Mountain ultramarathon induces extreme physiological stress for the human body. For instance, a decrease in total hemoglobin mass (Hbmass) due to severe hemolysis is historically suspected. Nevertheless, hematological changes following a 330-km mountain ultramarathon have to date never been investigated.
Blood volumes were determined before (pre-) and after (post-) a 330-km race completed by thirteen participants, through the automated carbon monoxide (CO)-rebreathing method. Native and normalized blood viscosity were determined using a cone/plate viscometer at five different speeds (11.25 to 225 s-1). Biomarkers of inflammation, erythropoiesis, and hemolysis were additionally quantified.
Following the race, an 18% rise in PV (3338 ± 568 vs. 3928 ± 590 mL; p = 0.001) was observed, while absolute Hbmass (802 ± 102 vs. 833 ± 111 g; p = 0.09) did not change significantly. A decrease in native viscosity was reported at all speeds (p < 0.001) with a significant reduction for normalized viscosity at low to intermediate speeds only (i.e. 11.25, 22.5, and 45 s-1). Marked inflammation was suggested by upregulated interleukin-6 (7.1 ± 8 vs. 16.5 ± 14 ng⋅L-1, p = 0.011) and C-reactive protein levels (12.3 ± 14 vs. 51.6 ± 14 μg⋅mL-1, p = 0.001). Besides, the increased erythropoietin (5.7 ± 3 vs. 12 ± 6 mU⋅mL-1, p = 0.021) and erythroferrone levels (6.5 ± 4 vs. 8.5 ± 4 ng⋅L-1, p = 0.001) may indicate enhanced erythropoiesis.
Overall, these findings suggest an enhanced red blood cell turnover, probably triggered by limited exercise-induced hemolysis (although still supported by the decrease in corrected viscosity), likely balanced through accelerated erythropoiesis.