Anemia in astronauts has been noted since the first space missions, but the mechanisms contributing to anemia in space flight have remained unclear."

Source : Nature Medicine volume 28, pages 59–62 (2022)

Anomalies in the amount of hemoglobin degradation in astronauts (around 12% higher than normal) was first discovered in the 1980s. The degradation leads to anemia, and tends to take hold in the first 10 days or so of micro-gravity spaceflight. After-effects appear to be semi-permanent in some cases.

One year after landing, erythrocytic effects persisted, including increased levels of hemolysis, reticulocytosis and hemoglobin. These findings suggest that the destruction of red blood cells, termed hemolysis, is a primary effect of microgravity in space flight and support the hypothesis that the anemia associated with space flight is a hemolytic condition that should be considered in the screening and monitoring of both astronauts and space tourists."

[ Source as above ]

Despite many decades of research, there is currently no solid agreement on exactly what causes the effects.

Space flights are now known to lead to a wide range of potentially serious health challenges, including loss in bone density and muscle mass, spaceflight-associated neuro-ocular syndrome (SANS), perturbed immune function, and spaceflight anemia. For full technical details, see the 2024 Space Omics and Medical Atlas.

Also see:
Apollo Syndromeplugin-autotooltip__plain plugin-autotooltip_bigApollo Syndrome

A 2016 study into cardiovascular disease (CVD) amongst members of the NASA Apollo missions found that the CVD mortality rate in Apollo lunar astronauts was 4–5 times higher than in non-flight and Low Earth Orbit astronauts.

"[...
Microgravity Immune Suppressionplugin-autotooltip__plain plugin-autotooltip_bigMicrogravity Immune Suppression

"Spaceflight causes alterations in human immunity, a finding which has been well documented immediately following spaceflight. Limited in-flight studies have also confirmed that to some degree immunity is compromised
Spaceflight-associated neuro-ocular syndrome (SANS)plugin-autotooltip__plain plugin-autotooltip_bigSpaceflight-associated neuro-ocular syndrome (SANS)

Approximately 60% of the crew members of the International Space Station have reported altered visual acuity after long-duration exposure to micro-gravity.

"Astronauts on long-duration spaceflight missions may develop changes in ocular structure and function, which can persist for years after the return to normal gravity.