Acute Effects of Marathon and Ultramarathon Running on Body Composition in Trained Male Athletes

<i>Background and Objectives:</i> This study aimed to compare acute changes in body composition parameters following marathon (42.195 km) and ultramarathon (61 km) runs in trained male athletes, with particular focus on hydration dynamics and metabolic stress. <i>Materials and Methods:</i> Sixteen male amateur endurance runners were assigned to two groups: marathon (<i>n</i> = 8) and ultramarathon (<i>n</i> = 8). Body composition was assessed at three time points pre-race, immediately post-race, and 24 h post-race using bioelectrical impedance analysis. Measurements included body weight (BW), body mass index (BMI), total body water (TBW), total body fat (TBF), lean body mass (LBM), right arm fat (RAF), left arm fat (LAF), right leg fat (RLF), left leg fat (LLF), and torso fat (TF). <i>Results:</i> Both groups exhibited significant reductions in BW and BMI post-race (<i>p</i> < 0.05), with more pronounced changes observed in the ultramarathon group. Partial restoration of these metrics occurred within 24 h, primarily due to glycogen resynthesis and fluid retention. TBW remained stable immediately post-race but increased notably during recovery, particularly in ultramarathon runners, suggesting more effective hydration responses. Muscle and fat-free mass changes were minimal but more favorable in the ultramarathon group. Both total and regional fat percentages declined significantly post-race in both groups, with ultramarathon runners showing greater reductions. <i>Conclusions:</i> Endurance running induces short-term but substantial alterations in body composition, with ultramarathon participation eliciting more pronounced metabolic and fluid balance responses. These findings highlight the importance of race-specific nutritional and hydration strategies tailored to event type and duration.