Characteristics, changes and influence of body composition during a 4486km transcontinental ultramarathon: results from the Transeurope Footrace mobile whole body MRI-project. At Transseuropalauf 2009, a group of runners with a mobile MRI was examined before, during and after the run. The individual body compartments were examined in detail. The crucial results were that the runners lost about half of their body fat during the race.Also bold mass (muscle mass) was lost.
Surprisingly, the visceral fat took the most and fastest from the subcutaneous fat and the fat-free mass. The whole study can be found under https://pubmed.ncbi.nlm.nih.gov/23657091/