The goal of this observational study is to learn about the physical capacity and metabolic characteristics of male marathon runners with type 1 diabetes mellitus (T1DM). The main question it aims to answer is:

Do male marathon runners with T1DM have different physical fitness and metabolic profiles compared to healthy runners without diabetes?

Participants with T1DM and healthy controls who have completed a marathon will undergo a single in-person assessment, including body composition analysis, resting metabolic rate measurement, and a maximal exercise test

This observational, cross-sectional case-control study aims to compare physical capacity and metabolic parameters between male marathon runners with type 1 diabetes mellitus (T1DM) and healthy male marathon runners without diabetes. The study focuses on evaluating differences in cardiopulmonary fitness, resting metabolic rate, and body composition between the two groups.

Participants attended a single study visit after overnight fasting. The visit included anthropometric measurements, assessment of medical history, and structured questionnaires on diabetes management (for T1DM participants) and physical activity habits. Body composition was assessed using dual-energy X-ray absorptiometry (DXA), providing detailed analysis of total and regional fat mass, lean tissue mass, and bone mineral density. Daily calibration of the DXA scanner ensured data accuracy and consistency.

Resting metabolic rate (RMR) was measured by indirect calorimetry using a ventilated hood system. Measurements were performed in a controlled environment with standardized temperature and low light to minimize variability. Data collection spanned 25 minutes, with the first 5 minutes discarded for acclimatization and the subsequent 20 minutes analyzed. Energy expenditure and substrate oxidation rates were calculated using established equations (Weir equation and Zuntz table).

Cardiorespiratory fitness was assessed through a maximal exercise test on a cycle ergometer with continuous respiratory gas analysis. The test protocol began with a resting phase and warm-up, followed by incremental workload increases until volitional exhaustion. Parameters such as VO₂max, ventilatory efficiency, gas exchange threshold, and peak workload were measured using a calibrated metabolic cart.

Data analysis involved descriptive statistics, Mann-Whitney U tests for numerical comparisons, and Chi-square tests for categorical variables. Dedicated manufacturer software was used for CPET data processing, and statistical analyses were conducted using R software (version 4.4.1).

Inclusion Criteria:

• A diagnosis of T1DM for at least 1 year.
• Age ≥ 18 years.
• Written informed consent and adherence to the study protocol.

Exclusion Criteria:

• Pregnancy.

• Use of medications significantly affecting metabolism or exercise performance

• Presence of chronic medical or psychiatric conditions limiting safe participation

  •Presence of advanced chronic complications of diabetes: proliferative retinopathy, diabetic kidney disease in stages III-V, cardiovascular diseases

• Inability to safely complete maximal exercise testing.

Adult marathon runners aged ≥18 years. Participants in the T1DM group with a confirmed diagnosis of T1DM for at least one year. The control group comprises healthy people with similar physical activity levels but no chronic health conditions.

IPD will not be shared due to the small sample size and the potential risk of participant re-identification, even with de-identified data. The study was not designed with broad data sharing in mind, and no participant consent was obtained for public data release.