returned to Earth, five out of 13 astronauts (one didn't get blood drawn on landing) had reached clinically diagnosable levels of anemia, defined as the condition where the body doesn't have enough red blood cells for its physiological needs.

About three or four months after landing, their red blood cell levels returned to normal. But even a year after their space flight was done, the astronauts' bodies were still destroying 30 percent more red blood cells than before their trip to space.

The study didn't measure red blood cell production, but given that no astronaut suffered severe anemia, despite the significant losses of red blood cells, their bodies may also have been producing more red blood cells than normal while in space.

If that turns out to be true, astronaut diets will need to be adjusted accordingly. An increase in red blood cell production can put added pressure on bone marrow function, and this necessarily requires higher energy consumption.

If astronauts are not properly protected, they could risk damage to their heart, lungs, bones, brain and muscle systems when they 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."

The study was published in Nature Medicine.

A version of this article was first published in January 2022.