Urinary changes in athletes have become a critical concern due to the increasing number of people engaging in exercise the past few decades (Scheer, 2019). The reported incidence of renal dysfunction during exercise varies widely, ranging from 11% to 100%, depending on the type and intensity of exercise and the athlete’s hydration status (Bellinghieri et al., 2008). Among these changes, athletic pseudonephritis—characterized by elevated urinary erythrocytes, casts, and protein—is the most frequently observed renal dysfunction presentation (Van Biervliet et al., 2013). Given the well-established significance of hematuria and proteinuria as markers of urological and nephrological disorders, their evaluation is essential in cases of evident exercise-induced hematuria.

Prolonged strenuous exercise leads to a decrease in creatinine clearance, resulting in reduced urine flow. Several factors—such as local vascular changes, local hypoxia, lactate accumulation, oxidative stress, hormonal changes, and a general septic reaction—may influence glomerular membrane and tubular cells function in the kidneys during exercise (Shephard, 2016; Wołyniec et al., 2020). These pathophysiological changes result in increased glomerular permeability and decreased renal tubule reabsorption of erythrocytes, casts, and protein (Abarbanel et al., 1990; Fan et al., 2020; Haugen et al., 1980). In addition, myoglobin, a heme-containing protein found in muscle tissues, can contribute to renal stress. During intense or prolonged exercise, the risk of exertional rhabdomyolysis increases due to excessive skeletal muscle breakdown, resulting in myoglobin in circulation and myoglobinuria when the renal threshold is crossed. The overflow of myoglobin can also contribute to proteinuria (Knochel, 1982; Milne, 1988; Rojas-Valverde et al., 2021).

Post-exercise hematuria and proteinuria are usually temporary and can be resolved within a few days without medical supports (Heathcote et al., 2009; Saeed et al., 2012; Varma et al., 2014). However, acute kidney injury (AKI) diagnosis criteria are met in approximately 16%–50% of athletes participating in long-distance races, with endurance runners accounting for 97% of these cases (Rojas-Valverde et al., 2020; Tidmas et al., 2022). The cross-country ultra-marathon (CCUM), which exceeds the classical marathon distance of 42.195 km and takes place on open-air courses over natural terrain, has been growing in popularity. The greater intensity and longer duration of the race results in more evident pathologically and physiologically changes in the competitors’ organs and body systems.

Considering the increasing number of ultra-marathon participants, this observational study aimed to investigate the acute changes in the urinary erythrocytes, casts, and protein in Asian males after a 100 km CCUM race. The secondary aim was to evaluate whether post-exercise hematuria and proteinuria were associated with the participants’ average running speed of these finishers. We believe that these data could provide a practical reference for sports physicians and coaches to make decisions.

Exercise-induced hematuria is common among individuals who participate in a variety of sport categories. Although self-limiting, the repeated erythrocytes loss through the urine can be a contributing factor to the anemia (Abarbanel et al., 1990; Dang, 2001). The incidence of hematuria in ultra-marathoners following 90–100 km races are reported to range from 12% to 54%, with studies indicating that it typically resolves within a few hours to 7 days (Alvarez et al., 1987; Chiu et al., 2015; Kallmeyer and Miller, 1993; Varma et al., 2014). The 60% incidence of hematuria in finishers included in the present study differs from the 24.4% reported in Kallmeyer’s recruits (Kallmeyer and Miller, 1993), but it is similar to the 53.8% observed by Alvarez (Alvarez et al., 1987). Exercise-induced hematuria is influenced by both the duration and intensity of exercise (Bellinghieri et al., 2008; McInnis et al., 1998). However, no association between running speed and hematuria was observed in the present study.

As for cylindruria, hyaline casts can be found in normal cases of paraphysiological conditions such as after strenuous exercise, and in non-renal disorders such as dehydration. During intense physical activity, renal blood flow can decrease markedly—from approximately 1,000 mL/min at rest to as low as 200 mL/min (Bellinghieri et al., 2008). In addition, a reduction in urine output below 25 mL/h was associated with a significant increase in urinary casts (Haugen et al., 1980). Notably, 2/20 (10%) finishers presented hyaline casts after the race. One of the finishers who exhibited hyaline casts >25/LPF also exhibited granular casts. The presence of granular casts is commonly associated with renal tubular damage (Caleffi and Lippi, 2015; Chawla et al., 2008). Taken together, these results might imply that acute adverse effects on glomerular function occur in response to long-distance running.

Morteza-Khodaee’s study showed that 30% of runners had proteinuria (≥1+) after completing a 161-km ultra-marathon (Khodaee et al., 2021). In the present study, 45% of the included finishers exhibited proteinuria (spot urine PCR >0.2 mg/mg), based on the most reported cutoff for detecting proteinuria (Kamińska et al., 2020). Albuminuria is an early and sensitive marker of kidney risk (de Zeeuw, 2004) and albumin is the main protein lost during exercise (Heathcote et al., 2009; Poortmans et al., 1989). After running, albuminuria can reach a 10 to 25-fold increase in healthy individuals (Poortmans et al., 1997; Poortmans et al., 1989). Wołyniec and colleagues have reported that ACR increased from 5.37 to 49.64 mg/g after a 100 km ultra-marathon (Wołyniec et al., 2020). The ACR of the finishers included in the present study increased from 0 to 31.8 mg/g after the 100 km cross-country run. This data, although far from conclusive, provides additional insights into urine ACR and PCR—important biomarkers of kidney damage—in long-distance running sports.

In the literature discussing during very long runs, the post-exercise proteinuria is said to be related to the duration but not the intensity of exercise (Wołyniec et al., 2020). The data of the present study showed no correlation between running speed and the degree of proteinuria and myoglobinuria. Notably, 13 (65%) of the finishers observed in the present study presented with urinary myoglobin levels above 5 ng/mL after the run. The wide variability in post-race myoglobin levels suggests inter-individual differences that may be influenced by factors such as hydration status, muscle mass, or baseline conditioning. However, myoglobinuria may be asymptomatic, transient, and reversible, and visible myoglobinuria (tea colored or brown colored urine) only occurs when urinary myoglobin exceeds 250 μg/mL (Scalco et al., 2016; Warren et al., 2002). Ultra-marathoners frequently compete in endurance events, where repeated myoglobin overflow may contribute to renal injury through renal vasoconstriction, proximal tubular necrosis, and distal tubular obstruction (Bosch et al., 2009).

As measurements during CCUMs are challenging, the time course of urinary data during ultra-endurance is limited. The strength of our study is that it presents acute urinary changes among finishers of a 100 km CCUM event. The present study has some limitations. First, the small sample size resulted in decreased power to detect significant post-race changes in the evaluated parameters. As CCUM is an extreme sport, it is difficult to enroll many individuals. Second, only male runners were included in the final analysis, which may account for sex differences in the urine data. Therefore, the data presented might not be representative of potential urinary parameter changes in female runners. Third, follow-up urinary tests on participants beyond the immediately post-race period were not performed. Hence, future studies measuring the long-term effects of CCUM races are necessary.

To date, no prior literature has explored acute urinary changes of athletic pseudonephritis in Asian ultra-marathoners during a CCUM. Although our study focuses on immediate post-race changes, it highlights important questions about the potential long-term consequences of repeated episodes of exertional hematuria and proteinuria. Incorporating longitudinal studies and post-race follow-ups would position this work within a broader clinical and research context. Such future investigations could shed light on whether these acute urinary changes fully resolve or contribute to cumulative renal stress over time. We anticipate that further data on ultra-marathons could provide additional insights into this issue.