How Much Muscle You Lose in a Marathon — What the Data Say
A study published in 2025 in Medicina (MDPI) measured the body composition of 16 recreational runners — eight marathoners, eight ultramarathoners — before the race, right after, and at 24 hours. The result pushes back on a common fear: muscle mass stayed essentially unchanged, while body weight dropped noticeably. If you've ever wondered what you actually lose during a long race, the answer matters more than it seems.
What actually dropped during the race
Both groups lost weight and showed a lower BMI right after the race — 1.65% for the marathoners, 3.46% for the ultramarathoners. The difference makes sense: more kilometers, more time on your feet, more fluid lost.
But once the researchers broke down the components, the picture changed. Lean mass — mostly muscle — didn't drop significantly in either group, at any of the three measurement points. What dropped substantially was total body fat: 9.54% in the marathoners, 15.32% in the ultramarathoners.
In other words, the kilos that vanished off the scale weren't muscle. They were fat mobilized for energy and water lost through sweat and breathing.
Why the weight comes back the next day
At 24 hours post-race, weight and BMI had partly recovered. The authors attribute this to glycogen resynthesis — muscle holds onto water as it rebuilds its carb stores — not to any rebuilding of muscle tissue, which would take much longer.
Basically, the sudden drop on the scale on race day is mostly a snapshot of dehydration and depleted glycogen, not a real change in body composition. That's a good reason not to draw conclusions from a single post-race weigh-in.
What this study doesn't tell you
The limitations are clear and worth stating plainly: 16 men, measured via bioelectrical impedance — a method sensitive to acute shifts in hydration — and a single race. The study shows what happens in the hours around one event, not what happens over an entire training block spanning several months.
The authors also note that other studies on longer ultramarathons have found real drops in muscle mass, likely due to a much larger and more prolonged caloric deficit than what happens in a single race. The practical takeaway: the real risk to your muscle doesn't seem to be race day itself, but the months of underfueling or skipped strength training during prep.
What you see in your own data, not in a study
A study of 16 runners tells you what happens on average, during one race. What matters to you is your own trajectory over an entire training block — not just the day after your marathon. With Trainr, two photos get you a 3D scan, and you track your waist, arms, chest, and body-fat estimate over time. If your strength training tailed off in the months before the race, it'll show up in your arm and chest trend, not in a single number on the scale.
That's the difference between guessing and measuring: an acute post-race weight drop doesn't mean anything to worry about, but a slow, steady decline in muscle mass over three or four months of distance training is worth noticing. Your first scan is free — useful for exactly this kind of check, before you head into a long training cycle.
Scan yourself — the first one is freeThe race itself doesn't eat your muscle — months of underfueled training do.
Trainr is a wellness tool, not a medical device. Measurements and body-composition figures are estimates for self-tracking; for medical questions, talk to a professional.