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

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

New research shows that even experienced ultramarathon runners face significant energy loss, muscle loss and hormonal changes in real-world races, with the longest distances taking the harshest physiological toll.

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

The study results showed significant energy deficits (averaging nearly 6,800 kcal) as well as significant muscle damage and hormonal changes occurring across all distances, with some markers showing the largest changes in the 230km group rather than consistent deterioration with distance traveled.

These results highlight the urgent need for personalized recovery and energy strategies for extreme endurance athletes and highlight that while severe physiological stress occurs as early as 100 km, the biological cost of running 230 km is different and significantly higher than that of running 100 km.

Growing interest in ultra-endurance events

Ultra-endurance sports have seen continued growth over the past decade, with thousands of athletes now competing in events lasting longer than 24 hours. While the physiological disadvantages of these breeds, particularly their extreme demands for energy availability and immune function, are well known, most existing research has focused on shorter time periods or controlled laboratory environments that lack ecological validity and the ability to reflect real breed conditions.

Consequently, understanding how levels of physiological stress vary with distance remains a significant gap in current exercise science.

Furthermore, there is little data on key appetite-stimulating hormones, such as leptin and ghrelin, during such events. Understanding these physiological fluctuations is critical because sustained negative energy balance can impair endocrine function and delay recovery, potentially jeopardizing long-term health.

Study design and athlete monitoring

The present study aims to fill these knowledge gaps and inform future sport policy by using data from the TorTour de Ruhr 2024, a grueling non-stop ultramarathon event in Germany. Study data was collected from 43 experienced endurance athletes (16 women and 27 men) who were divided into three groups based on their race distance: 100 km, 160.9 km and 230 km. Crucially, these athletes were very experienced and had completed an average of 37 ultramarathons.

Study data included a comprehensive physiological profile of all enrolled 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 that control energy metabolism, including leptin, ghrelin, insulin, glucagon, GLP-1, and irisin, were also recorded and included in subsequent statistical analyses.

Glucose monitoring:A subset of 17 participants were provided continuous glucose monitoring (CGM) systems to track their interstitial glucose levels in real time during their respective races.

Diet 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. In addition, they completed the General Assessment of Side Effects (GASE) questionnaire to assess physical symptoms such as nausea and muscle pain.

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

Extreme deficits and hormonal changes

Study analyzes showed that despite eating a high-carbohydrate diet (which accounted for nearly 79% of intake), study participants were unable to meet their calorie needs and instead experienced severe deficits. Specifically, the mean estimated energy deficit across all distances was calculated to be 6,797 kcal. Notably, this deficit varied significantly by distance, with the 230 km group having a deficit of up to 18,364 kcal. This extreme calorie deprivation was observed to trigger a cascade of hormonal adaptations, although not all hormones showed statistically significant distance-dependent differences.

Key findings included:

Appetite regulationLeptin decreased significantly at the overall group level, with the largest decrease occurring in the 230 km group, while there was only a trend toward reduction in the 100 km group and no significant change in the 160.9 km group. In contrast, ghrelin, the hunger hormone, increased (p = 0.0083).

Metabolic shifts: insulinLevels decreased (p = 0.0033), while glucagon levels increased (p = 0.0139). This mutual shift has already 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 normal ranges, demonstrating the body's remarkable ability to maintain homeostasis under stress.

Irisin release:The study also found a significant increase in irisin (p = 0.0160), a muscle hormone (myokine) related to fat metabolism, suggesting that extreme exercise stimulates adaptive metabolic remodeling.

GLP-1, another hormone examined in the study, showed no significant effects before and after exercise, further highlighting the heterogeneous hormonal responses to extreme endurance exercise.

Impact on ultra-endurance recovery

The present study notes the serious disruptions 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 increases in GASE levels (reported increases in nausea, loss of appetite, muscle pain and fatigue).

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

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