Body Weight, Serum Sodium Levels, and Renal Function in an Ultra-Distance Mountain Run An Australian group has studied the difference between a planned and free fluid supply at an ultramarathon over 85 km. A hyponatemia was only for 2% of the runners and the sodium mirror in the blood after the run correlated with the fluid supply.
A moderate fluid intake seems to be the best way to run into a hyponatemia. The work is under https://pubmed.ncbi.nlm.nih.gov/25010155/