Body Mass Change and Ultraendurance Performance: A Decrease in Body Mass Is Associated With an Increased Running Speed in Male 100-km Ultramarathoners. Generally, there is the opinion that you have to drink as much as possible in endurance charges, so you do not dehydrate.In particular, the dehydration, expressed as a decrease in body weight, should restrict performance. In ultra skills over 100 km, the situation seems to be a bit different.The fast runners drink more than the slow runners, probably because they are supported by a crew.The increased fluid supply but does not lead to a weight gain resp.Stabilization of the weight.
Much more the fast runners take more weight than the slow runners. From practical importance, a decrease in body weight at a 100 km run seems to improve performance and not worsen. The details can be read below https://pubmed.ncbi.nlm.nih.gov/22614141/