Background <p>Individuals with type 1 diabetes mellitus (T1DM) participating in extreme physical exertion, such as marathon running, pose unique challenges for both participants and their healthcare teams. Several reports have documented the successful completion of marathons by individuals and groups with T1DM. Understanding these athletes’ metabolic characteristics and physical performance could lead to safer and more effective management strategies. This study aimed to evaluate the cardiorespiratory fitness and body composition of individuals with T1DM who ran a marathon.</p> Methods <p>Five men with T1DM and five healthy controls who had completed a marathon took part in this study. Each participant underwent dual-energy X-ray absorptiometry (DXA) to assess body composition, indirect calorimetry to measure resting metabolic rate, and a maximal exercise test on a cycle ergometer to determine maximal oxygen uptake (VO2max).</p> Results <p>We included men aged 44.0 (34.00–48.0) years with diabetes duration of 10.0 (6.0–14.0) years. Their median body mass index was 22.5 (22.0-23.3) and glycated hemoglobin was 5.8 (5.6–6.9)%. Three out of five participants achieved a time in range exceeding 70% for the 90 days preceding the marathon. Their glycemic variability coefficient was 34.6 (27.8–39.5)%. The median fat tissue content was 21.8 (20.2–24.7)%. and muscle tissue content was 73.6 (70.5–74.1)%. The median basal metabolic rate was 1932.0 (1859.0-2046.0) kcal. and the VO2 max was 44.2 (36.5–44.3) ml•kg-1•min-1. Healthy controls did not differ significantly in VO2max and metabolic parameters.</p> Conclusions <p>People with type 1 diabetes who complete a marathon demonstrate good metabolic control and adequate cardiorespiratory fitness for recreational marathon running.</p> Trial registration <p>The trial was registered in ClinicalTrials.gov on April 13, 2025. The registration identification number is NCT06935903.</p>