Fueling Strategies and Glucose Dynamics in an Ultra-Endurance Tetrathlon

Introduction

This study presents a detailed case report on the fueling strategies and physiological responses of an amateur athlete undertaking a demanding 18.5-hour ultra-endurance tetrathlon. The research provides insights into how carbohydrate intake, glucose dynamics, and metabolic markers behave during and after such an extreme physical challenge, emphasizing the practical aspects of nutrition in multi-discipline endurance events.

The Study in Detail

The study, authored by Trinh J, Edin F, Andersson-Hall U, and Pettersson S, and published in Frontiers in Sports and Active Living, focuses on a single 37-year-old amateur athlete. This athlete completed a condensed "Swedish Classic" tetrathlon within 18.5 hours, involving 316 km of road cycling, 3 km of open-water swimming, 84 km of roller-skiing, and 30 km of trail running, with helicopter transfers between stages. The total active exercise time was 15 hours.

The methodology involved comprehensive monitoring:

• Baseline and Post-Event Testing: Fasting laboratory tests, including venous blood sampling, DXA scans, and a 75-g oral glucose tolerance test (OGTT) with indirect calorimetry, were performed the mornings before and after the event.
• During-Event Monitoring: Weighed-back nutrition and hydration, continuous glucose monitoring (CGM), heart rate (HR), body mass (BM), and detailed stage logistics were recorded throughout the tetrathlon.

Key findings include:

• Energy and Carbohydrate Intake: Total energy intake was 5,825 kcal, with 1,051 g of carbohydrates (13.8 g·kg^-1). This averaged 57 g·h^-1 including transfers and 50 g·h^-1 during active exercise.
• Body Mass Change: The athlete experienced a 2.5 kg (3.8%) decrease in body mass.
• Glucose Dynamics: CGM data indicated stable glucose levels during cycling, swimming, and roller-skiing. Transient hypoglycemia was observed only during the final trail running stage.
• Physiological Response: Mean relative heart rate across all events was 72 ± 6%. Gastrointestinal symptoms were minimal.
• Next-Day Metabolism: The total glucose AUC during the next-day OGTT was unchanged, but showed a higher early peak and earlier nadir. Indirect calorimetry revealed a 28% reduction in carbohydrate oxidation and a 47% increase in fat oxidation.
• Biomarkers: Inflammatory and muscle-damage markers increased, while cardiac troponin I remained within reference limits.

Assessment

This case report provides a unique, integrated dataset on ultra-endurance performance, combining detailed real-world fueling data with continuous physiological monitoring. The primary strength lies in its comprehensive, multi-faceted approach to data collection, including weighed-back nutrition, CGM, and detailed metabolic assessments before and after the event. This offers a granular view of how an athlete's body responds to extreme stress and specific fueling strategies.

The finding that discipline-specific feeding opportunities largely dictated achievable carbohydrate delivery, rather than generic hourly targets, is crucial. It suggests that the practicalities of an event's structure can significantly influence nutritional intake. The observation of stable glucose levels for most of the event, with transient hypoglycemia only in the final stage, indicates that the fueling strategy was largely effective, but highlights potential challenges during prolonged, high-intensity phases like trail running.

Limitations include the study's case report design, meaning the findings are specific to one individual and cannot be generalized to all ultra-endurance athletes. Individual responses to training, nutrition, and stress can vary significantly. However, as a detailed reference observation, it lays important groundwork for future prospective studies.

Practical Relevance

For athletes and coaches involved in ultra-endurance events, this study underscores several practical implications:

• Strategic Fueling: The study emphasizes the need for a highly individualized and discipline-specific fueling plan. Athletes should identify and leverage opportunities within each stage of a multi-sport event to consume carbohydrates, rather than adhering rigidly to hourly targets that might not be feasible during certain disciplines.
• Carbohydrate Intake Targets: The athlete's intake of 50-57 g·h^-1, while substantial, was effective in maintaining glycemic stability for most of the event. This provides a data point for consideration, though individual tolerance and requirements may vary.
• Monitoring Glucose: The use of continuous glucose monitoring (CGM) proved valuable in identifying periods of glycemic stability and transient hypoglycemia. This suggests that real-time CGM, potentially with predefined decision rules for carbohydrate intake, could be a useful tool for optimizing fueling strategies during events, particularly in later stages.
• Post-Event Recovery: The observed shift towards increased fat oxidation and reduced carbohydrate oxidation, alongside elevated inflammatory and muscle damage markers, highlights the significant metabolic stress induced by such events. This reinforces the importance of structured recovery nutrition and rest to facilitate physiological adaptation and repair.

Conclusion

This detailed case report on an amateur athlete completing a condensed ultra-endurance tetrathlon provides valuable insights into the interplay between stage-specific fueling, glucose dynamics, and post-event metabolic responses. It demonstrates that practical, discipline-aware fueling strategies can largely maintain glycemic stability, although challenges may arise in terminal stages. The findings serve as a foundational observation for developing more effective, logistics-aware fueling plans and for guiding future research into real-time glucose monitoring in ultra-endurance sports.