What Happens to Body Composition During a Marathon or Ultramarathon
Endurance races impose an immediate and measurable stress on the body. While long-term training adaptations are well studied, the short-term effects of a single race on body composition and hydration are less clearly understood. A 2025 study published in Medicina examined these acute changes by measuring trained runners before, immediately after, and 24 hours after completing either a marathon or an ultramarathon.
The results show that body weight, fat mass, and body water shift rapidly around race day — and that these changes differ between marathon and ultramarathon distances.
How the Study Was Conducted
Sixteen trained male endurance runners participated. Eight completed a marathon (42.2 km) and eight completed an ultramarathon (61 km). All participants were experienced amateur runners.
Body composition was assessed at three time points:
- pre-race
- immediately post-race
- 24 hours post-race
Measurements were performed using bioelectrical impedance analysis, allowing estimation of body weight, body mass index, total body water, fat mass, fat percentage, and fat-free mass.
Immediate Effects After Racing
Body weight decreased in both race distances
Immediately after finishing, both marathon and ultramarathon runners showed a significant reduction in body weight and BMI. The reduction was greater in ultramarathon runners, reflecting the longer duration of exercise.
Total body water did not decrease
Despite the loss of body weight, total body water did not significantly decline immediately after either race. The authors note that this suggests overall body hydration was maintained during competition, despite prolonged physical stress.
Fat mass declined
Both groups showed reductions in fat mass and fat percentage after racing. These changes were more pronounced in the ultramarathon group and were observed across multiple body regions.
Fat-free mass changed only slightly
Changes in fat-free mass were small in both groups. The study did not show large acute losses of lean mass immediately following either race distance.
Changes Observed 24 Hours After the Race
By 24 hours post-race, body weight partially recovered in both groups. This rebound was more evident in ultramarathon runners.
Total body water increased at this time point, particularly in the ultramarathon group. The authors associate this increase with post-race recovery processes, including replenishment of energy stores and fluid balance.
Fat mass remained lower than baseline at 24 hours post-race, while fat-free mass values remained close to pre-race levels.
Marathon vs. Ultramarathon: A Consistent Pattern
Across nearly all measured variables, ultramarathon runners experienced larger absolute changes than marathon runners:
- greater body weight loss immediately after racing
- greater reductions in fat mass
- greater increase in total body water during recovery
This pattern reflects the greater physiological demand imposed by longer race duration, as documented by the measured outcomes.
What the Study Shows — and What It Does Not
This study demonstrates that endurance races produce rapid and reversible changes in body composition, observable immediately after finishing and evolving within the first 24 hours of recovery.
However, the study:
- does not assess performance outcomes
- does not prescribe nutrition or hydration strategies
- does not quantify substrate use or metabolic rates
- does not examine long-term adaptations
All findings are limited to acute, short-term responses measured with bioelectrical impedance in trained male runners.
Bottom Line
A single marathon or ultramarathon leads to measurable reductions in body weight and fat mass, without a corresponding immediate decrease in total body water. Within 24 hours, body weight and body water partially rebound, especially after ultramarathon racing, while fat mass remains reduced. These findings highlight how quickly body composition can shift around endurance races — and how these shifts depend on race distance.
Source:
Düz S, İlbak İ, Öğüt AEK, Sagat P, Bartik P. Acute Effects of Marathon and Ultramarathon Running on Body Composition in Trained Male Athletes. Medicina (Kaunas). 2025 Nov 28;61(12):2123. doi: 10.3390/medicina61122123. PMID: 41470124; PMCID: PMC12734748.