Author(s): KRUMM, B., RABERIN, A., CALLOVINI, A., SCHORDERET, F., REMACLE, N., MILLET, G.P., Institution: UNIVERSITY OF LAUSANNE, Country: SWITZERLAND, Abstract-ID: 167

INTRODUCTION:
Substantial hemolysis has been suggested during mountain ultramarathons (MUM), primarily due to repeated ground impact and muscle contractions. However, new findings indicate that this hemolysis is insufficient to reduce total hemoglobin mass (Hbmass). Instead, recents results suggest an accelered red blood cell (RBC) turnover. Indeed, although not reaching statistical significance, increases in Hbmass of +12 g (1.4%) and +31 g (3.9%) have been reported following 170-km and 330-km MUMs, respectively. Consequently, one may speculate that Hbmass could increase significantly with increasing MUM distances. Finally, plasma volume (PV) and related total blood volume (BV) expansion has been shown to be proportional to the distance, with no evidence of a physiological plateau to date. The aim of the present study was to investigate changes in circulating blood volumes and erythropoietic variables across a 700-km MUM.
METHODS:
Eighteen participants (of 30) completed the SwissPeaks 2025 race (685 km; 48'020 m of cumulative elevation gain) with an average finisher time of 195 ± 21 h. Participants were tested four times; i.e., before (PRE), after 90 km, 300 km, and immediately after the finish (POST). At each time point, Hbmass, BV, PV, and red blood cell volume (RBCV) were measured using an optimized carbon monoxide rebreathing technique (Detalo Performance). In addition, venous blood samples were collected and analyzed for complete blood count and reticulocyte parameters (Sysmex XN-1000).
RESULTS:
Although unaffected during the first part of the race, significant increases in Hbmass (+32 ± 26 g [3.6%], p < 0.001) and RBCV (+97 ± 85 mL [3.7%], p < 0.001) were observed at POST. Increases in PV and BV were already evident after 90 km (+275 ± 362 mL [7.5%] and +271 ± 362 mL [4.3%], respectively; p = 0.001), further expanding after 300 km (+657 ± 388 mL [18.2%] and +685 ± 397 mL [11%], respectively; p < 0.001) before stabilizing toward the end of the race. Immature reticulocyte fraction (IRF) increased after 300 km (+4.5 ± 11.4%, p = 0.02), while reticulocyte percentage (RET%) rose progressively, reaching 1.3 ± 0.5% (p = 0.03) at km 300 and 1.6 ± 0.5% (p = 0.001) at POST. Finally, mean corpuscular volume (MCV) decreased at km 90 (–0.86 ± 0.80 fL [0.98%], p < 0.001) but gradually increased thereafter, together with red cell distribution width - standard deviation (RDW-SD). Both MCV (+1.09 ± 0.88 fL [1.23%], p < 0.001) and RDW-SD (+2.3 ± 1.6%, p < 0.001) exceeding PRE values at POST.
CONCLUSION:
The hypothesis of increases in Hbmass and RBCV after the race was confirmed. These findings are supported by the sequential rise in IRF and RET%, indicating stimulated erythropoiesis. Similarly, the post-race increases in MCV and RDW-SD reflect a larger average RBC size and greater heterogeneity, likely due to an elevated proportion of young RBC. Finally, results suggest that a physiological plateau in PV expansion during MUMs is reached after approximately 300 km.