Background: A Brazilian athlete has proposed setting a new world record for consecutive marathons by running 366 marathons in 366 consecutive days. The impact of such a feat on the cardiovascular system is unknown.

Objective: To monitor the cardiovascular system to assess the athlete's cardiovascular adaptations or maladaptations over the period.

Methods: During the pre-study evaluation, we conducted the pre-participation clinical evaluation (PPE) composed of anamnesis, electrocardiogram, blood test, and functional capacity by maximum cardiopulmonary exercise test (CPET). At follow-up, serial CPET, body composition assessment, blood sample, and echocardiogram were periodically performed for 12 months.

Results: At PPE, male, 43-year-old, height: 1.83 m, weight: 76.9 kg, maximum oxygen consumption (VO2max): 52 ml/kg/min, body fat: 12.6%, systolic and diastolic blood pressure: 120/80 mmHg, blood glucose: 92 mg/dL, total cholesterol (TC): 185 mg/dL, high-sensitivity C-reactive protein (hs-CRP): 0.08 mg/dL, creatine phosphokinase (CPK): 183 U/L, and high-sensitivity troponin T (hs-TnT): 7.1 ng/L. At follow-up, the average of VO2max remained at 48.7 ± 1.2 ml/kg/min, left ventricular ejection fraction (LVEF) at 62 ± 2%, LV strain global longitudinal at 19 ± 1%, LV mass index at 83 ± 7 g/m2, hs-CRP at 0.07 ± 0.01 mg/L, CPK at 169 ± 36 U/L, hs-TnT at 8.2 ± 1.4 ng/L, and no malignant arrhythmias were observed.

Conclusion: The athlete's cardiovascular system had adapted to an extremely high volume of consecutive marathons at moderate intensity for one year and remained functioning at normal range. In addition, the athlete set a new world record for most consecutive days to run a marathon, recognized by Guinness World Records.