Letter to: Effects of Altitude Descent: Hemodilution in Young Cyclists Int J Sports Med eFirst DOI: 10.1055/a-2858-0984 10.1055/a-2858-0984 Dear Editor, We read with great interest the recent report by Cristancho and colleagues on hemodilution in young cyclists descending from 2,600 m to 450 m for several days, and its potential impact on aerobic capacity, i.e., maximal oxygen uptake (VO 2 max). 1 The authors present valuable and interesting data on the shift in fluid volume that occurs when descending from a higher to a lower altitude under relatively warm conditions. Unfortunately, the impact of these changes on exercise performance was not measured. However, calculations were performed in the ‘Practical consequences for performance’ section, which suggest that the advantage of a higher blood volume on the VO 2 max is completely offset by the reduced hemoglobin Hb values. Based on our experience and previous findings, we argue that these calculations may be inadequate. Specifically, we think that the benefits of higher arterial oxygen saturation (SaO 2 ) during maximal exercise at low versus high altitude have been underestimated. This is a crucial factor in establishing whether athletes living at high altitudes could benefit from training at lower altitudes. Cardiac output ( Q ) and Hb change in parallel in opposite directions after descent, so higher exercising SaO 2 will have a beneficial effect on aerobic power (i.e., VO 2 max). This has been convincingly demonstrated by Vogel et al, who reported hemodynamic and respiratory responses to maximal exercise from high-altitude natives after they had descended from 4,350 m to the sea level over a period of 8 to 13 days ( Table 1 ). 2 Table 1 Physiological responses to maximal cycle exercise of male high-altitude residents descending from 8 to 13 days to the sea level (SL) Reference Altitude (m) VO 2 max (mL/kg/min) Q (L/min) HR (b/min) SV (mL) Hb (g/dL) SaO 2 (%) CaO 2 (vol%) av-O 2 diff (vol%) [ 2 ] 4,350 49.0 16.0 175 91.4 19.0 79.2 20.0 18.6 SL 53.6 17.2 180 95.6 17.6 96.9 23.1 18.9 +7.5% +2.9% +4.6% −7.4% +22.3% +15.5% +1.6% Abbreviations: av-O 2 diff, arterio-venous oxygen difference; CaO 2 , arterial oxygen content; HR, heart rate; Q, cardiac output; SaO 2 , arterial oxygen saturation; SV, stroke volume; VO 2 max, maximal oxygen uptake. These data clearly show how the various components of the oxygen transport system change when descending from a high to a low altitude. They indicate that the improvements in SaO 2 outweigh the reduction in Hb levels. It could be argued that this is due to the participants’ higher living altitude. However, this is contradicted by data presented by Tucker et al, who showed that trained runners living at a moderate altitude of 1,520 m (VO 2 max: 67.7 mL/kg/min) experienced a decrease in SaO 2 from 93.3% at rest to 84.5% during maximum exercise. 3 The SaO 2 of healthy athletes usually does not decrease below 94 to 95% at low altitudes/sea level when measured from arterial blood. 4 Consequently, Q×Hb remains unchanged and SaO 2 can be expected to be higher when descending from a high altitude resulting in higher VO 2 max at a low altitude. If true, high-altitude athletes will benefit from low-altitude training, in a similar way as known from living-high and training-low strategies. 5 However, well-designed studies aimed at addressing this research question are long overdue. Publication History Received: 22 May 2026 Accepted: 24 June 2026 Article published online: 08 July 2026 © 2026. Thieme. All rights reserved. Georg Thieme Verlag KG Oswald-Hesse-Straße 50, 70469 Stuttgart, Germany
Burtscher et al. (Wed,) studied this question.