Do not turn one field study into a universal hydration target
Ten well-trained athletes were measured during a warm Ironman. Average body mass fell by about 3%, while plasma electrolytes did not materially change and finishing core temperature was not related to the change in body mass. In this small field sample, that level of loss was tolerated without observed thermoregulatory failure.
The result argues against forcing fluid solely to prevent any body-mass loss. It does not prescribe a 3% target or show that dehydration is harmless. Build a race plan from practiced drinking, course access, heat, duration and symptoms; avoid both rigid over-drinking and an untested low-fluid strategy.
The athletes were well trained, the conditions were specific and the sample was small. Individual medical risk and extreme weather can change the decision. Read the full design in the PubMed record.