Hormonal
Prolonged ultra-endurance exercise leads to a significant decrease in hemoglobin and hematocrit, which is primarily due to plasma volume expansion (hemodilution) rather than a loss of red blood cell mass.
Don't panic if your hemoglobin drops after a long race. It's likely due to your blood plasma expanding to help you regulate temperature and blood pressure, not because you lost blood cells. This usually resolves with rest and hydration.
Haemoglobin decreased from 13.7 g/dL on day 1 to 11.6 g/dL on day 7... Haematocrit followed the same trend... (probably related to haemodilution and not a result of low red blood cell count; see discussion for explanation.
Why this rating
Single case study; mechanism is well-supported by literature cited but not directly measured via gold-standard blood volume analysis in this specific study.
Source
Physiological Responses and Nutritional Intake during a 7-Day Treadmill Running World Record
Nicolas Berger et al. · International Journal of Environmental Research and Public Health · 2020
DOI 10.3390/ijerph17165962
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