What happens to your body during an ultramarathon? New study reveals key metabolic changes

By Hugo Francisco de SouzaReviewed by Susha Cheriyedath, M.Sc.Dec 10 2025

New research shows that even seasoned ultramarathoners face profound energy loss, muscle breakdown, and shifting hormones during real-world races, with the longest distances driving the most punishing physiological toll.

Study: Does Distance Matter? Metabolic and Muscular Challenges of a Non-Stop Ultramarathon with Sub-Analysis Depending on Running Distance. Image Credit: lzf / Shutterstock

A recent prospective observational study published in the journal Nutrients tracked ultramarathon athletes over 100 km, 160.9 km (100 miles), and 230 km to assess metabolic, hormonal, and muscular stress in real-world conditions.

Study findings revealed substantial energy deficits (averaging nearly 6,800 kcal) alongside significant muscle damage and hormonal changes that occurred across all distances, with some markers showing the largest alterations in the 230 km group rather than a uniform worsening with mileage.

These findings underscore the critical need for personalized recovery and fueling strategies for extreme endurance athletes, highlighting that while severe physiological strain occurs even at 100 km, the biological cost of running 230 km is distinct from, and substantially more severe than, running 100 km.

Growing Interest in Ultra-Endurance Events

Ultra-endurance sports have witnessed continued growth over the past decade, with thousands of athletes now competing in events lasting longer than 24 hours. While the physiological demerits of these races, especially their extreme demands on energy availability and immune function, are well established, most existing research has focused on shorter timeframes or controlled laboratory settings, which lack ecological validity, the ability to reflect real-world race conditions.

Consequently, understanding how the magnitude of physiological stress scales with distance remains a significant gap in current sport science. 

Furthermore, data on key appetite-regulating hormones, for example, leptin and ghrelin, during such events are scarce. Understanding these physiological fluctuations is vital, as sustained negative energy balance can impair endocrine function and delay recovery, potentially risking long-term health.

Study Design and Athlete Monitoring

The present study aims to address these knowledge gaps and inform future athletic policy by leveraging data from the 2024 TorTour de Ruhr, a gruelling non-stop ultramarathon event held in Germany. Study data were obtained from 43 experienced endurance athletes (16 women and 27 men), divided into three groups based on their race distance: 100 km, 160.9 km, and 230 km. Crucially, these athletes were highly experienced, having completed an average of 37 previous ultramarathons.

The study data comprised a comprehensive physiological profile of all included participants derived from a mix of blood biomarkers, digital monitoring, and surveys:

Biochemical Analysis: Blood and saliva samples were collected immediately before the race and at the finish line to measure and compare markers of muscle damage, specifically creatine kinase muscle type (CKM) and lactate dehydrogenase (LDH). Hormones governing energy metabolism, including leptin, ghrelin, insulin, glucagon, GLP-1, and irisin, were also recorded and included in subsequent statistical analyses.

Glucose Monitoring: A subgroup of 17 participants was provided with continuous glucose monitoring (CGM) systems to track their interstitial glucose levels in real-time during their respective races.

Dietary and Symptom Tracking: Participants were required to track and report their food and fluid intake using the Food Database GmbH, Bremen, Germany (FDDB) database app. Additionally, they completed the General Assessment of Side Effects (GASE) questionnaire to rate physical symptoms such as nausea and muscle pain.

Notably, only 39 of the 43 included participants completed their respective races, and their datasets formed the basis for statistical analyses, including descriptive statistics, the Kolmogorov-Smirnov normality test, and the Wilcoxon matched-pairs signed-rank test.

Extreme Deficits and Hormonal Shifts

Study analyses highlighted that, despite consuming a carbohydrate-heavy diet (accounting for nearly 79% of intake), study participants were unable to meet their calorific needs, instead demonstrating severe deficits. Specifically, the mean estimated energy deficit across all distances was computed at 6,797 kcal. Notably, this deficit varied substantially by distance, with the 230 km group showing a deficit of up to 18,364 kcal. This extreme caloric shortfall was observed to trigger a cascade of hormonal adjustments, although not all hormones demonstrated statistically significant distance-dependent differences.

Key findings included:

Appetite regulation, Leptin dropped significantly at the whole-group level, with the most pronounced decrease occurring in the 230 km group, while showing only a trend toward reduction in the 100 km group and no significant change in the 160.9 km group. Conversely, ghrelin, the hunger hormone, increased (p = 0.0083).

Metabolic shifts: insulin levels decreased (p = 0.0033), while glucagon increased (p = 0.0139). This reciprocal shift has previously been shown to help the body mobilize stored fat and sugar to fuel the brain and muscles. Surprisingly, despite the massive calorie deficits, CGM data showed that glucose levels remained stable and within a normal range, demonstrating the body's remarkable ability to maintain homeostasis under stress.

Irisin Release: The study also noted a significant rise in irisin (p = 0.0160), a muscle-derived hormone (myokine) linked to fat metabolism, suggesting extreme exertion stimulates adaptive metabolic remodeling.

GLP-1, another hormone assessed in the study, did not show significant pre- and post, further highlighting the heterogeneous hormonal responses to extreme endurance exercise.

Implications for Ultra-Endurance Recovery

The present study establishes the severe disturbances in metabolic and structural integrity induced by ultramarathon running, supported by observations of significant increases in CKM and LDH (markers of muscle damage) and post-race surges in GASE scores (reported increases in nausea, loss of appetite, muscle pain, and exhaustion).

Future nutritional protocols should likely emphasize balanced carbohydrate, fat, and protein strategies, including adequate protein intake to support muscle resilience and recovery, while maintaining sufficient carbohydrate availability to stabilize energy supply and endocrine function, thereby improving not only athletic performance but also physiological well-being.