High-altitude chronic hypoxia can induce excessive erythrocytosis (EE, defined as a haemoglobin concentration of ≥21 g/dL in men), leading to hyperviscosity and promoting endothelial dysfunction. We aimed to assess whether EE affects the retinal vascular phenotype and the ophthalmological vascular response to CO2. We conducted an ophthalmological cross-sectional study among highlanders permanently living at 5100 m (La Rinconada, Peru). The central retinal artery equivalent (CRAE), central retinal vein equivalent (CRVE) and retinal vessel tortuosity were measured using semi-automatic imaging software (VAMPIRE) from the diameters of the six largest arteries and veins on fundus images. Choroidal blood flow was assessed using laser Doppler flowmetry. Measurements were performed at rest and during a hypercapnic challenge (+10.1 ± 1.4 mmHg end-tidal CO2). Among the 62 included highlanders, 38 (61%) had EE. Compared with non-EE, highlanders with EE exhibited higher CRVE (278 ± 25 vs. 249 ± 24 µm, P < 0.001), with no other significant ophthalmological differences. Resting CRVE was significantly correlated (all P-values < 0.001) with haematocrit (r = 0.56), haemoglobin concentration (r = 0.65), blood volume (r = 0.55) and the arterial partial pressure of carbon dioxide (r = 0.46). Hypercapnia led to a moderate overall decrease in CRVE (-8.6 ± 22.0 µm, P = 0.02), without a specific effect of EE, and induced no other retinal vascular or choroidal blood flow changes in either group. We observed larger retinal vein diameters in EE highlanders compared with non-EE. Although hypercapnia is known to increase retinal vessel diameter in healthy lowlanders, it selectively decreased CRVE in highlanders, irrespective of EE status. These findings suggest a retinal vascular dysfunction in highlanders, probably induced by chronic exposure to severe hypoxia.

This study investigated whether implementing heat training during the final week of a 3-week altitude training camp affects the hematological adaptations and variables monitored during chronic hypoxia. Twenty-three well-trained triathletes (19 males, 4 females) underwent 21 days of altitude training camp in spring (Live-High-Train-High, 1850 m). From day 14 to day 21, participants were split into two groups: a control group (HYPOXIA), which remained in the same condition, while the second group (COMBINED) had a ~1 h cycling session per day conducted in the heat (36°C ± 1°C, 40% ± 5% RH). Training load was the same for both groups. Hematological responses (including hemoglobin mass (Hbmass) and plasma volume (PV)) were assessed. Monitoring included physiological responses at rest (nocturnal oxygen saturation (SpO2) and heart rate, Hooper and Spiegel questionnaires, hydration status) and during exercise (incl. power, SpO2 through 45 min cycling at fixed heart rate). Both Hbmass (3.2% ± 3.1% for HYPOXIA, 3.7% ± 2.9% for COMBINED, p < 0.001) and PV (p = 0.003) increased during the camp, without interaction between groups (p ≥ 0.266). Exercise SpO2 decreased between the end of the second and the third week (p = 0.034, d = 0.583) in the COMBINED group. No difference between groups was detected for the other physiological or perceptual parameters. Adding heat training during the third week of an altitude training camp did not affect the increase in Hbmass or monitoring variables at rest. However, it did not confer any additional PV expansion and was associated with a reduced exercise SpO2.

Ultra-endurance running imposes extreme demands on oxygen transport, yet how red blood cells (RBCs) respond at the molecular level remains poorly defined. We integrated plasma and RBC multi-omics with hematology and hemorheology in athletes sampled before and after two trail races of distinct duration: a 40-km marathon (MCC) and a 171-km ultramarathon (UTMB). Both races elicited systemic inflammation, but UTMB was distinguished by marked IL-6 and kynurenine increases, acute-phase protein induction, and profound lipid remodeling. In RBCs, acylcarnitine accumulation, pantothenate depletion, and oxidized lipid species indicated Lands cycle activation, while purine salvage and carboxylate metabolism reflected redox-sensitive rerouting of energy pathways. Proteomics revealed non-random oxidation, particularly methionine oxidation of antioxidant enzymes, metabolic proteins, and proteasome components, correlating with impaired deformability as gleaned by testing of rheological properties. Elevated copper provided an additional correlate of reduced RBC mechanics. Despite minimal signatures of intravascular hemolysis, plasma bilirubin and hypoxanthine rose, consistent with extravascular clearance of damaged RBCs. Collectively, these results demonstrate that ultra-running accelerates RBC aging through inflammatory and oxidative pathways beyond mechanical trauma, linking systemic cytokine responses to molecular lesions, biomechanical dysfunction, and splenic sequestration. These findings not only identify actionable biomarkers of exercise-induced hemolysis but also provide translational insight into oxidative lesions that similarly limit RBC survival in transfusion and inflammatory disease settings.

Hypoxia research has significantly advanced our understanding of how the human body responds to low-oxygen environments, yet women are still frequently studied without adequate consideration of hormonal status or reproductive life-course stage. This review examines the complex interactions between hormonal status across the female life course and the specific physiological responses of women to acute and chronic hypoxia, focusing on the menstrual cycle, hormonal contraception, pregnancy, menopausal status, hormone replacement therapy, gender-affirming hormone therapy, and reproductive health in high-altitude environments. Estrogen and progesterone significantly modulate ventilatory, cardiovascular, hematological, vascular, and muscular/metabolic responses to hypoxia, with distinct effects across different menstrual cycle phases, contraceptive regimens, and life-course states. Menopause introduces additional complexities, as declining hormone levels alter the body’s ability to acclimatize to low-oxygen conditions. Additionally, we examine how chronic and lifelong hypoxia impacts reproductive health, including fertility and pregnancy outcomes, in women living at high altitudes, highlighting both physiological adaptations and contextual factors. While current research has made progress, further studies are needed to better understand these sex-specific responses. We propose that future research should integrate stratified approaches, accounting for hormonal status (cycle phase, contraceptive use, pregnancy, HRT, GAHT) and reproductive status, to optimize health and performance recommendations for women exposed to hypoxic environments. Hormonal status and reproductive life-course stage may modulate women’s physiological responses to hypoxia. Hypoxic exposure models differ in dose, duration, mechanisms, and physiological interpretation. Menstrual cycle phase, hormonal contraception, pregnancy, menopause, hormone replacement therapy, and gender-affirming hormone therapy represent distinct hormonal contexts. Hormonal modulation may affect ventilatory, cardiovascular, vascular, hematological, metabolic, and placental responses. Stratified study designs should report hormonal status, hypoxic dose, oxygen saturation response, exposure duration, and rest, exercise, or sleep context.

Introduction:Ultra-marathon races present a prolonged cardiopulmonary physiological stress but the magnitude, persistence, and impact of race environment on respiratory function are incompletely understood. The aim of this study is to observe the effect of ultra-marathon trail races on both lung function and airway impedance. Methods:Fifty-seven individuals (49/8, M/F) participated in either the Courmayeur-Champex-Chamonix (CCC®, 101.5km), Ultra Trail du Mont-Blanc (UMTB®, 171.5 km), or the Hong Kong 100 (HK, 100 km) races. Participant demographics were measured pre-race and included: age (40.7±10.3 yrs); height (175.5±7.1 cm), body mass (69.7±8.3 kg). Spirometry, airway impedance (via forced oscillation technique, FOT), maximal inspiratory (MIP) and expiratory pressure (MEP), and exhaled nitric oxide (ExNO) were assessed before, immediately (18-4 hours after), and 188-24 hours post-race. Mixed effects analysis with Dunnett's post-hoc correction were conducted to evaluate the effects of race and time. Results:Results demonstrated significant effects of time for: forced vital capacity (FVC), forced expiratory flow over one second (FEV1), peak expiratory flow (PEF), forced expiratory flow between 25-75% (FEF25-75), forced expiratory flow at 25% and at 50% (FEF25 and FEF50, respectively), MIP, MEP, airway reactance (X) at all frequencies from FOT (X5, X11, X19), respiratory rate, and ExNO (all p<0.05). A significant effect of race was also observed for ExNO (p=0.002). Discussion:Findings indicate declines in lung function and airway impedance immediately after ultra-marathon races, some of which, persisted up to 24 hours post-race. Further investigations are required to better understand the etiology of changes in lung function following ultra-endurance events.

Anemia, a global health challenge affecting a quarter of the global population, results from diverse causes such as nutritional deficiencies, chronic diseases, and genetic factors. It disproportionately impacts women of reproductive age and children, leading to significant morbidity and mortality. While high-altitude populations face unique diagnostic challenges due to natural hemoglobin increases, the current World Health Organization cutoffs often overestimate anemia in these regions. Altitude corrections significantly alter prevalence rates, particularly in South American children, leading to misdiagnosis. Proposed solutions include population-specific thresholds and iron status markers like serum hepcidin, though economic constraints and limited test availability remain challenges. Tailored strategies informed by genetic research highlight adaptations in Tibetan and Ethiopian highlanders, demonstrating the need for region-specific approaches. Socioeconomic factors exacerbate anemia in high-altitude areas. Addressing anemia requires updated diagnostic criteria, personalized strategies, and increased awareness to ensure accurate assessments and interventions in diverse populations, especially those residing at high altitudes.

Altitude training camps are frequently used by endurance athletes to increase total hemoglobin mass (Hbmass) and potentially aerobic performance. However, the effects of such intervention on red blood cell (RBC) properties remain largely unexplored, although these factors could influence the physiological responses following altitude training. This self-controlled study investigated the effects of a "live high-train high" (LHTH) altitude training camp on RBC senescence and Hbmass in nine elite swimmers. Participants performed two 4-wk training camps, one at 1,850 m altitude (LHTH) and the other one at sea level. Hbmass, hematocrit, RBC senescence, and deformability were measured before and immediately after both training camps, and again 10 days after the LHTH camp. A 400-m freestyle "all-out" swimming test was performed before and 10 days after each training camp (n = 6). Hbmass, hematocrit, CD71-positive RBCs, RBC phosphatidylserine exposure, and the percentage of RBC retaining mitochondria were increased after LHTH camp. RBC reactive oxygen species content was increased after both interventions, whereas RBC deformability and CD47 exposure remained unchanged. Although LHTH camp was accompanied by an increase in RBC senescence markers, Hbmass remained increased 10 days post-LHTH. Although 400-m swimming performance was not significantly improved 10 days post-LHTH (-3.8 ± 5.1 s), its change was associated with Hbmass variations. These results indicate that a 4-wk LHTH camp (1,850 m) is accompanied by an increase in Hbmass, which persists for up to 10 days after the camp, despite the increase in RBC senescence markers, a persistence that could be explained by a prolonged RBC survival.NEW & NOTEWORTHY This self-controlled study investigated the effects of a "live high-train high" (LHTH) altitude training camp on RBC senescence and Hbmass in nine swimmers. Despite increased RBC senescence markers, Hbmass increases during LHTH camp and remained elevated 10 days postaltitude, a persistence that could be due to an extended RBC lifespan. The self-controlled design, where each athlete completed both an LHTH training camp and a sea-level training camp, enables a rigorous assessment of the altitude effects.

Volatile organic compounds (VOCs) in exhaled breath change significantly after ultramarathons and could help monitor athletes’ physiological status to optimize training. In this study, we investigated how breath VOCs are linked to clinical variables that reflect the cardiovascular and respiratory system. Correlation analysis was performed between blood and respiratory data collected in pre- and post-race samples from 24 elite runners who participated in the 2019 Ultra-Trail du Mont Blanc (UTMB®) ultramarathon. Correlation analysis was then performed between these clinical data and previously published breath VOC data collected from the same individuals. Post-race clinical data showed decreased lung function compared to pre-race. Notably, respiratory parameters, vital capacity (VC) and forced expiratory volume (FEV1), showed positive moderate correlation with VOC 2,3-butanediol (r = 0.53, r = 0.63), a compound produced by bacterial metabolism. We hypothesize that the increase in 2,3-butanediol in post-race breath results from exercise-induced changes in gut microbiome activity, potentially protecting against lung injury. Additionally, correlations between lung function and respiratory muscle function strengthened post-race (VC/FEV1, r = 0.67 to r = 0.84; forced vital capacity (FVC)/maximal expiratory pressure (MEP), r = 0.57 to r = 0.75; FEV1 and MEP, r < 0.5 to r = 0.73). This suggests that exercise-induced changes in gut microbiome activity may indirectly influence these functions. Our findings support the notion of an intricate relationship between exhaustive exercise, altered gut microbiome activity, and lung function, and together they can influence the physiological status and performance of athletes.

Rationale: Volatile organic compounds (VOCs) in exhaled breath change significantly after ultramarathons and could help monitor athletes’ physiological status to optimize training. While VOCs can reflect metabolic changes, the connection between VOCs and clinical blood and respiratory data remains unknown. In this study, we investigated how breath VOCs are linked to clinical variables that reflect the cardiovascular and respiratory system. Methods: Correlation analysis was performed between respiratory data collected in pre- and post-race samples from 24 runners who participated in the 2019 Ultra-Trail du Mont Blanc (UTMB®) ultra-marathon. Correlation analysis was then performed between these clinical data and previously published breath VOC data collected from the same individuals. Results: Clinical data indicated a slight decrease in runner's lung function post-race. Interestingly, in post-race samples, respiratory parameters vital capacity (VC) and forced expiratory volume in one second (FEV1) formed a moderate positive correlation with 2,3-butanediol (r = 0.53, r = 0.63, respectively), a compound produced by bacterial metabolism. We hypothesize that production of 2,3-butanediol results from the effect of exhaustive exercise on the gut microbiome, which may provide protection to ameliorate lung injury. Additionally, a strengthened correlation from moderate (r = 0.5 – 0.7) to strong (r > 0.7) between VC and FEV1 (r = 0.67 to r = 0.84), forced vital capacity (FVC) and maximal expiratory pressure (MEP) (r = 0.57 to r = 0.75), was observed from pre-race to post-race samples. Finally, FEV1 and MEP shifted from no correlation (r < 0.5) in pre-race to a strong correlation (r = 0.73) in post-race. Conclusion: The strengthened correlation between respiratory data in post-race samples suggests that the gut microbiome, under the effect of exercise, may have an indirect impact on both lung function and respiratory muscle function changes. Our findings support the notion that there is an intricate relationship between exhaustive exercise, altered gut microbiome activity, and lung function, and together they can influence athlete performance.

High-altitude training has been a well-integrated tool in elite endurance sport for decades. The underlying premise is that physiological responses to high-altitude exposure, such as an increase in haemoglobin mass, are correlated with exercise performance. In this review, we highlight that physiological adaptations to a given magnitude of hypoxic exposure induce a highly variable range of responses in humans. For haemoglobin mass, this can vary from near nothing to previously unseen augmentations. At present, however, it remains inconclusive whether responses observed after one high-altitude training camp can be reproduced at the individual level during a subsequent camp. Exercise training performed in a warm environment or by wearing heat-capturing garments in a cooler environment has been termed 'heat training' and has, in recent years, become an increasingly popular alternative to altitude training. After a 3- to 5-week period of heat training, a robust increase in haemoglobin mass has repeatedly been reported, although the associated performance gains are only small. Accordingly, heat training should probably not be implemented before all other steps involved in elite training are in place and, importantly, heat training should not be performed at the expense of other important aspects of elite life. Thus, heat training might serve as a substitute for altitude training. Ultimately, it can be recommended that elite athletes experiment with either approach and that the choice of implementation should be made at the individual level, ensuring that the applied stress (hypoxia or heat) is tolerated adequately by the athlete.

Carbon monoxide (CO) rebreathing is frequently used to determine hemoglobin mass (Hbmass) during hypoxic or heat training and high-altitude research. Accurate and reliable carboxyhemoglobin (HbCO) determination is crucial for reliable Hbmass measurements. The aim was therefore to explore the stability of HbCO and interchangeability of two Radiometer analyzers in the determination of Hbmass. Twelve subjects performed a CO rebreathing test. Five capillary blood samples were taken before and after the CO rebreathing test and either analyzed immediately on site (three capillary tubes, Day 1, ABL 90) or stored at room temperature and sent to another laboratory for analysis 4-8 days later (two capillary tubes, ABL 825). Intraclass correlation coefficient (ICC) and relative typical error (TE) were calculated to compare both measurements. A paired sample t test was performed to detect potential differences between Day 1 (ABL 90) and Days 4-8 (ABL 825). A trivial mean difference was observed between the two measurements for ΔHbCO (0.05%, p = 0.01, d = -0.12) and Hbmass (7.7 g, p = 0.01, d = 0.10). High reliability (ICC > 0.98) and low TE (< 0.91%) were found for ΔHbCO and Hbmass. Immediate analysis with the same analyzer remains recommended despite trivial differences between measurements. However, when logistical issues (analyzer breakdown, extreme, and/or remote locations) do not allow optimal procedures, delayed analysis, potentially with a different analyzer, might be used as a viable alternative.

Background Since vascular responses to hypoxia in both healthy high-altitude natives and chronic mountain sickness (a maladaptive high-altitude pathology characterised by excessive erythrocytosis and the presence of symptoms-CMS) remain unclear, the role of inflammation and oxidative/nitrosative stress on the endothelium-dependent and -independent responses in both the micro- and macrocirculation, in healthy Andeans at different altitudes and in CMS patients, was examined. Methods 94 men were included: 18 lowlanders (LL), 38 healthy highlanders permanently living at 3800 m (n = 21-HL-3800) or in La Rinconada, the highest city in the world (5100-5300 m) (n = 17-HL-5100/No CMS). Moreover, 14 participants with mild (Mild CMS) and 24 with moderate to severe CMS (Mod/Sev CMS) were recruited. All undertook two reactivity tests: i) local thermal hyperaemia (microcirculation) and ii) flow-mediated dilation (macrocirculation). Endothelium-independent function (glyceryl trinitrate) was also assessed only in La Rinconada. Findings Conductance and skin blood flow velocity during the microcirculation test, as well as macrocirculation progressively decreased with altitude (LL > HL-3800 > HL-5100/No CMS). CMS also induced a decrease in macrocirculation (HL-5100/No CMS > Mild CMS = Mod/Sev CMS), while glyceryl trinitrate restored vascular function. Both oxidative stress and nitric oxide metabolites increased with altitude only. Principal component analysis revealed that increasing inflammation with altitude was associated with a progressive decline in both micro- and macrovascular function in healthy highlanders. Interpretation Both micro and macrovascular function are affected by chronic exposure to hypoxia, the latter being further compounded by CMS.

AbstractChronic mountain sickness is a maladaptive syndrome that affects individuals living permanently at high altitude and is characterized primarily by excessive erythrocytosis (EE). Recent results concerning the impact of EE in Andean highlanders on clotting and the possible promotion of hypercoagulability, which can lead to thrombosis, were contradictory. We assessed the coagulation profiles of Andeans highlanders with and without excessive erythrocytosis (EE+ and EE−). Blood samples were collected from 30 EE+ and 15 EE− in La Rinconada (Peru, 5100–5300 m a.s.l.), with special attention given to the sampling pre‐analytical variables. Rotational thromboelastometry tests were performed at both native and normalized (40%) haematocrit using autologous platelet‐poor plasma. Thrombin generation, dosages of clotting factors and inhibitors were measured in plasma samples. Data were compared between groups and with measurements performed at native haematocrit in 10 lowlanders (LL) at sea level. At native haematocrit, in all rotational thromboelastometry assays, EE+ exhibited hypocoagulable profiles (prolonged clotting time and weaker clot strength) compared with EE− and LL (all P < 0.01). At normalized haematocrit, clotting times were normalized in most individuals. Conversely, maximal clot firmness was normalized only in FIBTEM and not in EXTEM/INTEM assays, suggesting abnormal platelet activity. Thrombin generation, levels of plasma clotting factors and inhibitors, and standard coagulation assays were mostly normal in all groups. No highlanders reported a history of venous thromboembolism based on the dedicated survey. Collectively, these results indicate that EE+ do not present a hypercoagulable profile potentially favouring thrombosis.

PurposeTo investigate the effects of a repeated-sprint training in hypoxia induced by voluntary hypoventilation at low lung volume (RSH-VHL) including end-expiratory breath holding (EEBH) of maximal duration.MethodsOver a 4-week period, twenty elite judo athletes (10 women and 10 men) were randomly split into two groups to perform 8 sessions of rowing repeated-sprint exercise either with RSH-VHL (each sprint with maximal EEBH) or with unrestricted breathing (RSN, 10-s sprints). Before (Pre-), 5 days after (Post-1) and 12 days after (Post-2) the last training session, participants completed a repeated-sprint ability (RSA) test on a rowing ergometer (8 × 25-s “all-out” repetitions interspersed with 25 s of passive recovery). Power output (PO), oxygen uptake, perceptual-motor capacity (turning off a traffic light with a predetermined code), cerebral (Δ[Hbdiff]) and muscle (Δ[Hb/Mb]diff) oxygenation, cerebral total haemoglobin concentration (Δ[THb]) and muscle total haemoglobin/myoglobin concentration (Δ[THb/Mb]) were measured during each RSA repetition and/or recovery period.ResultsFrom Pre-to Post-1 and Post-2, maximal PO, mean PO (MPO) of the first half of the test (repetitions 1–4), oxygen uptake, end-repetition cerebral Δ[Hbdiff] and Δ[THb], end-repetition muscle Δ[Hb/Mb]diff and Δ[THb/Mb] and perceptual-motor capacity remained unchanged in both groups. Conversely, MPO of the second half of the test (repetitions 5–8) was higher at Post-1 than at Pre-in RSH-VHL only (p < 0.01), resulting in a lower percentage decrement score over the entire RSA test (20.4% ± 6.5% vs. 23.9% ± 7.0%, p = 0.01). Furthermore, MPO (5–8) was greater in RSH-VHL than in RSN at Post-1 (p = 0.04). These performance results were accompanied by an increase in muscle Δ[THb/Mb] (p < 0.01) and a concomitant decrease in cerebral Δ[THb] (p < 0.01) during the recovery periods of the RSA test at Post-1 in RSH-VHL.ConclusionFour weeks of RSH-VHL including maximal EEBH improved the ability of elite judo athletes to repeat high-intensity efforts. The performance improvement, observed 5 days but not 12 days after training, may be due to enhanced muscle perfusion. The unchanged oxygen uptake and the decrease in cerebral regional blood volume observed at the same time suggest that a blood volume redistribution occurred after the RSH-VHL intervention to meet the increase in muscle perfusion.

PURPOSE:In competitive sport, classic methods of measuring drug prevalence, such as doping controls or questionnaires, are challenging. Here we describe a novel urine sampling method to measure drug use in athletes. We hypothesize that the prevalence of drug use in ultramarathon runners is measured more accurately with our sampling method than randomized-response questionnaires. METHODS:Urine samples and associated demographic data were collected from male participants using blind, automated urinals at the start of ultramarathon races. Various nonprohibited and prohibited substances were subsequently screened. Concomitantly, 2931 male and female runners participating in the same ultramarathons completed an anonymized, randomized-response questionnaire regarding drug use. RESULTS:Among 412 individual urine samples, 205 (49.8%) contained at least one substance, and 16.3% of the samples contained one or more prohibited substances. Substances detected in urine included nonsteroid anti-inflammatory drugs (NSAID) (22.1%), acetaminophen (15.5%), opioids (6.6%), diuretics (4.9%), hypnotics (4.4%), glucocorticoids (2.7%), beta-2 agonists (2.2%), cannabinoids (1.9%), and stimulants (1.2%). None of the samples contained erythropoietin-receptor agonists or suspicious testosterone. Drug use was not associated with the participants' characteristics or ranking. Respondents to the questionnaire reported using acetaminophen (13.6%) and NSAID (12.9%); however, no prohibited substances were declared. CONCLUSIONS:There was a high prevalence of drug use among male ultramarathon runners, in particular, NSAID and painkillers; however, performance-enhancing drugs were marginally used. Blind urine sampling highlighted prohibited drug use not declared in questionnaires, and it is useful to assess the prevalence of drug use and/or doping in competitive athletes.

INTRODUCTION:Endurance exercise at altitude can increase cardiac output and pulmonary vascular pressure to levels that may exceed the stress tolerability of the alveolar-capillary unit. This study examined the effect of ultramarathon trail racing at different altitudes (ranging from <1000 m to between 1500 and 2700 m) on alveolar-capillary recruitment and lung diffusion. METHODS:Cardiac and lung function were examined before and after an ultramarathon in 67 runners (age: 41 ± 9 yr, body mass index: 23 ± 2 kg·m -2 , 10 females), and following 12-24 h of recovery in a subset ( n = 27). Cardiac biomarkers (cTnI and BNP) were assessed from whole blood, whereas lung fluid accumulation (comet tails), stroke volume (SV), and cardiac output ( Q ) were quantified via echocardiography. Lung diffusing capacity for carbon monoxide (DLco) and its components, alveolar membrane conductance (Dm) and capillary blood volume (Vc), were determined via a single-breath method at rest and during three stages of submaximal semirecumbent cycling (20, 30, and 40 W). RESULTS:Average race time was 25 ± 12 h. From pre- to post-race, there was an increase in cardiac biomarkers (cTnI: 0.04 ± 0.02 vs 0.13 ± 0.03 ng·mL -1 , BNP: 20 ± 2 vs 112 ± 21 pg·mL -1 ; P < 0.01) and lung comet tails (2 ± 1 vs 7 ± 6, P < 0.01), a decrease in resting and exercise SV (76 ± 2 vs 69 ± 2 mL, 40 W: 93 ± 2 vs 88 ± 2 mL; P < 0.01), and an elevation in Q at rest (4.1 ± 0.1 vs 4.6 ± 0.2 L·min -1 , P < 0.01; 40 W: 7.3 ± 0.2 vs 7.4 ± 0.3 L·min -1 , P = 0.899). Resting DLco and Vc decreased after the race ( P < 0.01), whereas Dm was unchanged ( P = 0.465); however, during the three stages of exercise, DLco, Vc, and Dm were all reduced from pre- to post-race (40 W: 36.3 ± 0.9 vs 33.0 ± 0.8 mL·min -1 ·mm Hg -1 , 83 ± 3 vs 73 ± 2 mL, 186 ± 6 vs 170 ± 7 mL·min -1 ·mm Hg -1 , respectively; P < 0.01). When corrected for alveolar volume and Q , DLco decreased from pre- to post-race ( P < 0.01), and changes in DLco were similar for all ultramarathon events ( P > 0.05). CONCLUSIONS:Competing in an ultramarathon leads to a transient increase in cardiac injury biomarkers, mild lung-fluid accumulation, and impairments in lung diffusion. Reductions in DLco are predominantly caused by a reduced Vc and possible pulmonary capillary de-recruitment at rest. However, impairments in alveolar-capillary recruitment and Dm both contribute to a fall in exertional DLco following an ultramarathon. Perturbations in lung diffusion were evident across a range of event distances and varying environmental exposures.