How Much Muscle Runners Lose in a Marathon
A 2025 study in Medicina (MDPI) put 16 amateur male runners through body-composition scans before, immediately after, and 24 hours after a marathon or ultramarathon. The headline number many runners fear — losing hard-earned muscle to endurance training — barely showed up. What did show up was fat loss and a temporary fluid shuffle.
What actually dropped on race day
Researchers split eight runners into a marathon group (42.195 km) and eight into an ultramarathon group (61 km), measuring body composition with bioelectrical impedance at three points: before the race, right after, and a day later. Both groups lost significant body weight and BMI immediately post-race, and the ultramarathon runners lost more of both — unsurprising given the extra distance and time on their feet.
But when the researchers looked specifically at muscle and fat-free mass, the changes were minimal in both groups. If anything, the ultramarathon runners showed slightly smaller muscle losses than the marathon runners, despite covering nearly 19 extra kilometers. That is not what most runners expect to hear.
Where the weight really went: fat and fluid, not muscle
Total and regional body fat percentage fell significantly in both groups after the race, with the steeper drop in the ultramarathon group. That tracks with the sheer caloric demand of hours of running. Total body water, meanwhile, stayed essentially flat right after the finish line — but rose over the following 24 hours, especially in the ultramarathon group, alongside a partial rebound in weight and BMI.
The researchers attribute that rebound to glycogen resynthesis and fluid retention: the body restocking its fuel stores and pulling water back in as part of that process. In other words, a chunk of the number that disappears from the scale on race day is water bound to glycogen, not tissue — and it comes back within a day as the body recovers.
What this study can and can't tell you
The sample is small — eight runners per group — and all were amateur men, so these exact numbers won't generalize perfectly to every runner. The study also used bioelectrical impedance, a convenient field method but not as precise as a gold-standard scan like DXA. And critically, it measured the acute shock of a single race, not the cumulative effect of a full 12- or 16-week training block.
That last point matters. This research doesn't prove that months of marathon training leave muscle untouched — it shows that the muscle loss visible around one race effort is small and tangled up with fluid shifts, not a clear signal that endurance work is quietly eating muscle. The two questions — what happens in one race, and what happens over a training cycle — are different, and only the first has been measured here.
Why tracking beats guessing
If you're mid-training-block and worried about muscle loss, a single before-and-after scale reading won't tell you much — this study shows how much of that number is fluid, not tissue. What matters is the trend over weeks: is your waist holding steady while your mileage climbs, are your arm measurements stable, is your estimated body-fat percentage trending down without your fat-free mass collapsing with it.
Trainr builds a 3D body scan from two phone photos and tracks those measurements — waist, chest, arms, body-fat estimate — over time, so you're watching a trend instead of reacting to one noisy data point after a long run. The first scan is free, and it's most useful as a baseline: something to compare against a month into training, not a single verdict on race day.
Scan yourself — the first one is freeThe marathon takes your glycogen and your water first — your muscle, this study suggests, mostly stays behind.
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.