Case Report: Stage-by-stage fueling, glucose dynamics, and next-day metabolism and biomarker responses after baseline testing in an 18.5-hour Swedish classic tetrathlon

Background Amateur ultra-endurance (UE) athletes often exhibit suboptimal fueling, particularly inadequate carbohydrate (CHO) intake, during competition. Integrated real-world datasets that combine weighed-back, stage-specific fueling with blinded continuous glucose monitoring (CGM) and next-day oral glucose tolerance test (OGTT) including indirect calorimetry in a condensed multi-discipline UE setting are scarce. Case presentation A 37-year-old amateur athlete completed a tightly timed “Swedish Classic” on official race courses within 18:30 h:min (18.5 h)—316 km road cycling, 3 km open-water swimming, 84 km roller-skiing, and 30 km trail running—with helicopter transfers between stages [433 km total; 15:01 h:min (15.0 h) active exercise]. Fasting laboratory tests were performed at 07:30 on the mornings before and after the attempt, including venous blood sampling, dual-energy x-ray absorptiometry (DXA), and a 75-g oral glucose tolerance test (OGTT; 120 min) with indirect calorimetry. Weighed-back nutrition and hydration, CGM, heart rate (HR), body mass (BM), and stage logistics were recorded throughout. Results Total energy intake was 5,825 kcal with 1,051 g CHO (13.8 g·kg −1 ), averaging 57 g·h −1 including transfers and 50 g·h −1 during exercise. BM decreased by 2.5 kg (−3.8%). CGM showed stable glucose during cycling, swimming, and roller-skiing, with transient hypoglycaemia confined to the final run. Mean relative HR across all events was 72 ± 6%, and gastrointestinal symptoms were minimal. Next-day OGTT total glucose AUC was unchanged but showed a higher early peak and earlier nadir. Indirect calorimetry indicated reduced CHO oxidation (−28%) and increased fat oxidation (+47%). Inflammatory and muscle-damage markers increased, while cardiac troponin I remained within reference limits. Conclusion In this condensed UE tetrathlon case, discipline-specific feeding opportunities, rather than generic hourly targets, largely determined achievable CHO delivery, and glycaemic excursions occurred only during the terminal running stage. This integrated case provides a reference observation to inform logistics-aware fueling plans and to motivate prospective studies evaluating whether real-time CGM with predefined decision rules improves late-stage fueling and glycaemic stability in UE events.