Physiological, nutritional, and thermoregulatory responses of a world-class mountain-ultramarathon athlete during the 2025 Western States Endurance Run 100

Although physiological responses during ultra-endurance events are becoming better understood in recreational runners, very little is known about how these responses manifest in elite athletes. This case study reports the physiological, nutritional, and thermoregulatory responses of an elite ultra-endurance athlete who completed the 2025 Western States Endurance Run (WSER 100) in 14:19:22, finishing third overall and within 10 min of the course record. This case study provides the first comprehensive in-race assessment of energy expenditure and intake, hydration, and renal responses in a world-class athlete under competitive race conditions. Measurements included within-event total energy expenditure (doubly labeled water), energy intake, heart rate, gastrointestinal temperature (telemetric ingestible pill), body mass, renal biomarkers [neutrophil gelatinase-associated lipocalin (NGAL) and kidney injury molecule-1 (KIM-1)], and durability assessed from GPS-derived pacing data. Total energy expenditure was 16,104 kcal. Energy intake totaled 6,720 kcal (∼86 g carbohydrate·h^-1). The athlete consumed 12.5 L of fluids (0.87 L·h^-1; 18.5 g sodium) and lost 4.3% body mass. Mean gastrointestinal temperature was 37.1°C and peaked at 39.4°C. Urinary biomarkers showed transient renal stress, with NGAL concentrations increasing from 9.4 to 25.4 ng·mL^-1 and KIM-1 from 0.30 to 1.70 pg·mL^-1, alongside mild proteinuria and hematuria. Pacing analysis showed a mean normalized speed of 84.8% of predicted critical speed, with a 15% decline across the race, demonstrating exceptional fatigue resistance. This case defines the upper range of energy expenditure (∼18.8 kcal·min^-1) and carbohydrate ingestion sustainable in ultra-marathon running.<b>NEW & NOTEWORTHY</b> This case study provides the first comprehensive, in-race assessment of physiological, nutritional, and thermoregulatory responses in a world-class ultra-endurance athlete during the 2025 Western States Endurance Run. Using doubly labeled water, ingestible telemetry, and renal biomarkers, this study quantifies the upper limits of energy expenditure (18.8 kcal·min^-1), carbohydrate intake (86 g·h^-1), and fatigue resistance achievable in competitive ultra-endurance performance under extreme environmental conditions.