The purpose of the present study was to characterize the progression of red blood cell volume (RBCV) expansion and potential volumetric and endocrine regulators of erythropoiesis during endurance training (ET). Nine healthy, untrained volunteers (age = 27 ± 4 yr) underwent supervised ET consisting of 3-4 × 60 min cycle ergometry sessions per week for 8 wk. Plasma volume (PV), RBCV, and overnight fasting hematological markers were determined before and at ,, and of ET. In addition, plasma erythropoietin (EPO), cortisol, copeptin, and proatrial natriuretic peptide concentrations were measured during a 3-h morning period at baseline and postexercise at and PV increased from baseline (2,405 ± 335 ml) at, , and (+374 ± 194, +505 ± 156, and +341 ± 160 ml, respectively, < 0.001). Increases in RBCV from baseline (1,737 ± 442 ml) were manifested at (+109 ± 114 ml, = 0.030) and (+205 ± 109 ml, = 0.001). Overnight fasting plasma EPO concentration increased from baseline (11.3 ± 4.8 mIU/ml) at (+2.5 ± 2.8 mIU·ml, = 0.027) and returned to baseline concentration at and Higher 3-h-postexercise EPO concentration was observed at (11.6 mIU/ml) compared with (8.4 ± 3.9 mIU/ml, = 0.009) and baseline (9.0 ± 4.2 mIU/ml, = 0.019). Linear relationships between EPO concentration and hematocrit (β = -56.2, < 0.001) and cortisol (β = 0.037, < 0.001) were detected throughout the ET intervention. In conclusion, ET leads to mild, transient increases in circulating EPO concentration, concurring with early PV expansion and lowered hematocrit, preceding gradual RBCV enhancement.

Heat acclimation is associated with plasma volume (PV) expansion that occurs within the first week of exposure. However, prolonged effects on hemoglobin mass (Hbmass) are unclear as intervention periods in previous studies have not allowed sufficient time for erythropoiesis to manifest. Therefore, Hbmass, intravascular volumes, and blood volume (BV)-regulating hormones were assessed with 5½ weeks of exercise-heat acclimation (HEAT) or matched training in cold conditions (CON) in 21 male cyclists [(mean ± SD) age: 38 ± 9 years, body weight: 80.4 ± 7.9 kg, VO2peak: 59.1 ± 5.2 ml/min/kg]. HEAT (n = 12) consisted of 1 h cycling at 60% VO2peak in 40°C for 5 days/week in addition to regular training, whereas CON (n = 9) trained exclusively in cold conditions (<15°C). Before and after the intervention, Hbmass and intravascular volumes were assessed by carbon monoxide rebreathing, while reticulocyte count and BV-regulating hormones were measured before, after 2 weeks and post intervention. Total training volume during the intervention was similar (p = 0.282) between HEAT (509 ± 173 min/week) and CON (576 ± 143 min/week). PV increased (p = 0.004) in both groups, by 303 ± 345 ml in HEAT and 188 ± 286 ml in CON. There was also a main effect of time (p = 0.038) for Hbmass with +34 ± 36 g in HEAT and +2 ± 33 g in CON and a tendency toward a higher increase in Hbmass in HEAT compared to CON (time × group interaction: p = 0.061). The Hbmass changes were weakly correlated to alterations in PV (r = 0.493, p = 0.023). Reticulocyte count and BV-regulating hormones remained unchanged for both groups. In conclusion, Hbmass was slightly increased following prolonged training in the heat and although the mechanistic link remains to be revealed, the increase could represent a compensatory response in erythropoiesis secondary to PV expansion.

Elite endurance athletes possess a high capacity for whole-body maximal fat oxidation (MFO). The aim was to investigate the determinants of a high MFO in endurance athletes. The hypotheses were that augmented MFO in endurance athletes is related to concomitantly increments of skeletal muscle mitochondrial volume density (Mito ) and mitochondrial fatty acid oxidation (FAO ), that is, quantitative mitochondrial adaptations as well as intrinsic FAO per mitochondria, that is, qualitative adaptations. Eight competitive male cross-country skiers and eight untrained controls were compared in the study. A graded exercise test was performed to determine MFO, the intensity where MFO occurs (Fat ), and . Skeletal muscle biopsies were obtained to determine Mito (electron microscopy), FAO , and OXPHOS (high-resolution respirometry). The following were higher (P < 0.05) in endurance athletes compared to controls: MFO (mean [95% confidence intervals]) (0.60 g/min [0.50-0.70] vs 0.32 [0.24-0.39]), Fat (46% [44-47] vs 35 [34-37]), (71 mL/min/kg [69-72] vs 48 [47-49]), Mito (7.8% [7.2-8.5] vs 6.0 [5.3-6.8]), FAO (34 pmol/s/mg muscle ww [27-40] vs 21 [17-25]), and OXPHOS (108 pmol/s/mg muscle ww [104-112] vs 69 [68-71]). Intrinsic FAO (4.0 pmol/s/mg muscle w.w/Mito [2.7-5.3] vs 3.3 [2.7-3.9]) and OXPHOS (14 pmol/s/mg muscle ww/Mito [13-15] vs 11 [10-13]) were, however, similar in the endurance athletes and untrained controls. MFO and Mito correlated (r = 0.504, P < 0.05) in the endurance athletes. A strong correlation between Mito and MFO suggests that expansion of Mito might be rate-limiting for MFO in the endurance athletes. In contrast, intrinsic mitochondrial changes were not associated with augmented MFO.