The human body did not evolve to handle life in space.
Researchers have noticed a loss of red blood cells in astronauts since our species first started to spend extended periods of time outside of our planet.
Until recently, the cause of'space anemia' was a mystery. Some experts believe space anemia is a short-term phenomenon that compensates for the fluid changes in our bodies under microgravity.
A new study points towards a more destructive mechanism.
The human body destroys more red blood cells than it normally would, according to researchers.
The readings come from the breath and blood of 14 astronauts on the International Space Station.
"This is the best description we have of red blood cell control in space and after return to Earth," says epidemiologist Guy Trudel.
We had no idea if we were going to find anything, but these findings are amazing. We were rewarded for our curiosity.
Blood tests of iron and breath tests were used to make the measurements. Red blood cell loss is an approximation of carbon monoxide loss because a molecule of red blood cells is destroyed for every molecule of carbon monoxide exhaled.
The astronauts in the study created and destroyed 2 million red blood cells a second. Their bodies destroyed 3 million blood cells a second.
In microgravity, the human body loses about 10 percent of its liquid flow as blood accumulates in our head and chest. That's the reason why astronauts look swollen in their videos.
For a long time, this was the explanation for space fatigue. The loss of red blood cells may have been our way of making up for the loss of blood volume.
That's not what the study found. Red blood cells are lost in space flight instead of being equalized.
The loss of red blood cells continued even after 120 days, when all the cells had been created in space.
Trudel says that the study shows that the astronauts' red blood cells are destroyed when they arrive in space.
The astronauts lost a lot of red blood cells in space, which may have led to a higher-than-normal circulation of iron in their blood.
Five out of 13 astronauts had reached clinically diagnosable levels of anemia, which is when the body doesn't have enough red blood cells for its needs.
Their red blood cell levels returned to normal after a few months. The astronauts' bodies were still killing more red blood cells a year after they returned from space.
The study didn't measure red blood cell production, but it is possible that the astronauts produced more red blood cells than normal, despite the significant losses of red blood cells.
If that is true, the astronauts' diet will need to be adjusted.
When astronauts return to Earth, they could be at risk of injury to their heart, lungs, bones, brain and muscle systems.
Trudel says that having fewer red blood cells in space isn't a problem.
When you land on Earth or on another planet or moon, you can be affected by anemia, which can affect your energy, endurance, and strength. You must deal with gravity again after you land because the effects of anemia are only felt once.
Nature Medicine published the study.